When was statistics used in psychology research first?

When was statistics used in psychology research first?

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I am wondering about the origin of statistics' usage in psychology. Psychology was rooted in philosophy. At that time there were no experiments and analysing the data, but I guess that there must be the very first time when someone came up with the idea to use statistics in the analysis. (and that's my question).

This paper suggests it was a psychophysics experiment by Fencher in 1860.


The word psychology derives from the Greek word psyche, for spirit or soul. The latter part of the word "psychology" derives from -λογία -logia, which refers to "study" or "research". [7] The Latin word psychologia was first used by the Croatian humanist and Latinist Marko Marulić in his book, Psichiologia de ratione animae humanae in the late 15th century or early 16th century. [8] The earliest known reference to the word psychology in English was by Steven Blankaart in 1694 in The Physical Dictionary. The dictionary refers to "Anatomy, which treats the Body, and Psychology, which treats of the Soul." [9]

In 1890, William James defined psychology as "the science of mental life, both of its phenomena and their conditions." [10] This definition enjoyed widespread currency for decades. However, this meaning was contested, notably by radical behaviorists such as John B. Watson, who in his 1913 manifesto defined the discipline of psychology as the acquisition of information useful to the control of behavior. Since James defined "psychology," the term more strongly implicates scientific experimentation. [11] [12] Folk psychology refers to the understanding of ordinary people, as contrasted with that of psychology professionals. [13]

The ancient civilizations of Egypt, Greece, China, India, and Persia all engaged in the philosophical study of psychology. In Ancient Egypt the Ebers Papyrus mentioned depression and thought disorders. [14] Historians note that Greek philosophers, including Thales, Plato, and Aristotle (especially in his De Anima treatise), [15] addressed the workings of the mind. [16] As early as the 4th century BC, Greek physician Hippocrates theorized that mental disorders had physical rather than supernatural causes. [17]

In China, psychological understanding grew from the philosophical works of Laozi and Confucius, and later from the doctrines of Buddhism. This body of knowledge involves insights drawn from introspection and observation, as well as techniques for focused thinking and acting. It frames the universe in term of a division of physical reality and mental reality as well as the interaction between the physical and the mental. Chinese philosophy also emphasized purifying the mind in order to increase virtue and power. An ancient text known as The Yellow Emperor's Classic of Internal Medicine identifies the brain as the nexus of wisdom and sensation, includes theories of personality based on yin–yang balance, and analyzes mental disorder in terms of physiological and social disequilibria. Chinese scholarship that focused on the brain advanced during the Qing Dynasty with the work of Western-educated Fang Yizhi (1611–1671), Liu Zhi (1660–1730), and Wang Qingren (1768–1831). Wang Qingren emphasized the importance of the brain as the center of the nervous system, linked mental disorder with brain diseases, investigated the causes of dreams and insomnia, and advanced a theory of hemispheric lateralization in brain function. [18]

Influenced by Hinduism, Indian philosophy explored distinctions in types of awareness. A central idea of the Upanishads and other Vedic texts that formed the foundations of Hinduism was the distinction between a person's transient mundane self and their eternal, unchanging soul. Divergent Hindu doctrines and Buddhism have challenged this hierarchy of selves, but have all emphasized the importance of reaching higher awareness. Yoga encompasses a range of techniques used in pursuit of this goal. Theosophy, a religion established by Russian-American philosopher Helena Blavatsky, drew inspiration from these doctrines during her time in British India. [19] [20]

Psychology was of interest to Enlightenment thinkers in Europe. In Germany, Gottfried Wilhelm Leibniz (1646–1716) applied his principles of calculus to the mind, arguing that mental activity took place on an indivisible continuum. He suggested that the difference between conscious and unconscious awareness is only a matter of degree. Christian Wolff identified psychology as its own science, writing Psychologia Empirica in 1732 and Psychologia Rationalis in 1734. Immanuel Kant advanced the idea of anthropology as a discipline, with psychology an important subdivision. Kant, however, explicitly rejected the idea of an experimental psychology, writing that "the empirical doctrine of the soul can also never approach chemistry even as a systematic art of analysis or experimental doctrine, for in it the manifold of inner observation can be separated only by mere division in thought, and cannot then be held separate and recombined at will (but still less does another thinking subject suffer himself to be experimented upon to suit our purpose), and even observation by itself already changes and displaces the state of the observed object." In 1783, Ferdinand Ueberwasser (1752-1812) designated himself Professor of Empirical Psychology and Logic and gave lectures on scientific psychology, though these developments were soon overshadowed by the Napoleonic Wars. [21] At the end of the Napoleonic era, Prussian authorities discontinued the Old University of Münster. [22] Having consulted philosophers Hegel and Herbart, however, in 1825 the Prussian state established psychology as a mandatory discipline in its rapidly expanding and highly influential educational system. However, this discipline did not yet embrace experimentation. [23] In England, early psychology involved phrenology and the response to social problems including alcoholism, violence, and the country's crowded "lunatic" asylums. [24]

Beginning of experimental psychology

Gustav Fechner began conducting psychophysics research in Leipzig in the 1830s. He articulated the principle that human perception of a stimulus varies logarithmically according to its intensity. [25] The principle became known as the Weber–Fechner law. Fechner's 1860 Elements of Psychophysics challenged Kant's stricture against the conducting of quantitative research on the mind. [26] [23] In Heidelberg, Hermann von Helmholtz conducted parallel research on sensory perception, and trained physiologist Wilhelm Wundt. Wundt, in turn, came to Leipzig University, establishing the psychological laboratory that brought experimental psychology to the world. Wundt focused on breaking down mental processes into the most basic components, motivated in part by an analogy to recent advances in chemistry, and its successful investigation of the elements and structure of materials. [27] Paul Flechsig and Emil Kraepelin soon created another influential laboratory at Leipzig, a psychology-related lab, that focused more on experimental psychiatry. [23]

The German psychologist Hermann Ebbinghaus, a researcher at the University of Berlin, was another 19th-century contributor to the field. He pioneered the experimental study of memory and developed quantitative models of learning and forgetting. [28] In the early twentieth century, Wolfgang Kohler, Max Wertheimer, and Kurt Koffka co-founded the school of Gestalt psychology (not to be confused with the Gestalt therapy of Fritz Perls). The approach of Gestalt psychology is based upon the idea that individuals experience things as unified wholes. Rather than reducing thoughts and behavior into smaller component elements, as in structuralism, the Gestaltists maintained that whole of experience is important, and differs from the sum of its parts.

Psychologists in Germany, Denmark, Austria, England, and the United States soon followed Wundt in setting up laboratories. [29] G. Stanley Hall, an American who studied with Wundt, founded a psychology lab that became internationally influential. The lab was located at Johns Hopkins University. Hall, in turn, trained Yujiro Motora, who brought experimental psychology, emphasizing psychophysics, to the Imperial University of Tokyo. [30] Wundt's assistant, Hugo Münsterberg, taught psychology at Harvard to students such as Narendra Nath Sen Gupta—who, in 1905, founded a psychology department and laboratory at the University of Calcutta. [19] Wundt's students Walter Dill Scott, Lightner Witmer, and James McKeen Cattell worked on developing tests of mental ability. Cattell, who also studied with eugenicist Francis Galton, went on to found the Psychological Corporation. Witmer focused on the mental testing of children Scott, on employee selection. [31]

Another student of Wundt, the Englishman Edward Titchener, created the psychology program at Cornell University and advanced "structuralist" psychology. The idea behind structuralism was to analyze and classify different aspects of the mind, primarily through the method of introspection. [32] William James, John Dewey, and Harvey Carr advanced the idea of functionalism, an expansive approach to psychology that underlined the Darwinian idea of a behavior's usefulness to the individual. In 1890, James wrote an influential book, The Principles of Psychology, which expanded on the structuralism. He memorably described "stream of consciousness." James's ideas interested many American students in the emerging discipline. [32] [33] [34] Dewey integrated psychology with societal concerns, most notably by promoting progressive education, inculcating moral values in children, and assimilating immigrants. [35]

A different strain of experimentalism, with a greater connection to physiology, emerged in South America, under the leadership of Horacio G. Piñero at the University of Buenos Aires. [36] In Russia, too, researchers placed greater emphasis on the biological basis for psychology, beginning with Ivan Sechenov's 1873 essay, "Who Is to Develop Psychology and How?" Sechenov advanced the idea of brain reflexes and aggressively promoted a deterministic view of human behavior. [37] The Russian-Soviet physiologist Ivan Pavlov discovered in dogs a learning process that was later termed "classical conditioning" and applied the process to human beings. [38]

Consolidation and funding

One of the earliest psychology societies was La Société de Psychologie Physiologique in France, which lasted from 1885 to 1893. The first meeting of the International Congress of Psychology sponsored by the International Union of Psychological Science took place in Paris, in August 1889, amidst the World's Fair celebrating the centennial of the French Revolution. William James was one of three Americans among the four hundred attendees. The American Psychological Association (APA) was founded soon after, in 1892. The International Congress continued to be held at different locations in Europe and with wide international participation. The Sixth Congress, held in Geneva in 1909, included presentations in Russian, Chinese, and Japanese, as well as Esperanto. After a hiatus for World War I, the Seventh Congress met in Oxford, with substantially greater participation from the war-victorious Anglo-Americans. In 1929, the Congress took place at Yale University in New Haven, Connecticut, attended by hundreds of members of the APA. [29] Tokyo Imperial University led the way in bringing new psychology to the East. New ideas about psychology diffused from Japan into China. [18] [30]

American psychology gained status upon the U.S.'s entry into World War I. A standing committee headed by Robert Yerkes administered mental tests ("Army Alpha" and "Army Beta") to almost 1.8 million soldiers. [39] Subsequently, the Rockefeller family, via the Social Science Research Council, began to provide funding for behavioral research. [40] [41] Rockefeller charities funded the National Committee on Mental Hygiene, which disseminated the concept of mental illness and lobbied for applying ideas from psychology to child rearing. [39] [42] Through the Bureau of Social Hygiene and later funding of Alfred Kinsey, Rockefeller foundations helped establish research on sexuality in the U.S. [43] Under the influence of the Carnegie-funded Eugenics Record Office, the Draper-funded Pioneer Fund, and other institutions, the eugenics movement also influenced American psychology. In the 1910s and 1920s, eugenics became a standard topic in psychology classes. [44] In contrast to the US, in the UK psychology was met with antagonism by the scientific and medical establishments, and up until 1939, there were only six psychology chairs in universities in England. [45]

During World War II and the Cold War, the U.S. military and intelligence agencies established themselves as leading funders of psychology by way of the armed forces and in the new Office of Strategic Services intelligence agency. University of Michigan psychologist Dorwin Cartwright reported that university researchers began large-scale propaganda research in 1939–1941. He observed that "the last few months of the war saw a social psychologist become chiefly responsible for determining the week-by-week-propaganda policy for the United States Government." Cartwright also wrote that psychologists had significant roles in managing the domestic economy. [46] The Army rolled out its new General Classification Test to assess the ability of millions of soldiers. The Army also engaged in large-scale psychological research of troop morale and mental health. [47] In the 1950s, the Rockefeller Foundation and Ford Foundation collaborated with the Central Intelligence Agency (CIA) to fund research on psychological warfare. [48] In 1965, public controversy called attention to the Army's Project Camelot—the "Manhattan Project" of social science—an effort which enlisted psychologists and anthropologists to analyze the plans and policies of foreign countries for strategic purposes. [49] [50]

In Germany after World War I, psychology held institutional power through the military and subsequently expanded along with the rest of the military under the Third Reich. [23] Under the direction of Hermann Göring's cousin Matthias Göring, the Berlin Psychoanalytic Institute was renamed the Göring Institute. Freudian psychoanalysts were expelled and persecuted under the anti-Jewish policies of the Nazi Party, and all psychologists had to distance themselves from Freud and Adler, founders of psychoanalysis who were also Jewish. [51] The Göring Institute was well-financed throughout the war with a mandate to create a "New German Psychotherapy." This psychotherapy aimed to align suitable Germans with the overall goals of the Reich. As described by one physician, "Despite the importance of analysis, spiritual guidance and the active cooperation of the patient represent the best way to overcome individual mental problems and to subordinate them to the requirements of the Volk and the Gemeinschaft." Psychologists were to provide Seelenführung [lit., soul guidance], the leadership of the mind, to integrate people into the new vision of a German community. [52] Harald Schultz-Hencke melded psychology with the Nazi theory of biology and racial origins, criticizing psychoanalysis as a study of the weak and deformed. [53] Johannes Heinrich Schultz, a German psychologist recognized for developing the technique of autogenic training, prominently advocated sterilization and euthanasia of men considered genetically undesirable, and devised techniques for facilitating this process. [54]

After the war, new institutions were created although some psychologists, because of their Nazi affiliation, were discredited. Alexander Mitscherlich founded a prominent applied psychoanalysis journal called Psyche. With funding from the Rockefeller Foundation, Mitscherlich established the first clinical psychosomatic medicine division at Heidelberg University. In 1970, psychology was integrated into the required studies of medical students. [55]

After the Russian Revolution, the Bolsheviks promoted psychology as a way to engineer the "New Man" of socialism. Consequently, university psychology departments trained large numbers of students in psychology. At the completion of training, positions were made available for those students at schools, workplaces, cultural institutions, and in the military. The Russian state emphasized pedology and the study of child development. Lev Vygotsky became prominent in the field of child development. [37] The Bolsheviks also promoted free love and embraced the doctrine of psychoanalysis as an antidote to sexual repression. [56] Although pedology and intelligence testing fell out of favor in 1936, psychology maintained its privileged position as an instrument of the Soviet Union. [37] Stalinist purges took a heavy toll and instilled a climate of fear in the profession, as elsewhere in Soviet society. [57] Following World War II, Jewish psychologists past and present, including Lev Vygotsky, A.R. Luria, and Aron Zalkind, were denounced Ivan Pavlov (posthumously) and Stalin himself were celebrated as heroes of Soviet psychology. [58] Soviet academics experienced a degree of liberalization during the Khrushchev Thaw. The topics of cybernetics, linguistics, and genetics became acceptable again. The new field of engineering psychology emerged. The field involved the study of the mental aspects of complex jobs (such as pilot and cosmonaut). Interdisciplinary studies became popular and scholars such as Georgy Shchedrovitsky developed systems theory approaches to human behavior. [59]

Twentieth-century Chinese psychology originally modeled itself on U.S. psychology, with translations from American authors like William James, the establishment of university psychology departments and journals, and the establishment of groups including the Chinese Association of Psychological Testing (1930) and the Chinese Psychological Society (1937). Chinese psychologists were encouraged to focus on education and language learning. Chinese psychologists were drawn to the idea that education would enable modernization. John Dewey, who lectured to Chinese audiences between 1919 and 1921, had a significant influence on psychology in China. Chancellor T'sai Yuan-p'ei introduced him at Peking University as a greater thinker than Confucius. Kuo Zing-yang who received a PhD at the University of California, Berkeley, became President of Zhejiang University and popularized behaviorism. [60] After the Chinese Communist Party gained control of the country, the Stalinist Soviet Union became the major influence, with Marxism–Leninism the leading social doctrine and Pavlovian conditioning the approved means of behavior change. Chinese psychologists elaborated on Lenin's model of a "reflective" consciousness, envisioning an "active consciousness" (pinyin: tzu-chueh neng-tung-li ) able to transcend material conditions through hard work and ideological struggle. They developed a concept of "recognition" (pinyin: jen-shih ) which referred to the interface between individual perceptions and the socially accepted worldview failure to correspond with party doctrine was "incorrect recognition." [61] Psychology education was centralized under the Chinese Academy of Sciences, supervised by the State Council. In 1951, the Academy created a Psychology Research Office, which in 1956 became the Institute of Psychology. Because most leading psychologists were educated in the United States, the first concern of the Academy was the re-education of these psychologists in the Soviet doctrines. Child psychology and pedagogy for the purpose of a nationally cohesive education remained a central goal of the discipline. [62]


In 1920, Édouard Claparède and Pierre Bovet created a new applied psychology organization called the International Congress of Psychotechnics Applied to Vocational Guidance, later called the International Congress of Psychotechnics and then the International Association of Applied Psychology. [29] The IAAP is considered the oldest international psychology association. [63] Today, at least 65 international groups deal with specialized aspects of psychology. [63] In response to male predominance in the field, female psychologists in the U.S. formed the National Council of Women Psychologists in 1941. This organization became the International Council of Women Psychologists after World War II and the International Council of Psychologists in 1959. Several associations including the Association of Black Psychologists and the Asian American Psychological Association have arisen to promote the inclusion of non-European racial groups in the profession. [63]

The International Union of Psychological Science (IUPsyS) is the world federation of national psychological societies. The IUPsyS was founded in 1951 under the auspices of the United Nations Educational, Cultural and Scientific Organization (UNESCO). [29] [64] Psychology departments have since proliferated around the world, based primarily on the Euro-American model. [19] [64] Since 1966, the Union has published the International Journal of Psychology. [29] IAAP and IUPsyS agreed in 1976 each to hold a congress every four years, on a staggered basis. [63]

IUPsyS recognizes 66 national psychology associations and at least 15 others exist. [63] The American Psychological Association is the oldest and largest. [63] Its membership has increased from 5,000 in 1945 to 100,000 in the present day. [32] The APA includes 54 divisions, which since 1960 have steadily proliferated to include more specialties. Some of these divisions, such as the Society for the Psychological Study of Social Issues and the American Psychology–Law Society, began as autonomous groups. [63]

The Interamerican Psychological Society, founded in 1951, aspires to promote psychology across the Western Hemisphere. It holds the Interamerican Congress of Psychology and ha had 1,000 members in year 2000. The European Federation of Professional Psychology Associations, founded in 1981, represents 30 national associations with a total of 100,000 individual members. At least 30 other international organizations represent psychologists in different regions. [63]

In some places, governments legally regulate who can provide psychological services or represent themselves as a "psychologist." [65] The APA defines a psychologist as someone with a doctoral degree in psychology. [66]


Early practitioners of experimental psychology distinguished themselves from parapsychology, which in the late nineteenth century enjoyed popularity (including the interest of scholars such as William James). Some people considered parapsychology to be part of "psychology." Parapsychology, hypnotism, and psychism were major topics at the early International Congresses. But students of these fields were eventually ostractized, and more or less banished from the Congress in 1900–1905. [29] Parapsychology persisted for a time at Imperial University in Japan, with publications such as Clairvoyance and Thoughtography by Tomokichi Fukurai, but here too it was mostly shunned by 1913. [30]

As a discipline, psychology has long sought to fend off accusations that it is a "soft" science. Philosopher of science Thomas Kuhn's 1962 critique implied psychology overall was in a pre-paradigm state, lacking agreement on the type of overarching theory found in mature sciences such as chemistry and physics. [67] Because some areas of psychology rely on research methods such as surveys and questionnaires, critics asserted that psychology is not an objective science. Skeptics have suggested that personality, thinking, and emotion cannot be directly measured and are often inferred from subjective self-reports, which may be problematic. Experimental psychologists have devised a variety of ways to indirectly measure these elusive phenomenological entities. [68] [69] [70]

Divisions still exist within the field, with some psychologists more oriented towards the unique experiences of individual humans, which cannot be understood only as data points within a larger population. Critics inside and outside the field have argued that mainstream psychology has become increasingly dominated by a "cult of empiricism," which limits the scope of research because investigators restrict themselves to methods derived from the physical sciences. [71] Feminist critiques have argued that claims to scientific objectivity obscure the values and agenda of (historically) mostly male researchers. [39] Jean Grimshaw, for example, argues that mainstream psychological research has advanced a patriarchal agenda through its efforts to control behavior. [72]


Psychologists generally consider biology the substrate of thought and feeling, and therefore an important area of study. Behaviorial neuroscience, also known as biological psychology, involves the application of biological principles to the study of physiological and genetic mechanisms underlying behavior in humans and other animals. The allied field of comparative psychology is the scientific study of the behavior and mental processes of non-human animals. [73] A leading question in behavioral neuroscience has been whether and how mental functions are localized in the brain. From Phineas Gage to H.M. and Clive Wearing, individual people with mental deficits traceable to physical brain damage have inspired new discoveries in this area. [74] Modern behavioral neuroscience could be said to originate in the 1870s, when in France Paul Broca traced production of speech to the left frontal gyrus, thereby also demonstrating hemispheric lateralization of brain function. Soon after, Carl Wernicke identified a related area necessary for the understanding of speech. [75]

The contemporary field of behavioral neuroscience focuses on the physical basis of behavior. Behaviorial neuroscientists use animal models, often relying on rats, to study the neural, genetic, and cellular mechanisms that underlie behaviors involved in learning, memory, and fear responses. [76] Cognitive neuroscientists, by using neural imaging tools, investigate the neural correlates of psychological processes in humans. Neuropsychologists conduct psychological assessments to determine how an individual's behavior and cognition are related to the brain. The biopsychosocial model is a cross-disciplinary, holistic model that concerns the ways in which interrelationships of biological, psychological, and socio-environmental factors affect health and behavior. [77]

Evolutionary psychology approaches thought and behavior from a modern evolutionary perspective. This perspective suggests that psychological adaptations evolved to solve recurrent problems in human ancestral environments. Evolutionary psychologists attempt to find out how human psychological traits are evolved adaptations, the results of natural selection or sexual selection over the course of human evolution. [78]

The history of the biological foundations of psychology includes evidence of racism. The idea of white supremacy and indeed the modern concept of race itself arose during the process of world conquest by Europeans. [79] Carl von Linnaeus's four-fold classification of humans classifies Europeans as intelligent and severe, Americans as contented and free, Asians as ritualistic, and Africans as lazy and capricious. Race was also used to justify the construction of socially specific mental disorders such as drapetomania and dysaesthesia aethiopica—the behavior of uncooperative African slaves. [80] After the creation of experimental psychology, "ethnical psychology" emerged as a subdiscipline, based on the assumption that studying primitive races would provide an important link between animal behavior and the psychology of more evolved humans. [81]


A tenet of behavioral research is that a large part of both human and lower-animal behavior is learned. A principle associated with behavioral research is that the mechanisms involved in learning apply to humans and non-human animals. Behavioral researchers have developed a treatment known as behavior modification, which is used to help individuals replace undesirable behaviors with desirable ones.

Early behavioral researchers studied stimulus–response pairings, now known as classical conditioning. They demonstrated that when a biologically potent stimulus (e.g., food that elicits salivation) is paired with a previously neutral stimulus (e.g., a bell) over several learning trials, the neutral stimulus by itself can come to elicit the response the biologically potent stimulus elicits. Ivan Pavlov—known best for inducing dogs to salivate in the presence of a stimulus previously linked with food—became a leading figure in the Soviet Union and inspired followers to use his methods on humans. [37] In the United States, Edward Lee Thorndike initiated "connectionist" studies by trapping animals in "puzzle boxes" and rewarding them for escaping. Thorndike wrote in 1911: "There can be no moral warrant for studying man's nature unless the study will enable us to control his acts." [82] From 1910–1913 the American Psychological Association went through a sea change of opinion, away from mentalism and towards "behavioralism." In 1913 John B. Watson coined the term behaviorism for this school of thought. [83] Watson's famous Little Albert experiment in 1920 was at first thought to demonstrate that repeated use of upsetting loud noises could instill phobias (aversions to other stimuli) in an infant human, [12] [84] although such a conclusion was likely an exaggeration. [85] Karl Lashley, a close collaborator with Watson, examined biological manifestations of learning in the brain. [74]

Clark L. Hull, Edwin Guthrie, and others did much to help behaviorism become a widely used paradigm. [32] A new method of "instrumental" or "operant" conditioning added the concepts of reinforcement and punishment to the model of behavior change. Radical behaviorists avoided discussing the inner workings of the mind, especially the unconscious mind, which they considered impossible to assess scientifically. [86] Operant conditioning was first described by Miller and Kanorski and popularized in the U.S. by B.F. Skinner, who emerged as a leading intellectual of the behaviorist movement. [87] [88]

Noam Chomsky published an influential critique of radical behaviorism on the grounds that behaviorist principles could not adequately explain the complex mental process of language acquisition and language use. [89] [90] The review, which was scathing, did much to reduce the status of behaviorism within psychology. [91] Martin Seligman and his colleagues discovered that they could condition "learned helplessness" in dogs, a state that was not predicted by the behaviorist approach to psychology. [92] [93] Edward C. Tolman advanced a hybrid "cognitive behavioral" model, most notably with his 1948 publication discussing the cognitive maps used by rats to guess at the location of food at the end of a maze. [94] Skinner's behaviorism did not die, in part because it generated successful practical applications. [90]

The Association for Behavior Analysis International was founded in 1974 and by 2003 had members from 42 countries. The field has gained a foothold in Latin America and Japan. [95] Applied behavior analysis is the term used for the application of the principles of operant conditioning to change socially significant behavior (it supersedes the term behavior modification). [96]


Green Red Blue
Purple Blue Purple

Blue Purple Red
Green Purple Green

The Stroop effect is the fact that naming the color of the first set of words is easier and quicker than the second.

Cognitive psychology involves the study of mental processes, including perception, attention, language comprehension and production, memory, and problem solving. [97] Researchers in the field of cognitive psychology are sometimes called cognitivists. They rely on an information processing model of mental functioning. Cognitivist research is informed by functionalism and experimental psychology.

Starting in the 1950s, the experimental techniques developed by Wundt, James, Ebbinghaus, and others re-emerged as experimental psychology became increasingly cognitivist and, eventually, constituted a part of the wider, interdisciplinary cognitive science. [98] Some called this development the cognitive revolution because it rejected the anti-mentalist dogma of behaviorism as well as the strictures of psychoanalysis. [98]

Albert Bandura helped along the transition in psychology from behaviorism to cognitive psychology. Bandura and other social learning theorists advanced the idea of vicarious learning. In other words, they advanced the view that a child can learn by observing his or her social environment and not necessarily from having been reinforced for enacting a behavior, although they did not rule out the influence of reinforcement on learning a behavior. [99]

Technological advances also renewed interest in mental states and mental representations. English neuroscientist Charles Sherrington and Canadian psychologist Donald O. Hebb used experimental methods to link psychological phenomena to the structure and function of the brain. The rise of computer science, cybernetics, and artificial intelligence underlined the value of comparing information processing in humans and machines.

A popular and representative topic in this area is cognitive bias, or irrational thought. Psychologists (and economists) have classified and described a sizeable catalogue of biases which recur frequently in human thought. The availability heuristic, for example, is the tendency to overestimate the importance of something which happens to come readily to mind. [100]

Elements of behaviorism and cognitive psychology were synthesized to form cognitive behavioral therapy, a form of psychotherapy modified from techniques developed by American psychologist Albert Ellis and American psychiatrist Aaron T. Beck.

On a broader level, cognitive science is an interdisciplinary enterprise involving cognitive psychologists, cognitive neuroscientists, linguists, and researchers in artificial intelligence, human–computer interaction, and computational neuroscience. The discipline of cognitive science covers cognitive psychology as well as philosophy of mind, computer science, and neuroscience. [101] Computer simulations are sometimes used to model phenomena of interest.


Social psychology is concerned with how behaviors, thoughts, feelings, and the social environment influence human interactions. [102] Social psychologists study such topics as the influence of others on an individual's behavior (e.g. conformity, persuasion) and the formation of beliefs, attitudes, and stereotypes about other people. Social cognition fuses elements of social and cognitive psychology for the purpose of understanding how people process, remember, or distort social information. The study of group dynamics involves research on the nature of leadership, organizational communication, and related phenomena. In recent years, many social psychologists have become increasingly interested in implicit measures, mediational models, and the interaction of person and social factors in accounting for behavior. Some concepts that sociologists have applied to the study of psychiatric disorders, concepts such as the social role, sick role, social class, life events, culture, migration, and total institution, have influenced social psychologists. [103]


Psychoanalysis refers to the theories and therapeutic techniques applied to the unconscious mind and its impact on everyday life. These theories and techniques inform treatments for mental disorders. [104] [105] [106] Psychoanalysis originated in the 1890s, most prominently with the work of Sigmund Freud. Freud's psychoanalytic theory was largely based on interpretive methods, introspection, and clinical observation. It became very well known, largely because it tackled subjects such as sexuality, repression, and the unconscious. [107] Freud pioneered the methods of free association and dream interpretation. [108] [109]

Psychoanalytic theory is not monolithic. Other well known psychoanalytic thinkers who, to a greater or lesser degree, diverged with Freud include Alfred Adler, Carl Jung, Erik Erikson, Melanie Klein, D.W. Winnicott, Karen Horney, Erich Fromm, John Bowlby, Freud's daughter Anna Freud, and Harry Stack Sullivan. These individuals ensured that psychoanalysis would evolve into diverse schools of thought. Among these schools are ego psychology, object relations, and interpersonal, Lacanian, and relational psychoanalysis.

Psychologists such as Hans Eysenck and philosophers including Karl Popper sharply criticized psychoanalysis. Popper argued that psychoanalysis had been misrepresented as a scientific discipline, [110] whereas Eysenck advanced the view that psychoanalytic tenets had been contradicted by experimental data. By the end of 20th century, psychology departments in American universities mostly marginalized Freudian theory, dismissing it as a "desiccated and dead" historical artifact. [111] Researchers such as António Damásio, Oliver Sacks, and Joseph LeDoux, and individuals in the emerging field of neuro-psychoanalysis, however, have defended some of Freud's ideas on scientific grounds. [112]

Existential-humanistic theories

Humanistic psychology, which has been influenced by existentialism and phenomenology, [114] stresses free will and self-actualization. [115] It emerged in the 1950s as a movement within academic psychology, in reaction to both behaviorism and psychoanalysis. [116] The humanistic approach seeks to view the whole person, not just fragmented parts of the personality or isolated cognitions. [117] Humanistic psychology also focuses on personal growth, self-identity, death, aloneness, and freedom. It emphasizes subjective meaning, the rejection of determinism, and concern for positive growth rather than pathology. Some founders of the humanistic school of thought were American psychologists Abraham Maslow, who formulated a hierarchy of human needs, and Carl Rogers, who created and developed client-centered therapy.

Later, positive psychology opened up humanistic themes to scientific study. Positive psychology is the study of factors which contribute to human happiness and well-being, focusing more on people who are currently healthy. In 2010, Clinical Psychological Review published a special issue devoted to positive psychological interventions, such as gratitude journaling and the physical expression of gratitude. It is, however, far from clear that positive psychology is effective in making people happier. [118] [119] Positive psychological interventions have been limited in scope, but their effects are thought to be somewhat better than placebo effects. The evidence, however, is far from clear that interventions based on positive psychology increase human happiness or resilience. [118] [119]

The American Association for Humanistic Psychology, formed in 1963, declared:

Humanistic psychology is primarily an orientation toward the whole of psychology rather than a distinct area or school. It stands for respect for the worth of persons, respect for differences of approach, open-mindedness as to acceptable methods, and interest in exploration of new aspects of human behavior. As a "third force" in contemporary psychology, it is concerned with topics having little place in existing theories and systems: e.g., love, creativity, self, growth, organism, basic need-gratification, self-actualization, higher values, being, becoming, spontaneity, play, humor, affection, naturalness, warmth, ego-transcendence, objectivity, autonomy, responsibility, meaning, fair-play, transcendental experience, peak experience, courage, and related concepts. [120]

Existential psychology emphasizes the need to understand a client's total orientation towards the world. Existential psychology is opposed to reductionism, behaviorism, and other methods that objectify the individual. [115] In the 1950s and 1960s, influenced by philosophers Søren Kierkegaard and Martin Heidegger, psychoanalytically trained American psychologist Rollo May helped to develop existential psychology. Existential psychotherapy, which follows from existential psychology, is a therapeutic approach that is based on the idea that a person's inner conflict arises from that individual's confrontation with the givens of existence. Swiss psychoanalyst Ludwig Binswanger and American psychologist George Kelly may also be said to belong to the existential school. [121] Existential psychologists tend to differ from more "humanistic" psychologists in the former's relatively neutral view of human nature and relatively positive assessment of anxiety. [122] Existential psychologists emphasized the humanistic themes of death, free will, and meaning, suggesting that meaning can be shaped by myths and narratives meaning can be deepened by the acceptance of free will, which is requisite to living an authentic life, albeit often with anxiety with regard to death. [123]

Austrian existential psychiatrist and Holocaust survivor Viktor Frankl drew evidence of meaning's therapeutic power from reflections upon his own internment. [124] He created a variation of existential psychotherapy called logotherapy, a type of existentialist analysis that focuses on a will to meaning (in one's life), as opposed to Adler's Nietzschean doctrine of will to power or Freud's will to pleasure. [125]


Personality psychology is concerned with enduring patterns of behavior, thought, and emotion. Theories of personality vary across different psychological schools of thought. Each theory carries different assumptions about such features as the role of the unconscious and the importance of childhood experience. According to Freud, personality is based on the dynamic interactions of the id, ego, and super-ego. [126] By contrast, trait theorists have developed taxonomies of personality constructs in describing personality in terms of key traits. Trait theorists have often employed statistical data-reduction methods, such as factor analysis. Although the number of proposed traits has varied widely, Hans Eysenck's early biologically-based model suggests at least three major trait constructs are necessary to describe human personality, extraversion–introversion, neuroticism-stability, and psychoticism-normality. Raymond Cattell empirically derived a theory of 16 personality factors at the primary-factor level and up to eight broader second-stratum factors. [127] [128] [129] [130] Since the 1980s, the Big Five (openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism) emerged as an important trait theory of personality. [131] Dimensional models of personality are receiving increasing support, and a version of dimensional assessment has been included in the DSM-V. However, despite a plethora of research into the various versions of the "Big Five" personality dimensions, it appears necessary to move on from static conceptualizations of personality structure to a more dynamic orientation, acknowledging that personality constructs are subject to learning and change over the lifespan. [132] [133]

An early example of personality assessment was the Woodworth Personal Data Sheet, constructed during World War I. The popular, although psychometrically inadequate, Myers–Briggs Type Indicator [134] was developed to assess individuals' "personality types" according to the personality theories of Carl Jung. The Minnesota Multiphasic Personality Inventory (MMPI), despite its name, is more a dimensional measure of psychopathology than a personality measure. [135] California Psychological Inventory contains 20 personality scales (e.g., independence, tolerance). [136]

Unconscious mind

Study of the unconscious mind, a part of the psyche outside the individual's awareness but that is believed to influence conscious thought and behavior, was a hallmark of early psychology. In one of the first psychology experiments conducted in the United States, C.S. Peirce and Joseph Jastrow found in 1884 that research subjects could choose the minutely heavier of two weights even if consciously uncertain of the difference. [137] Freud popularized the concept of the unconscious mind, particularly when he referred to an uncensored intrusion of unconscious thought into one's speech (a Freudian slip) or to his efforts to interpret dreams. [138] His 1901 book The Psychopathology of Everyday Life catalogues hundreds of everyday events that Freud explains in terms of unconscious influence. Pierre Janet advanced the idea of a subconscious mind, which could contain autonomous mental elements unavailable to the direct scrutiny of the subject. [139]

The concept of unconscious processes has remained important in psychology. Cognitive psychologists have used a "filter" model of attention, according to which much information processing takes place below the threshold of consciousness, and only certain stimuli, limited by their nature and number, make their way through the filter. Much research has shown that subconscious priming of certain ideas can covertly influence thoughts and behavior. [139] Because of the unreliability of self-reporting, a major hurdle in this type of research involves demonstrating that a subject's conscious mind has not perceived a target stimulus. For this reason, some psychologists prefer to distinguish between implicit and explicit memory. In another approach, one can also describe a subliminal stimulus as meeting an objective but not a subjective threshold. [140]

The automaticity model of John Bargh and others involves the ideas of automaticity and unconscious processing in our understanding of social behavior, [141] [142] although there has been dispute with regard to replication. [143] [144] Some experimental data suggest that the brain begins to consider taking actions before the mind becomes aware of them. [145] The influence of unconscious forces on people's choices bears on the philosophical question of free will. John Bargh, Daniel Wegner, and Ellen Langer describe free will as an illusion. [141] [142] [146]


Some psychologists study motivation or the subject of why people or lower animals initiate a behavior at a particular time. It also involves the study of why humans and lower animals continue or terminate a behavior. Psychologists such as William James initially used the term motivation to refer to intention, in a sense similar to the concept of will in European philosophy. With the steady rise of Darwinian and Freudian thinking, instinct also came to be seen as a primary source of motivation. [147] According to drive theory, the forces of instinct combine into a single source of energy which exerts a constant influence. Psychoanalysis, like biology, regarded these forces as demands originating in the nervous system. Psychoanalysts believed that these forces, especially the sexual instincts, could become entangled and transmuted within the psyche. Classical psychoanalysis conceives of a struggle between the pleasure principle and the reality principle, roughly corresponding to id and ego. Later, in Beyond the Pleasure Principle, Freud introduced the concept of the death drive, a compulsion towards aggression, destruction, and psychic repetition of traumatic events. [148] Meanwhile, behaviorist researchers used simple dichotomous models (pleasure/pain, reward/punishment) and well-established principles such as the idea that a thirsty creature will take pleasure in drinking. [147] [149] Clark Hull formalized the latter idea with his drive reduction model. [150]

Hunger, thirst, fear, sexual desire, and thermoregulation constitute fundamental motivations in animals. [149] Humans seem to exhibit a more complex set of motivations—though theoretically these could be explained as resulting from desires for belonging, positive self-image, self-consistency, truth, love, and control. [151] [152]

Motivation can be modulated or manipulated in many different ways. Researchers have found that eating, for example, depends not only on the organism's fundamental need for homeostasis—an important factor causing the experience of hunger—but also on circadian rhythms, food availability, food palatability, and cost. [149] Abstract motivations are also malleable, as evidenced by such phenomena as goal contagion: the adoption of goals, sometimes unconsciously, based on inferences about the goals of others. [153] Vohs and Baumeister suggest that contrary to the need-desire-fulfilment cycle of animal instincts, human motivations sometimes obey a "getting begets wanting" rule: the more you get a reward such as self-esteem, love, drugs, or money, the more you want it. They suggest that this principle can even apply to food, drink, sex, and sleep. [154]


Developmental psychology refers to the scientific study of how and why humans change over the course of their lives. [155] Given the discipline's origins in the work of Jean Piaget, developmental psychologists originally focused primarily on the development of cognition from infancy to adolescence. Later, developmental psychology extended itself to the study cognition over the life span. In addition to studying cognition, developmental psychologists have also come to focus on affective, moral, social, and neural development.

Developmental psychologists who study children use a number of research methods. For example, they make observations of children in natural settings such as preschools [156] and engage them in experimental tasks. [157] Such tasks often resemble specially designed games and activities that are both enjoyable for the child and scientifically useful. Developmental researchers have even devised clever methods to study the mental processes of infants. [158] In addition to studying children, developmental psychologists also study aging and processes throughout the life span, including old age. [159] These psychologists draw on the full range of psychological theories to inform their research. [155]

Genes and environment

All researched psychological traits are influenced by both genes and environment, to varying degrees. [160] [161] These two sources of influence are often confounded in observational research of individuals and families. An example of this confounding can be shown in the transmission of depression from a depressed mother to her offspring. A theory based on environmental transmission would hold that an offspring, by virtue of his or her having a problematic rearing environment managed by a depressed mother, is at risk for developing depression. On the other hand, a hereditarian theory would hold that depression risk in an offspring is influenced to some extent by genes passed to the child from the mother. Genes and environment in these simple transmission models are completely confounded. A depressed mother may both carry genes that contribute to depression in her offspring and also create a rearing environment that increases the risk of depression in her child.

Behavioral genetics researchers have employed methodologies that help to disentangle this confound and understand the nature and origins of individual differences in behavior. [78] Traditionally the research has involved twin studies and adoption studies, two designs where genetic and environmental influences can be partially un-confounded. More recently, gene-focused research has contributed to understanding genetic contributions to the development of psychological traits.

The availability of microarray molecular genetic or genome sequencing technologies allows researchers to measure participant DNA variation directly, and test whether individual genetic variants within genes are associated with psychological traits and psychopathology through methods including genome-wide association studies. One goal of such research is similar to that in positional cloning and its success in Huntington's: once a causal gene is discovered biological research can be conducted to understand how that gene influences the phenotype. One major result of genetic association studies is the general finding that psychological traits and psychopathology, as well as complex medical diseases, are highly polygenic, [162] [163] [164] [165] [166] where a large number (on the order of hundreds to thousands) of genetic variants, each of small effect, contribute to individual differences in the behavioral trait or propensity to the disorder. Active research continues to work toward understanding the genetic and environmental bases of behavior and their interaction.

Psychology encompasses many subfields and includes different approaches to the study of mental processes and behavior.

Psychological testing

Psychological testing has ancient origins, dating as far back as 2200 BC, in the examinations for the Chinese civil service. Written exams began during the Han dynasty (202 BC – AD 200). By 1370, the Chinese system required a stratified series of tests, involving essay writing and knowledge of diverse topics. The system was ended in 1906. [167] In Europe, mental assessment took a different approach, with theories of physiognomy—judgment of character based on the face—described by Aristotle in 4th century BC Greece. Physiognomy remained current through the Enlightenment, and added the doctrine of phrenology: a study of mind and intelligence based on simple assessment of neuroanatomy. [168]

When experimental psychology came to Britain, Francis Galton was a leading practitioner. By virtue of his procedures for measuring reaction time and sensation, he is considered an inventor of modern mental testing (also known as psychometrics). [169] James McKeen Cattell, a student of Wundt and Galton, brought the idea of psychological testing to the United States, and in fact coined the term "mental test". [170] In 1901, Cattell's student Clark Wissler published discouraging results, suggesting that mental testing of Columbia and Barnard students failed to predict academic performance. [170] In response to 1904 orders from the Minister of Public Instruction, French psychologists Alfred Binet and Théodore Simon developed and elaborated a new test of intelligence in 1905–1911. They used a range of questions diverse in their nature and difficulty. Binet and Simon introduced the concept of mental age and referred to the lowest scorers on their test as idiots. Henry H. Goddard put the Binet-Simon scale to work and introduced classifications of mental level such as imbecile and feebleminded. In 1916 (after Binet's death), Stanford professor Lewis M. Terman modified the Binet-Simon scale (renamed the Stanford–Binet scale) and introduced the intelligence quotient as a score report. [171] Based on his test findings, and reflecting the racism common to that era, Terman concluded that mental retardation "represents the level of intelligence which is very, very common among Spanish-Indians and Mexican families of the Southwest and also among negroes. Their dullness seems to be racial." [172]

Following the Army Alpha and Army Beta tests for soldiers in World War I, mental testing became popular in the US, where it was soon applied to schoolchildren. The federally created National Intelligence Test was administered to 7 million children in the 1920s. In 1926, the College Entrance Examination Board created the Scholastic Aptitude Test to standardize college admissions. [173] The results of intelligence tests were used to argue for segregated schools and economic functions, including the preferential training of Black Americans for manual labor. These practices were criticized by Black intellectuals such a Horace Mann Bond and Allison Davis. [172] Eugenicists used mental testing to justify and organize compulsory sterilization of individuals classified as mentally retarded. [44] In the United States, tens of thousands of men and women were sterilized. Setting a precedent that has never been overturned, the U.S. Supreme Court affirmed the constitutionality of this practice in the 1927 case Buck v. Bell. [174]

Today mental testing is a routine phenomenon for people of all ages in Western societies. [175] Modern testing aspires to criteria including standardization of procedure, consistency of results, output of an interpretable score, statistical norms describing population outcomes, and, ideally, effective prediction of behavior and life outcomes outside of testing situations. [176] Developments in psychometrics include work on test and scale reliability and validity. [177] Developments in item-response theory, [178] structural equation modeling, [179] and bifactor analysis [180] have helped in strengthening test and scale construction.

Mental health care

The provision of psychological health services is generally called clinical psychology in the U.S. Sometimes, however, members of the school psychology and counseling psychology professions engage in practices that resemble that of clinical psychologists. Clinical psychologists typically include people who have graduated from doctoral programs in clinical psychology. In Canada, some of the members of the abovementioned groups usually fall within the larger category of professional psychology. In Canada and the U.S., practitioners get bachelor's degrees and doctorates doctoral students in clinical psychology usually spend one year in a predoctoral internship and one year in postdoctoral internship. In Mexico and most other Latin American and European countries, psychologists do not get bachelor's and doctoral degrees instead, they take a three-year professional course following high school. [66] Clinical psychology is at present the largest specialization within psychology. [181] It includes the study and application of psychology for the purpose of understanding, preventing, and relieving psychological distress, dysfunction, and/or mental illness. Clinical psychologists also try to promote subjective well-being and personal growth. Central to the practice of clinical psychology are psychological assessment and psychotherapy although clinical psychologists may also engage in research, teaching, consultation, forensic testimony, and program development and administration. [182]

Credit for the first psychology clinic in the United States typically goes to Lightner Witmer, who established his practice in Philadelphia in 1896. Another modern psychotherapist was Morton Prince, an early advocate for the establishment of psychology as a clinical and academic discipline. [181] In the first part of the twentieth century, most mental health care in the United States was performed by psychiatrists, who are medical doctors. Psychology entered the field with its refinements of mental testing, which promised to improve the diagnosis of mental problems. For their part, some psychiatrists became interested in using psychoanalysis and other forms of psychodynamic psychotherapy to understand and treat the mentally ill. [39] [183]

Psychotherapy as conducted by psychiatrists blurred the distinction between psychiatry and psychology, and this trend continued with the rise of community mental health facilities. Some in the clinical psychology community adopted behavioral therapy, a thoroughly non-psychodynamic model that used behaviorist learning theory to change the actions of patients. A key aspect of behavior therapy is empirical evaluation of the treatment's effectiveness. In the 1970s, cognitive-behavior therapy emerged with the work of Albert Ellis and Aaron Beck. Although there are similarities between behavior therapy and cognitive-behavior therapy, cognitive-behavior therapy required the application of cognitive constructs. Since the 1970s, the popularity of cognitive-behavior therapy among clinical psychologists increased. A key practice in behavioral and cognitive-behavioral therapy is exposing patients to things they fear, based on the premise that their responses (fear, panic, anxiety) can be deconditioned. [184]

Mental health care today involves psychologists and social workers in increasing numbers. In 1977, National Institute of Mental Health director Bertram Brown described this shift as a source of "intense competition and role confusion." [39] Graduate programs issuing doctorates in clinical psychology emerged in the 1950s and underwent rapid increase through the 1980s. The PhD degree is intended to train practitioners who could also conduct scientific research. The PsyD degree is more exclusively designed to train practitioners. [66]

Some clinical psychologists focus on the clinical management of patients with brain injury. This subspecialty is known as clinical neuropsychology. In many countries, clinical psychology is a regulated mental health profession. The emerging field of disaster psychology (see crisis intervention) involves professionals who respond to large-scale traumatic events. [185]

The work performed by clinical psychologists tends to be influenced by various therapeutic approaches, all of which involve a formal relationship between professional and client (usually an individual, couple, family, or small group). Typically, these approaches encourage new ways of thinking, feeling, or behaving. Four major theoretical perspectives are psychodynamic, cognitive behavioral, existential–humanistic, and systems or family therapy. There has been a growing movement to integrate the various therapeutic approaches, especially with an increased understanding of issues regarding culture, gender, spirituality, and sexual orientation. With the advent of more robust research findings regarding psychotherapy, there is evidence that most of the major therapies have equal effectiveness, with the key common element being a strong therapeutic alliance. [186] [187] Because of this, more training programs and psychologists are now adopting an eclectic therapeutic orientation. [188] [189] [190] [191] [192]

Diagnosis in clinical psychology usually follows the Diagnostic and Statistical Manual of Mental Disorders (DSM). [193] The study of mental illnesses is called abnormal psychology.


Educational psychology is the study of how humans learn in educational settings, the effectiveness of educational interventions, the psychology of teaching, and the social psychology of schools as organizations. The work of developmental psychologists such as Lev Vygotsky, Jean Piaget, and Jerome Bruner has been influential in creating teaching methods and educational practices. Educational psychology is often included in teacher education programs in places such as North America, Australia, and New Zealand.

School psychology combines principles from educational psychology and clinical psychology to understand and treat students with learning disabilities to foster the intellectual growth of gifted students to facilitate prosocial behaviors in adolescents and otherwise to promote safe, supportive, and effective learning environments. School psychologists are trained in educational and behavioral assessment, intervention, prevention, and consultation, and many have extensive training in research. [194]

Industrial and organizational (I/O) psychology involves research and practices that apply psychological theories and principles to organizations and individuals' work-lives. [195] In the field's beginnings, industrialists brought the nascent field of psychology to bear on the study of scientific management techniques for improving workplace efficiency. The field was at first called economic psychology or business psychology later, industrial psychology, employment psychology, or psychotechnology. [196] An influential early study examined workers at Western Electric's Hawthorne plant in Cicero, Illinois from 1924–1932. Western Electric experimented on factory workers to assess their responses to changes in illumination, breaks, food, and wages. The researchers came to focus on workers' responses to observation itself, and the term Hawthorne effect is now used to describe the fact that people work harder when they think they're being watched. [197] Although the Hawthorne research can be found in psychology textbooks, the research and its findings, however, were weak at best. [198] [199]

The name industrial and organizational psychology emerged in the 1960s. In 1973, it became enshrined in the name of the Society for Industrial and Organizational Psychology, Division 14 of the American Psychological Association. [196] One goal of the discipline is to optimize human potential in the workplace. Personnel psychology is a subfield of I/O psychology. Personnel psychologists apply the methods and principles of psychology in selecting and evaluating workers. Another subfield, organizational psychology, examines the effects of work environments and management styles on worker motivation, job satisfaction, and productivity. [200] Most I/O psychologists work outside of academia, for private and public organizations and as consultants. [196] A psychology consultant working in business today might expect to provide executives with information and ideas about their industry, their target markets, and the organization of their company. [201] [202]

Organizational behavior (OB) is an allied field involved in the study of human behavior within organizations. [203] One way to differentiate I/O psychology from OB is to note that I/O psychologists train in university psychology departments and OB specialists, in business schools.

Military and intelligence

One role for psychologists in the military has been to evaluate and counsel soldiers and other personnel. In the U.S., this function began during World War I, when Robert Yerkes established the School of Military Psychology at Fort Oglethorpe in Georgia. The school provided psychological training for military staff. [39] [204] Today, U.S Army psychologists perform psychological screening, clinical psychotherapy, suicide prevention, and treatment for post-traumatic stress, as well as provide prevention-related services, for example, smoking cessation. [205]

Psychologists may also work on a diverse set of campaigns known broadly as psychological warfare. Psychological warfare chiefly involves the use of propaganda to influence enemy soldiers and civilians. This so-called black propaganda is designed to seem as if it originates from a source other than the Army. [206] The CIA's MKULTRA program involved more individualized efforts at mind control, involving techniques such as hypnosis, torture, and covert involuntary administration of LSD. [207] The U.S. military used the name Psychological Operations (PSYOP) until 2010, when these activities were reclassified as Military Information Support Operations (MISO), part of Information Operations (IO). [208] Psychologists have sometimes been involved in assisting the interrogation and torture of suspects, staining the records of the psychologists involved. [209]

Health, well-being, and social change

Medical facilities increasingly employ psychologists to perform various roles. A prominent aspect of health psychology is the psychoeducation of patients: instructing them in how to follow a medical regimen. Health psychologists can also educate doctors and conduct research on patient compliance. [210] [211] Psychologists in the field of public health use a wide variety of interventions to influence human behavior. These range from public relations campaigns and outreach to governmental laws and policies. Psychologists study the composite influence of all these different tools in an effort to influence whole populations of people. [212]

An outstanding example of the contribution of psychologists to social change involves the research of Kenneth and Mamie Clark. These two African American psychologists studied segregation's the adverse psychological impact on children. Their research findings played a role in the desegregation case Brown v. Board of Education (1954). [213]

Occupational health psychology (OHP) is a branch of psychology that is very much interdisciplinary. It is concerned with the health and safety of workers. [47] [214] OHP addresses topic areas such as the impact of occupational stressors on physical and mental health, workplace mistreatment, work-family balance, the impact of involuntary unemployment on physical and mental health, safety/accidents, and interventions designed to improve/protect worker health. [47] [215] OHP grew out of health psychology and I/O psychology. [216] OHP has also been informed by disciplines outside psychology, including occupational medicine, industrial engineering, and economics. [217] [218]

Quantitative psychological research lends itself to the statistical testing of hypotheses. Although the field makes abundant use of randomized and controlled experiments in laboratory settings, such research can only assess a limited range of short-term phenomena. Some psychologists rely on less rigorously controlled, but more ecologically valid, field experiments as well. Other research psychologists rely on statistical methods to glean knowledge from population data. [219] The statistical methods research psychologists employ include the Pearson product–moment correlation coefficient, the analysis of variance, multiple linear regression, logistic regression, structural equation modeling, and hierarchical linear modeling. The measurement and operationalization of important constructs is an essential part of these research designs.

Although this type of psychological research is much less abundant than quantitative research, some psychologists conduct qualitative research. This type of research can involve interviews, questionnaires, and first-hand observation. [220] While hypothesis testing is rare, virtually impossible, in qualitative research, qualitative studies can be helpful in theory and hypothesis generation, interpreting seemingly contradictory quantitative findings, and understanding why some interventions fail and others succeed. [221]

Controlled experiments

A true experiment with random assignment of research participants (sometimes called subjects) to rival conditions allows researchers to make strong inferences about causal relationships. When there are large numbers of research participants, the random assignment (also called random allocation) of those participants to rival conditions ensures that the individuals in those conditions will, on average, be similar on most characteristics, including characteristics that went unmeasured. In an experiment, the researcher alters one or more variables of influence, called independent variables, and measures resulting changes in the factors of interest, called dependent variables. Prototypical experimental research is conducted in a laboratory with a carefully controlled environment.

A quasi-experiment refers to a situation in which there are rival conditions under study but random assignment to the different conditions is not possible. Investigators must work with preexisting groups of people. Researchers can use common sense to consider how much the nonrandom assignment threatens the study's validity. [224] For example, in research on the best way to affect reading achievement in the first three grades of school, school administrators may not permit educational psychologists to randomly assign children to phonics and whole language classrooms, in which case the psychologists must work with preexisting classroom assignments. Psychologists will compare the achievement of children attending phonics and whole language classes and, perhaps, statistically adjust for any initial differences in reading level.

Experimental researchers typically use a statistical hypothesis testing model which involves making predictions before conducting the experiment, then assessing how well the data collected are consistent with the predictions. These predictions are likely to originate from one or more abstract scientific hypotheses about how the phenomenon under study actually works. [225]

Other types of studies

Surveys are used in psychology for the purpose of measuring attitudes and traits, monitoring changes in mood, and checking the validity of experimental manipulations (checking research participants' perception of the condition they were assigned to). Psychologists have commonly used paper-and-pencil surveys. However, surveys are also conducted over the phone or through e-mail. Web-based surveys are increasingly used to conveniently reach many subjects.

Observational studies are commonly conducted in psychology. In cross-sectional observational studies, psychologists collect data at a single point in time. The goal of many cross-sectional studies is the assess the extent factors are correlated with each other. By contrast, in longitudinal studies psychologists collect data on the same sample at two or more points in time. Sometimes the purpose of longitudinal research is to study trends across time such as the stability of traits or age-related changes in behavior. Because some studies involve endpoints that psychologists cannot ethically study from an experimental standpoint, such as identifying the causes of depression, they conduct longitudinal studies a large group of depression-free people, periodically assessing what is happening in the individuals' lives. In this way psychologists have an opportunity to test causal hypotheses regarding conditions that commonly arise in people's lives that put them at risk for depression. Problems that affect longitudinal studies include selective attrition, the type of problem in which bias is introduced when a certain type of research participant disproportionately leaves a study.

Exploratory data analysis refers to a variety of practices that researchers use to reduce a great many variables to a small number overarching factors. In Peirce's three modes of inference, exploratory data analysis corresponds to abduction. [226] Meta-analysis is the technique research psychologists use to integrate results from many studies of the same variables and arriving at a grand average of the findings. [227]

Technological assays

A classic and popular tool used to relate mental and neural activity is the electroencephalogram (EEG), a technique using amplified electrodes on a person's scalp to measure voltage changes in different parts of the brain. Hans Berger, the first researcher to use EEG on an unopened skull, quickly found that brains exhibit signature "brain waves": electric oscillations which correspond to different states of consciousness. Researchers subsequently refined statistical methods for synthesizing the electrode data, and identified unique brain wave patterns such as the delta wave observed during non-REM sleep. [228]

Newer functional neuroimaging techniques include functional magnetic resonance imaging and positron emission tomography, both of which track the flow of blood through the brain. These technologies provide more localized information about activity in the brain and create representations of the brain with widespread appeal. They also provide insight which avoids the classic problems of subjective self-reporting. It remains challenging to draw hard conclusions about where in the brain specific thoughts originate—or even how usefully such localization corresponds with reality. However, neuroimaging has delivered unmistakable results showing the existence of correlations between mind and brain. Some of these draw on a systemic neural network model rather than a localized function model. [229] [230] [231]

Psychiatric interventions such as transcranial magnetic stimulation and drugs also provide information about brain–mind interactions. Psychopharmacology is the study of drug-induced mental effects.

Computer simulation

Computational modeling is a tool used in mathematical psychology and cognitive psychology to simulate behavior. [232] This method has several advantages. Since modern computers process information quickly, simulations can be run in a short time, allowing for high statistical power. Modeling also allows psychologists to visualize hypotheses about the functional organization of mental events that couldn't be directly observed in a human. Computational neuroscience uses mathematical models to simulate the brain. Another method is symbolic modeling, which represents many mental objects using variables and rules. Other types of modeling include dynamic systems and stochastic modeling.

Animal studies

Animal experiments aid in investigating many aspects of human psychology, including perception, emotion, learning, memory, and thought, to name a few. In the 1890s, Russian physiologist Ivan Pavlov famously used dogs to demonstrate classical conditioning. Non-human primates, cats, dogs, pigeons, and rats and other rodents are often used in psychological experiments. Ideally, controlled experiments introduce only one independent variable at a time, in order to ascertain its unique effects upon dependent variables. These conditions are approximated best in laboratory settings. In contrast, human environments and genetic backgrounds vary so widely, and depend upon so many factors, that it is difficult to control important variables for human subjects. There are pitfalls, however, in generalizing findings from animal studies to humans through animal models. [233]

Comparative psychology refers to the scientific study of the behavior and mental processes of non-human animals, especially as these relate to the phylogenetic history, adaptive significance, and development of behavior. Research in this area explores the behavior of many species, from insects to primates. It is closely related to other disciplines that study animal behavior such as ethology. [234] Research in comparative psychology sometimes appears to shed light on human behavior, but some attempts to connect the two have been quite controversial, for example the Sociobiology of E.O. Wilson. [235] Animal models are often used to study neural processes related to human behavior, e.g. in cognitive neuroscience.

Qualitative research

Qualitative research is often designed to answer questions about the thoughts, feelings, and behaviors of individuals. Qualitative research involving first-hand observation can help describe events as they occur, with the goal of capturing the richness of everyday behavior and with the hope of discovering and understanding phenomena that might have been missed if only more cursory examinations have been made.

Qualitative psychological research methods include interviews, first-hand observation, and participant observation. Creswell (2003) identifies five main possibilities for qualitative research, including narrative, phenomenology, ethnography, case study, and grounded theory. Qualitative researchers [236] sometimes aim to enrich interpretations or critiques of symbols, subjective experiences, or social structures. Sometimes hermeneutic and critical aims can give rise to quantitative research, as in Erich Fromm's application of psychological and sociological theories, in his book Escape from Freedom, to understanding why many ordinary Germans supported Hitler. [237]

Just as Jane Goodall studied chimpanzee social and family life by careful observation of chimpanzee behavior in the field, psychologists conduct naturalistic observation of ongoing human social, professional, and family life. Sometimes the participants are aware they are being observed, and other times the participants do not know they are being observed. Strict ethical guidelines must be followed when covert observation is being carried out.

Program Evaluation

Program Evaluation is a systematic method for collecting, analyzing, and using information to answer questions about projects, policies and programs, particularly about their effectiveness. [239] [240] In both the public and private sectors, stakeholders often want to know whether the programs they are funding, implementing, voting for, receiving, or objecting to are producing their intended effect. While program evaluation first focuses on effectiveness, important considerations often include how much the program costs per participant, how the program could be improved, whether the program is worthwhile, whether there are better alternatives, if there are unintended outcomes, and whether the program goals are appropriate and useful. [241]


The field of metascience has revealed problems in psychological research. Some psychological research has suffered from bias, [242] problematic reproducibility, [243] and misuse of statistics. [244] These findings have led to calls for reform from within and from outside the scientific community. [245]

Confirmation bias

In 1959, statistician Theodore Sterling examined the results of psychological studies and discovered that 97% of them supported their initial hypotheses, implying a possible publication bias. [246] [247] [248] Similarly, Fanelli (2010) [249] found that 91.5% of psychiatry/psychology studies confirmed the effects they were looking for, and concluded that the odds of this happening (a positive result) was around five times higher than in fields such as space- or geosciences. Fanelli argues that this is because researchers in "softer" sciences have fewer constraints to their conscious and unconscious biases.


A replication crisis in psychology has emerged. Many notable findings in the field have not been replicated. Some researchers were even accused of publishing fraudulent results. [250] [251] [252] Systematic efforts, including efforts by the Reproducibility Project of the Center for Open Science, to assess the extent of the problem found that as many as two-thirds of highly publicized findings in psychology failed to be replicated. [253] Reproducibility has generally been stronger in cognitive psychology (in studies and journals) than social psychology [253] and subfields of differential psychology. [254] [255] Other subfields of psychology have also been implicated in the replication crisis, including clinical psychology, [256] [257] developmental psychology, [258] [259] [260] and a field closely related to psychology, educational research. [261] [262] [263] [264]

Focus on the replication crisis has led to other renewed efforts in the discipline to re-test important findings. [265] [266] In response to concerns about publication bias and data dredging (conducting a large number of statistical tests on a great many variables but restricting reporting to the results that were statistically significant), 295 psychology and medical journals have adopted result-blind peer review where studies are accepted not on the basis of their findings and after the studies are completed, but before the studies are conducted and upon the basis of the methodological rigor of their experimental designs and the theoretical justifications for their proposed statistical analysis before data collection or analysis is conducted. [267] [268] In addition, large-scale collaborations among researchers working in multiple labs in different countries have taken place. The collaborators regularly make their data openly available for different researchers to assess. [269] Allen et al. [270] estimated that 61 percent of result-blind studies have yielded null results, in contrast to an estimated 5 to 20 percent in traditional research.

Misuse of statistics

Some critics view statistical hypothesis testing as misplaced. Psychologist and statistician Jacob Cohen wrote in 1994 that psychologists routinely confuse statistical significance with practical importance, enthusiastically reporting great certainty in unimportant facts. [271] Some psychologists have responded with an increased use of effect size statistics, rather than sole reliance on p-values. [272]

WEIRD bias

In 2008, Arnett pointed out that most articles in American Psychological Association journals were about U.S. populations when U.S. citizens are only 5% of the world's population. He complained that psychologists had no basis for assuming psychological processes to be universal and generalizing research findings to the rest of the global population. [273] In 2010, Henrich, Heine, and Norenzayan reported a bias in conducting psychology studies with participants from "WEIRD" ("Western, Educated, Industrialized, Rich, and Democratic") societies. [274] [275] Henrich et al. found that "96% of psychological samples come from countries with only 12% of the world’s population" (p. 63). The article gave examples of results that differ significantly between people from WEIRD and tribal cultures, including the Müller-Lyer illusion. Arnett (2008), Altmaier, and Hall (2008) and Morgan-Consoli et al. (2018) view the Western bias in research and theory as a serious problem considering psychologists are increasingly applying psychological principles developed in WEIRD regions in their research, clinical work, and consultation with populations around the world. [273] [276] [277] In 2018, Rad, Martingano, and Ginges showed that nearly a decade after Henrich et al.'s paper, over 80% of the samples used in studies published in the journal Psychological Science employed WEIRD samples. Moreover, their analysis showed that several studies did not fully disclose the origin of their samples the authors offered a set of recommendations to editors and reviewers to reduce WEIRD bias. [278]

Unscientific mental health training

Some observers perceive a gap between scientific theory and its application—in particular, the application of unsupported or unsound clinical practices. [279] Critics say there has been an increase in the number of mental health training programs that do not instill scientific competence. [280] Practices such as "facilitated communication for infantile autism" memory-recovery techniques including body work and other therapies, such as rebirthing and reparenting, may be dubious or even dangerous, despite their popularity. [281] These practices, however, are outside the mainstream practices taught in clinical psychology doctoral programs.

Ethical standards in the discipline have changed over time. Some famous past studies are today considered unethical and in violation of established codes (the Canadian Code of Conduct for Research Involving Humans, and the Belmont Report). The American Psychological Association has advanced a set of ethical principles and a code of conduct for the profession. [282]

The most important contemporary standards include informed and voluntary consent. After World War II, the Nuremberg Code was established because of Nazi abuses of experimental subjects. Later, most countries (and scientific journals) adopted the Declaration of Helsinki. In the U.S., the National Institutes of Health established the Institutional Review Board in 1966, and in 1974 adopted the National Research Act (HR 7724). All of these measures encouraged researchers to obtain informed consent from human participants in experimental studies. A number of influential but ethically dubious studies led to the establishment of this rule such studies included the MIT-Harvard Fernald School radioisotope studies, the Thalidomide tragedy, the Willowbrook hepatitis study, and Stanley Milgram's studies of obedience to authority.


Universities have ethics committees dedicated to protecting the rights (e.g., voluntary nature of the research, privacy) and well-being (e.g., minimizing distress) of research participants. University ethics committees evaluate proposed research to ensure that researchers protect the rights and well-being of participants an investigator's research project cannot be conducted unless approved by such an ethics committee. [283]

The ethics code of the American Psychological Association originated in 1951 as "Ethical Standards of Psychologists". This code has guided the formation of licensing laws in most American states. It has changed multiple times over the decades since its adoption. In 1989, the APA revised its policies on advertising and referral fees to negotiate the end of an investigation by the Federal Trade Commission. The 1992 incarnation was the first to distinguish between "aspirational" ethical standards and "enforceable" ones. Members of the public have a five-year window to file ethics complaints about APA members with the APA ethics committee members of the APA have a three-year window. [284]

Some of the ethical issues considered most important are the requirement to practice only within the area of competence, to maintain confidentiality with the patients, and to avoid sexual relations with them. Another important principle is informed consent, the idea that a patient or research subject must understand and freely choose a procedure they are undergoing. [284] Some of the most common complaints against clinical psychologists include sexual misconduct. [284]

Other animals

Research on other animals is also governed by university ethics committees. Research on nonhuman animals cannot proceed without permission of the ethics committee of the researcher's home institution. Current ethical guidelines state that using non-human animals for scientific purposes is only acceptable when the harm (physical or psychological) done to animals is outweighed by the benefits of the research. [285] Keeping this in mind, psychologists can use certain research techniques on animals that could not be used on humans.

Criminal profiling: the reality behind the myth

Forensic psychologists are working with law enforcement officials to integrate psychological science into criminal profiling.

July/August 2004, Vol 35, No. 7

For 16 years, "mad bomber" George Metesky eluded New York City police. Metesky planted more than 30 small bombs around the city between 1940 and 1956, hitting movie theaters, phone booths and other public areas.

In 1956, the frustrated investigators asked psychiatrist James Brussel, New York State's assistant commissioner of mental hygiene, to study crime scene photos and notes from the bomber. Brussel came up with a detailed description of the suspect: He would be unmarried, foreign, self-educated, in his 50s, living in Connecticut, paranoid and with a vendetta against Con Edison--the first bomb had targeted the power company's 67th street headquarters.

While some of Brussel's predictions were simply common sense, others were based on psychological ideas. For instance, he said that because paranoia tends to peak around age 35, the bomber, 16 years after his first bomb, would now be in his 50s. The profile proved dead on: It led police right to Metesky, who was arrested in January 1957 and confessed immediately.

In the following decades, police in New York and elsewhere continued to consult psychologists and psychiatrists to develop profiles of particularly difficult-to-catch offenders. At the same time, though, much of the criminal profiling field developed within the law enforcement community--particularly the FBI.

Nowadays profiling rests, sometimes uneasily, somewhere between law enforcement and psychology. As a science, it is still a relatively new field with few set boundaries or definitions. Its practitioners don't always agree on methodology or even terminology. The term "profiling" has caught on among the general public, largely due to movies like "The Silence of the Lambs" and TV shows like "Profiler." But the FBI calls its form of profiling "criminal investigative analysis" one prominent forensic psychologist calls his work "investigative psychology" and another calls his "crime action profiling."

Despite the different names, all of these tactics share a common goal: to help investigators examine evidence from crime scenes and victim and witness reports to develop an offender description. The description can include psychological variables such as personality traits, psychopathologies and behavior patterns, as well as demographic variables such as age, race or geographic location. Investigators might use profiling to narrow down a field of suspects or figure out how to interrogate a suspect already in custody.

"In some ways, [profiling] is really still as much an art as a science," says psychologist Harvey Schlossberg, PhD, former director of psychological services for the New York Police Department. But in recent years, many psychologists--together with criminologists and law enforcement officials--have begun using psychology's statistical and research methods to bring more science into the art.

How does profiling work?

Informal criminal profiling has a long history. It was used as early as the 1880s, when two physicians, George Phillips and Thomas Bond, used crime scene clues to make predictions about British serial murderer Jack the Ripper's personality.

At the same time, profiling has taken root in the United States, where, until recent decades, profilers relied mostly on their own intuition and informal studies. Schlossberg, who developed profiles of many criminals, including David Berkowitz--New York City's "Son of Sam"--describes the approach he used in the late 1960s and 70s: "What I would do," he says, "is sit down and look through cases where the criminals had been arrested. I listed how old [the perpetrators] were, whether they were male or female, their level of education. Did they come from broken families? Did they have school behavioral problems? I listed as many factors as I could come up with, and then I added them up to see which were the most common."

In 1974, the FBI formed its Behavioral Science Unit to investigate serial rape and homicide cases. From 1976 to 1979, several FBI agents--most famously John Douglas and Robert Ressler--interviewed 36 serial murderers to develop theories and categories of different types of offenders.

Most notably, they developed the idea of the "organized/disorganized dichotomy": Organized crimes are premeditated and carefully planned, so little evidence is found at the scene. Organized criminals, according to the classification scheme, are antisocial but know right from wrong, are not insane and show no remorse. Disorganized crimes, in contrast, are not planned, and criminals leave such evidence as fingerprints and blood. Disorganized criminals may be young, under the influence of alcohol or drugs, or mentally ill.

Over the past quarter-century, the Behavioral Science Unit has further developed the FBI's profiling process--including refining the organized/disorganized dichotomy into a continuum and developing other classification schemes.

"The basic premise is that behavior reflects personality," explains retired FBI agent Gregg McCrary. In a homicide case, for example, FBI agents glean insight into personality through questions about the murderer's behavior at four crime phases:

Antecedent: What fantasy or plan, or both, did the murderer have in place before the act? What triggered the murderer to act some days and not others?

Method and manner: What type of victim or victims did the murderer select? What was the method and manner of murder: shooting, stabbing, strangulation or something else?

Body disposal: Did the murder and body disposal take place all at one scene, or multiple scenes?

Postoffense behavior: Is the murderer trying to inject himself into the investigation by reacting to media reports or contacting investigators?

A rape case is analyzed in much the same way, but with the additional information that comes from a living victim. Everything about the crime, from the sexual acts the rapist forces on the victim to the order in which they're performed, offers a clue about the perpetrator, McCrary says.

Psychology's contributions

Although the FBI approach has gained public attention, some psychologists have questioned its scientific solidity. Ressler, Douglas and the other FBI agents were not psychologists, and some psychologists who looked at their work found methodological flaws.

Former FBI agent McCrary agrees that some of the FBI's early research was rough: "Early on it was just a bunch of us [FBI agents] basing our work on our investigative experience," he says, "and hopefully being right more than we were wrong."

McCrary says he believes that they were right more than wrong, though, and emphasizes that FBI methods have improved since then. In the meantime, psychologists have also been helping to step up profiling's scientific rigor. Some psychologists have been conducting their own criminal profiling research, and they've developed several new approaches:

Offender profiling. Much of this work comes from applied psychologist David Canter, PhD, who founded the field of investigative psychology in the early 1990s and now runs the Centre for Investigative Psychology at the University of Liverpool.

Investigative psychology, Canter says, includes many areas where psychology can contribute to investigations--including profiling. The goal of investigative psychology's form of profiling, like all profiling, is to infer characteristics of a criminal based on his or her behavior during the crime. But, Canter says, the key is that all of those inferences should come from empirical, peer-reviewed research--not necessarily from investigative experience.

For example, Canter and his colleagues recently analyzed crime scene data from 100 serial homicides to test the FBI's organized/disorganized model. Their results, which will be published in an upcoming issue of APA's Psychology, Public Policy and Law, indicate that, in contrast to some earlier findings, almost all serial murderers show some level of organization.

Organized behaviors--like positioning or concealing a victim's body--are the "core variables" that tend to show up most frequently and co-occur with other variables most often, he found. The differences between murderers, the researchers say, instead lie in the types of disorganized behaviors they exhibit. The study suggests that serial murderers can be divided into categories based on the way they interact with their victims: through sexual control, mutilation, execution or plunder.

Canter says that research like this, which uses the statistical techniques of psychology to group together types of offender behaviors, is the only way to develop scientifically defensible descriptions and classifications of offenders.

"Our approach," he says, "is to consider all the information that may be apparent at the crime scene and to carry out theory-based studies to determine the underlying structures of that material."

In another study, he and his colleagues collected crime scene data from 112 rape cases and analyzed the relationship among different crime scene actions--from what types of sexual acts the rapist demanded to whether he bound the victim. The researchers found that the types of sexual violation and physical assault did not distinguish rapists from each other these were the core variables that occurred in most rape cases. Instead, what distinguished the rapists into categories were nonphysical interactions--things like whether they stole from or apologized to the victim.

Canter puts little faith in the investigative experience-derived offender descriptions developed by law-enforcement agents. As he sees it, psychologists need to work from the ground up to gather data and classify offenders in areas as various as arson, burglary, rape and homicide.

Crime action profiling. Forensic psychologist Richard Kocsis, PhD, and his colleagues have developed models based on large studies of serial murderers, rapists and arsonists that act as guides to profiling such crimes. The models, he says, are similar to the structured interviews clinical psychologists use to make clinical diagnoses. They come out of an Australian government-funded research program that Kocsis ran, in which he developed profiling methods in collaboration with police and fire agencies.

Now in private practice, Kocsis says crime action profiling models are rooted in knowledge developed by forensic psychologists, psychiatrists and criminologists. Part of crime action profiling also involves examining the process and practice of profiling.

"Everybody seems to be preoccupied with developing principles for profiling," Kocsis explains. "However, what seems to have been overlooked is any systematic examination of how to compose a profile. What type of information do, or should, profiles contain? What type of case material do you need to construct a profile? How does the presence or absence of material affect the accuracy of a profile?"

He has studied, for example, whether police officers perceive the same profile to be more accurate and useful when they believe it was written by a professional profiler rather than a layperson.

Kocsis agrees that the future of profiling lies in more empirically based research. He also believes, though, that just as some clinicians are better than others, there is also a skill element involved in profiling. Is profiling an art or a science? "Realistically, I think it is probably a bit of both," he says.

The psychology-law enforcement relationship

Among those in the profiling field, the tension between law enforcement and psychology still exists to some degree. "The difference is really a matter of the FBI being more oriented towards investigative experience than [academic psychologists] are," says retired FBI agent McCrary.

"But," he adds, "it's important to remember that we're all working toward the same thing."

In recent years, the FBI has begun to work closely with many forensic psychologists--in fact, it employs them. Psychologist Stephen Band, PhD, is the chief of the Behavioral Science Unit, and clinical forensic psychologist Anthony Pinizzotto, PhD, is one of the FBI's chief scientists.

The unit also conducts research with forensic psychologists at the John Jay College of Criminal Justice in New York. One recent collaborative study, for example, looked at the relationship between burglaries and certain types of sexual offenses--whether specific aspects of a crime scene differed in incidents that began as a burglary and ended in a sexual offense, as opposed to crimes that began as a sexual offense but included theft. Police looking at the first type of crime might want to look for convicted burglars in the area, Pinizzotto explains. The study will be published in an upcoming issue of Sex Offender Law Report, published by the Civic Research Institute.

One of the FBI's collaborators at John Jay College is Gabrielle Salfati, PhD, a graduate of the Centre for Investigative Psychology. "Whenever we do research, we try to bring in as many varied points of view as possible," Pinizzotto says. "Gabrielle Salfati's expertise on the statistical aspects of evaluating crime scenes is a great contribution."

More recently, the unit has also begun to collaborate with forensic psychologists at Marymount University in Arlington, Va.--another indication that law enforcement and psychology will continue to work together.

"I think," says Band, "that there is an incredible value added when applications of professional psychology enter into the mix of what we do."

Inferential statistics

We have seen that descriptive statistics are useful in providing an initial way to describe, summarize, and interpret a set of data. They are limited in usefulness because they tell us nothing about how meaningful the data are. The second step in analyzing data requires inferential statistics. Inferential statistics provide researchers with the tools to make inferences about the meaning of the results. Specifically, they allow researchers to generalize from the sample they used in their research to the greater population, which the sample represents. Keep in mind that psychologists, like other scientists, rely on relatively small samples to try to understand populations.

This is not a textbook about statistics, so we will limit the discussion of inferential statistics. However, all students of psychology should become familiar with one very important inferential statistic: the significance test. In the simplest, non-mathematical terms, the significance test is the researcher’s estimate of how likely it is that their results were simply the result of chance. Significance testing is not the same thing as estimating how meaningful or large the results are. For example, you might find a very small difference between two experimental conditions that is statistically significant.

Typically, most researchers use the convention that if significance testing shows that a result has a less than 5% probability of being due to chance alone, the result is considered to be real and to generalize to the population. If the significance test shows that the probability of chance causing the outcome is greater than 5%, it is considered to be a non-significant result and, consequently, of little value non-significant results are more likely to be chance findings and, therefore, should not be generalized to the population. Significance tests are reported as p values, for example, p< .05 means the probability of being caused by chance is less than 5%. P values are reported by all statistical programs so students no longer need to calculate them by hand. Most often, p values are used to determine whether or not effects detected in the research are present. So, if p< .05, then we can conclude that an effect is present, and the difference between the two groups is real.

Thus, p values provide information about the presence of an effect. However, for information about how meaningful or large an effect is, significance tests are of little value. For that, we need some measure of effect size. Effect size is a measure of magnitude for example, if there is a difference between two experimental groups, how large is the difference? There are a few different statistics for calculating effect sizes.

In summary, statistics are an important tool in helping researchers understand the data that they have collected. Once the statistics have been calculated, the researchers interpret their results. Thus, while statistics are heavily used in the analysis of data, the interpretation of the results requires a researcher’s knowledge, analysis, and expertise.

Key Takeaways

  • Descriptive statistics organize and summarize some important properties of the data set. Frequency distributions and histograms are effective tools for visualizing the data set. Measures of central tendency and dispersion are descriptive statistics.
  • Many human characteristics are normally distributed.
  • Measures of central tendency describe the central point around which the scores are distributed. There are three different measures of central tendency.
  • The range and standard deviation show the dispersion of scores as well as the shape of the distribution of the scores. The standard deviation of the normal distribution has some special properties.
  • Inferential statistics provide researchers with the tools to make inferences about the meaning of the results, specifically about generalizing from the sample they used in their research to the greater population, which the sample represents.
  • Significance tests are commonly used to assess the probability that observed results were due to chance. Effect sizes are commonly used to estimate how large an effect has been obtained.

Exercises and Critical Thinking

  1. Keep track of something you do over a week, such as your daily amount of exercise, sleep, cups of coffee, or social media time. Record your scores for each day. At the end of the week, construct a frequency distribution of your results, and draw a histogram that represents them. Calculate all three measures of central tendency, and decide which one best represents your data and why. Invite a friend or family member to participate, and do the same for their data. Compare your data sets. Whose shows the greatest dispersion around the mean, and how do you know?
  2. The data for one person cannot generalize to the population. Consider why people might have different scores than yours.

Image Attribution

Figure 2.5. Used under a CC BY-NC-SA 4.0 license.

Figure 2.6. Used under a CC BY-NC-SA 4.0 license.

Figure 2.7. Used under a CC BY-NC-SA 4.0 license.

Figure 2.8. Used under a CC BY-NC-SA 4.0 license.

Figure 2.9. Used under a CC BY-NC-SA 4.0 license.

Long Descriptions

Figure 2.7. Of the 25 families, 24 families have an income between $44,000 and $111,000, and only one family has an income of $3,800,000. The mean income is $223,960, while the median income is $73,000.


Charles Darwin's theory of evolution in 1859 became the scientific rationale for using animal experiments to learn more about humans. In the late nineteenth century, Ivan Pavlov's experiments in the development of "conditioned" responses in dogs (salivation) helped to foster an increasingly authoritative school of psychology known as behaviorism. The contemporary human treatment regimen known as behavior modification is fashioned from parallels drawn on these early experiments in operant conditioning.

In 1876, England passed the British Cruelty to Animals Act, which regulated animal experimentation. Still, behaviorist thinking at that time denied animals any psyche or emotion. Academic journals described animal behavior only in terms of physiologic response to stimuli, with no mention of any psychological consequence.

In later years, the behaviorist theories were overshadowed by the development and spread (from Europe to the United States) of ethology which concerns itself with genetic predisposition, or innate/instinctive behavior and knowledge. This theory continues to prevail in the United States, but in terms of relevance, it is tempered by the reality that between 85 and 90 percent of all animal experimentation is conducted on species not sufficiently similar to humans to draw dispositive parallels. The majority of all animal research in the field of psychology is conducted on various rodent species (rats, mice, hamsters, etc.) or birds as laboratory subjects.

Australian philosopher Peter Singer made the case for an end to animal experimentation with his 1975 book, Animal Liberation. Coinciding with his book was the comprehensive and sensitive research of such ethologists as Jane Goodall and Dian Fossey, who suggested that primates were capable of a full spectrum of emotions, including love, sorrow, jealousy, humor, and deceit. These animals also learned to communicate with humans by using over 300 learned signs in American Sign Language. Studies with other species produced similar results. During the late 1990s, an African gray parrot named Alex, who was being studied at the Arizona State University, fell ill and was required to spend the night alone at a veterinary clinic. When his keeper attempted to leave the room at the clinic, Alex cried out, "Come here, I love you, I'm sorry. Wanna go back." Such examples of the yet-unknown extent of emotional, psychological, and behavioral capacity in other species have cast new doubts on the scientific rationale for the continuation of captive animal experimentation.

When was statistics used in psychology research first? - Psychology

Research studies with small sample sizes, high variability, and sampling bias are usually not representative of the general population.

Learning Objectives

Explain the factors that can threaten the external validity of a study

Key Takeaways

Key Points

  • Sampling bias is when the sample in question is not representative of the general population.
  • Selection bias occurs when the participants in the sample are not equally and fairly selected for both the experimental and control groups this renders any results from the experiment meaningless.
  • Response bias is when only highly motivated people return a survey. When this occurs, the resulting data is biased toward those with the motivation to answer and submit the survey, and is therefore not representative of the population as a whole.
  • External validity is the ability to apply conclusions gathered from the results of an experiment to the general population.
  • Data sets with little variability have values that are similar to each other data sets with high variability have values that are more spread out.

Key Terms

  • reliability: The overall consistency of a measure.
  • external validity: In research, whether or not study findings can be generalized to real-world scenarios.
  • law of diminishing returns: The tendency for a continuing effort toward a particular goal to decline in efficacy after a certain amount of success has been achieved.
  • bias: An inclination, predisposition, or prejudice toward something.

Research studies often fall prey to experimental bias, in which the results are not representative of what they are supposed to measure. This limits the applicability of the results to anything beyond the experiment itself, which decreases or eliminates the value of those results.

External Validity

A study that is externally valid is one in which the data and conclusions gathered from the results of an experiment can be applied to the general population outside of the experiment itself. If the study’s data and conclusions cannot be applied to the general population, including general events or scenarios, then the experiment’s results are only relevant to that experiment, and nothing more. A study’s external validity can be threatened by such factors as small sample sizes, high variability, and sampling bias.

Small Sample Sizes

The smaller the sample size for an experiment, the less applicable the results will be to the general population. The world has some 7 billion individuals, and thus a representative sample in any experiment would have to be very large to be applied to this general population. Nonetheless, the larger the sample group is in relation to the general population to whom the results are to be applied, the more likely it is to be applicable.

This premise, however, can be negatively impacted by the law of diminishing returns, which states that effectiveness will decline after a certain amount of success has been achieved. This means that after a certain point, including more individuals in a study would gradually have less value to researchers. This could be caused by a multitude of factors, including cost and time put into the research. Generally it is best to attain a reasonable sample size that is representative of the population being studied.

High Variability

Variability, also known as dispersion or spread, refers to how spread out a group of data is, or how much the measures differ from each other. Data sets with similar values are considered to have little variability because the values are within a smaller spread, whereas data sets with values that are spread out have high variability because the values are within a larger spread. In many instances of high variability there are outliers, which are values that exist far outside of the area where the majority of values are found. In many cases these outliers, which increase the variability of the data set, are removed when conducting statistical analysis of the data.

Sampling Bias

Sampling bias occurs when the sample participating in the study is not representative of the general population. This may be the result of purposeful selection of participants by the researcher, but there are many other factors that can create sampling bias. One example is surveys taken during a presidential election. The results of the surveys often depend on the city, state, or area being surveyed. For example, people in cities tend to vote one way, while people in rural environments often vote another. Similarly, one’s geographic location (the Northeast, South, Midwest, etc.) can have an impact on who is being surveyed. If there is a high saturation of a given political party in an area surveyed, then the results will be skewed in the direction of the political party, and not be representative of the general population.

Selection Bias

Selection bias happens when the comparisons in data from the sample population have no meaning or value because the participants in the sample were not equally and fairly selected for both the experimental and control groups. Both the experimental and control groups should be representative of the general population, as well as representative of each other. One group should not show substantially higher characteristics of a given variable than the other, as this can distort the findings.

Response Bias

Response bias (also known as “self-selection bias”) occurs when only certain types of people respond to a survey or study. When this occurs, the resulting data is biased towards those with the motivation to answer and submit the survey or participate in the study. The resulting data, however, is not representative of the desired sample, nor the population at large. This is because only a select few have answered the survey and participated in the experiment. This data requires a disclaimer saying that out of all respondents, a certain characteristic is found. Regardless of a disclaimer, the results cannot be applied to the general population, nor the entire desired sample group.

For example, imagine that a university newspaper ran an ad asking for students to volunteer for a study in which intimate details of their sex lives would be discussed. Clearly, the sample of students who would volunteer for such a study would not be representative of all of the students at the university (many of whom would never want to volunteer for such a study due to privacy concerns). Similarly, an online survey about computer use is likely to attract people more interested in technology than is typical. In both of these examples, people who “self-select” themselves for the study are likely to differ in important ways from the population the experimenter wishes to draw conclusions about. Thus, the responses collected are biased and not representative of the general population of interest. Many of the admittedly “non-scientific” polls taken on television or websites suffer from response bias.

A response bias can also result when the non-random component occurs after the potential subject has enlisted in the experiment. Considering again the hypothetical experiment in which subjects are to be asked intimate details of their sex lives, and assume this time that the subjects did not know what the experiment was going to be about until they showed up. When they found out, many of the subjects would refuse to participate, leaving only those students who are very interested in discussing their sex lives, which results in a biased sample.


Another important issue to consider when collecting data is reliability. Reliability refers to the overall consistency of a measure. This means that any surveys, tasks, or measures the researcher administers during a study need to produce similar results each time they are used under similar conditions. If a measure is not reliable, it will produce different results, even under the same conditions. Consider a scale that measures how much you weigh. If one day the scale shows that you weigh 150 lbs yet the next day it shows you 170 lbs, it may be time to shop for a more reliable scale. Let’s look at another example. A researcher is running a study using a questionnaire that assesses emotion —
specifically negative affect. If the emotion questionnaire produces completely different results, even when very similar participants with identical levels of negative affect under identical experimental conditions complete the questionnaire, it is not reliable, and the data cannot be trusted. Ideally, the two similar participants with identical levels of negative affect should score very similarly on the emotion questionnaire. This would indicate the measure’s ability to produce consistent results under similar conditions: the measure would be considered reliable.

Therefore, when determining the reliability of a measure, a researcher must determine how much variability is stemming from measurement error (assumed to be random error) and how much is stemming from the “true score” or the actual, replicable aspects of the phenomenon being measured. This concept is sometimes referred to as “classical test theory.” Researchers typically do this by pre-testing their measures on preliminary samples of participants, and by running descriptive reliability analyses that indicate to them the measure’s overall consistency.

History and Uses of Survey Research

Survey research may have its roots in English and American “social surveys” conducted around the turn of the 20th century by researchers and reformers who wanted to document the extent of social problems such as poverty (Converse, 1987) [1] . By the 1930s, the US government was conducting surveys to document economic and social conditions in the country. The need to draw conclusions about the entire population helped spur advances in sampling procedures. At about the same time, several researchers who had already made a name for themselves in market research, studying consumer preferences for American businesses, turned their attention to election polling. A watershed event was the presidential election of 1936 between Alf Landon and Franklin Roosevelt. A magazine called Literary Digest conducted a survey by sending ballots (which were also subscription requests) to millions of Americans. Based on this “straw poll,” the editors predicted that Landon would win in a landslide. At the same time, the new pollsters were using scientific methods with much smaller samples to predict just the opposite—that Roosevelt would win in a landslide. In fact, one of them, George Gallup, publicly criticized the methods of Literary Digest before the election and all but guaranteed that his prediction would be correct. And of course it was. (We will consider the reasons that Gallup was right later in this chapter.) Interest in surveying around election times has led to several long-term projects, notably the Canadian Election Studies which has measured opinions of Canadian voters around federal elections since 1965. Anyone can access the data and read about the results of the experiments in these studies.

From market research and election polling, survey research made its way into several academic fields, including political science, sociology, and public health—where it continues to be one of the primary approaches to collecting new data. Beginning in the 1930s, psychologists made important advances in questionnaire design, including techniques that are still used today, such as the Likert scale. (See “What Is a Likert Scale?” in Section 9.2 “Constructing Survey Questionnaires” .) Survey research has a strong historical association with the social psychological study of attitudes, stereotypes, and prejudice. Early attitude researchers were also among the first psychologists to seek larger and more diverse samples than the convenience samples of university students that were routinely used in psychology (and still are).

Survey research continues to be important in psychology today. For example, survey data have been instrumental in estimating the prevalence of various mental disorders and identifying statistical relationships among those disorders and with various other factors. The National Comorbidity Survey is a large-scale mental health survey conducted in the United States . In just one part of this survey, nearly 10,000 adults were given a structured mental health interview in their homes in 2002 and 2003. Table 9.1 presents results on the lifetime prevalence of some anxiety, mood, and substance use disorders. (Lifetime prevalence is the percentage of the population that develops the problem sometime in their lifetime.) Obviously, this kind of information can be of great use both to basic researchers seeking to understand the causes and correlates of mental disorders as well as to clinicians and policymakers who need to understand exactly how common these disorders are.

Table 9.1 Some Lifetime Prevalence [2] Results From the National Comorbidity Study
Disorder Average Female Male
Generalized anxiety disorder 5.7 7.1 4.2
Obsessive-compulsive disorder 2.3 3.1 1.6
Major depressive disorder 16.9 20.2 13.2
Bipolar disorder 4.4 4.5 4.3
Alcohol abuse 13.2 7.5 19.6
Drug abuse 8.0 4.8 11.6

And as the opening example makes clear, survey research can even be used to conduct experiments to test specific hypotheses about causal relationships between variables. Such studies, when conducted on large and diverse samples, can be a useful supplement to laboratory studies conducted on university students. Although this approach is not a typical use of survey research, it certainly illustrates the flexibility of this method.

Research Methods

The aim of the study is a statement of what the researcher intents to investigate.

The hypothesis of the study is an idea, derived from psychological theory which contains a prediction which can be verified or disproved by some kind of investigation, usually an experiment.

A directional hypothesis indicates a direction in the prediction (one-tailed) e.g. ‘students with pets perform better than students without pets’.

A non-directional hypothesis does not indicate a direction in the prediction (two-tailed) e.g. ‘owning pets will affect students’ exam performances’.


A sample is the participants you select from a target population (the group you are interested in) to make generalisations about.

A Volunteer sample is where participants pick themselves through newspaper adverts, noticeboards or online.

Opportunity sampling uses people who are available at the time the study is carried out.

Random sampling is when every person in the target population has an equal chance of being selected.

Systematic sampling is when a system is used to select participants.

Stratified sampling is when you identify the subgroups and select participants in proportion with their occurrences.


Independent variable (IV) – the variable the experimenter manipulates, aassumed to have a direct effect on the DV.

Dependent variable (DV) – the variable the experimenter measures after making changes to the IV.

We must use operationalisation to ensure that variables are in a form that can be easily tested e.g. Educational attainment → GCSE grade in maths.

Extraneous variables are all variables, which are not the independent variable, but could affect the results of the experiment.There are two types: Situational variables (controlled through standardisation) and Participant variables (controlled through randomisation).

Experimental Design

In an independent measures design (between groups), a group of participants are recruited and divided into 2. The first group does the experimental task with the IV set for condition 1 and the second group does the experimental task with the IV set for condition 2. The DV is measured for each group and results are compared.

In a repeated measures design (within groups), a group of participants are recruited, and the group does the experimental task with the IV set for condition 1 and then the same for condition 2. The DV is measured for each group and results are compared.

In a matched pairs design, a group of participants are recruited. We find out what sorts of people we have in the group and recruit another group that matches them one for one. The experiment is then treated like an independent measures design and the results are compared.

Lab Experiment

This type of experiment is conducted in a well-controlled environment – not necessarily a laboratory – and therefore accurate and objective measurements are possible.

The researcher decides where the experiment will take place, at what time, with which participants, in what circumstances and using a standardized procedure.

Field Experiment

These are conducted in the everyday (i.e. natural) environment of the participants but the situations are still artificially set up.

The experimenter still manipulates the IV, but in a real-life setting (so cannot really control extraneous variables).

Natural Experiment

Natural experiments are when a naturally occurring IV is investigated that isn’t deliberately manipulated, it exists anyway.

Participants are not randomly allocated and the natural event may only occur rarely.

Case Study

Case studies are in-depth investigations of a single person, group, event or community.

Case studies are widely used in psychology and amongst the best-known ones carried out were by Sigmund Freud. He conducted very detailed investigations into the private lives of his patients in an attempt to both understand and help them overcome their illnesses.

Case studies provide rich qualitative data and have high levels of ecological validity.


Correlation means association - more precisely it is a measure of the extent to which two variables are related.

If an increase in one variable tends to be associated with an increase in the other then this is known as a positive correlation.

If an increase in one variable tends to be associated with a decrease in the other then this is known as a negative correlation.

A zero correlation occurs when there is no relationship between variables.


Unstructured (informal) interviews are like a casual conversation. There are no set questions and the participant is given the opportunity to raise whatever topics he/she feels are relevant and ask them in their own way. In this kind of interview much qualitative data is likely to be collected.

Structured (formal) interviews are like a job interview. There is a fixed, predetermined set of questions that are put to every participant in the same order and in the same way. The interviewer stays within their role and maintains social distance from the interviewee.


Questionnaires can be thought of as a kind of written interview. They can be carried out face to face, by telephone or post.

The questions asked can be open ended, allowing flexibility in the respondent's answers, or they can be more tightly structured requiring short answers or a choice of answers from given alternatives.

The choice of questions is important because of the need to avoid bias or ambiguity in the questions, ‘leading’ the respondent, or causing offence.


Covert observations are when the researcher pretends to be an ordinary member of the group and observes in secret. There could be ethical problems or deception and consent with this particular method of observation.

Overt observations are when the researcher tells the group he or she is conducting research (i.e. they know they are being observed).

Controlled: behavior is observed under controlled laboratory conditions (e.g. Bandura's Bobo doll study).

Natural: Here spontaneous behavior is recorded in a natural setting.

Participant: Here the observer has direct contact with the group of people they are observing.

Non-participant (aka "fly on the wall): The researcher does not have direct contact with the people being observed.

Pilot Study

A pilot study is an initial run-through of the procedures to be used in an investigation it involves selecting a few people and trying out the study on them. It is possible to save time, and in some cases, money, by identifying any flaws in the procedures designed by the researcher.

A pilot study can help the researcher spot any ambiguities (i.e. unusual things) or confusion in the information given to participants or problems with the task devised.

Sometimes the task is too hard, and the researcher may get a floor effect, because none of the participants can score at all or can complete the task – all performances are low. The opposite effect is a ceiling effect, when the task is so easy that all achieve virtually full marks or top performances and are “hitting the ceiling”.

Content Analysis

Content analysis is a research tool used to indirectly observe the presence of certain words, images or concepts within the media (e.g. advertisements, books films etc.). For example, content analysis could be used to study sex-role stereotyping.

Researchers quantify (i.e. count) and analyze (i.e. examine) the presence, meanings and relationships of words and concepts, then make inferences about the messages within the media, the writer(s), the audience, and even the culture and time of which these are a part.

To conduct a content analysis on any such media, the media is coded or broken down, into manageable categories on a variety of levels - word, word sense, phrase, sentence, or theme - and then examined.

Meta Analysis

A meta-analysis is a systematic review that involves identifying an aim and then searching for research studies that have addressed similar aims/hypotheses.

This is done by looking through various databases and then decisions are made about what studies are to be included/excluded.

Strengths: Increases the validity of the conclusions drawn as they’re based on a wider range.

Weaknesses: Research designs in studies can vary so they are not truly comparable.

Peer Review

A researcher submits an article to a journal. The choice of journal may be determined by the journal’s audience or prestige.

The journal selects two or more appropriate experts (psychologists working in a similar field) to peer review the article without payment. The peer reviewers assess: the methods and designs used, originality of the findings, the validity of the original research findings and its content, structure and language.

Feedback from the reviewer determines whether the article is accepted. The article may be: Accepted as it is, accepted with revisions, sent back to the author to revise and re-submit or rejected without the possibility of submission.

The editor makes the final decision whether to accept or reject the research report based on the reviewers comments/ recommendations.

Peer review is important because it prevent faulty data from entering the public domain, it provides a way of checking the validity of findings and the quality of the methodology and is used to assess the research rating of university departments.

Peer reviews may be an ideal, whereas in practice there are lots of problems. For example, it slows publication down and may prevent unusual, new work being published. Some reviewers might use it as an opportunity to prevent competing researchers from publishing work.

Some people doubt whether peer review can really prevent the publication of fraudulent research.

A The advent of the internet means that a lot of research and academic comment is being published without official peer reviews than before, though systems are evolving on the internet where everyone really has a chance to offer their opinions and police the quality of research.

Types of Data

Quantitative data is numerical data e.g. reaction time or number of mistakes. It represents how much or how long, how many there are of something. A tally of behavioral categories and closed questions in a questionnaire collect quantitative data.

Qualitative data is non-numerical data expressed in words e.g. an extract from a diary. It can’t be counted but can be turned into quantitative data by placing the data in categories and then counting frequency. Open questions in questionnaires and accounts from observational studies collect qualitative data.

Primary data is first hand data collected for the purpose of the investigation.

Secondary data is information that has been collected by someone other than the person who is conducting the research e.g. taken from journals, books or articles.


Validity is whether the observed effect in genuine and represents what is actually out there in the world.

Concurrent validity – the extent to which a psychological measure relates to an existing similar measure and obtains close results. For example, a new intelligence test compared to an established test.

Face validity – does the test measure what it’s supposed to measure ‘on the face of it’. This is done by ‘eyeballing’ the measuring or by passing it to an expert to check.

Ecological validity – the extent to which findings from a research study can be generalised to other settings / real life.

Temporal validity – the extent to which findings from a research study can be generalised to other historical times.


Reliability is a measure of consistency, if a particular measurement is repeated and the same result is obtained then it is described as being reliable.

Test-retest reliability – Assessing the same person on two different occasions which shows the extent to which the test produces the same answers.

Inter-observer reliability – the extent to which there is agreement between two or more observers.

Features of Science

Paradigm – A set of shared assumptions and agreed methods within a scientific discipline.

Paradigm shift – The result of scientific revolution: a significant change in the dominant unifying theory within a scientific discipline.

Objectivity – When all sources of personal bias are minimised so not to distort or influence the research process.

Empirical method – Scientific approaches that are based on the gathering of evidence through direct observation and experience.

Replicability – The extent to which scientific procedures and findings can be repeated by other researchers.

Falsifiability – The principle that a theory cannot be considered scientific unless it admits the possibility of being proved untrue.

Statistical Testing

A significant result is one where there is a low probability that chance factors were responsible for any observed difference, correlation or association in the variables tested.

If our test is significant, we can reject our null hypothesis and accept our alternative hypothesis.

If our test is not significant, we can accept our null hypothesis and reject our alternative hypothesis. A null hypothesis is a statement of no effect.

In Psychology, we use p < 0.05 (as it strikes a balance between making a type I and II error) but p < 0.01 is used in tests that could cause harm like introducing a new drug.

A type I error is when the null hypothesis is rejected when it should have been accepted (happens when a lenient significance level is used, an error of optimism).

A type II error is when the null hypothesis is accepted when it should have been rejected (happened when a stringent significance level is used, an error of pessimism).

Ethical Issues

Informed consent is when participants are able to make an informed judgement about whether to take part. It causes them to guess the aims of the study and change their behavior. To deal with it, we can gain presumptive consent or ask them to formally indicate their agreement to participate but it may invalidate the purpose of the study and it is not guaranteed that the participants would understand.

Deception should only be used when it approved by an ethics committee as it involves deliberately misleading or withholding information. Participants should be fully debriefed after the study but debriefing can’t turn the clock back.

All participants should be informed at the beginning that they have the Right to Withdraw if they ever feel distressed or uncomfortable. It causes bias as the ones that stayed are obedient and some may not withdraw as they may have been given incentives or feel like they’re spoiling the study. Researchers can offer the right to withdraw data after participation.

Participants should all have Protection from harm. The researcher should avoid risks greater than experienced in everyday life and they should stop the study if any harm is suspected. However, the harm may not be apparent at the time of the study.

Confidentiality concerns the communication of personal information. The researchers should not record any names but use numbers or false names though it may not be possible as it is sometimes possible to work out who the researchers were.

When was statistics used in psychology research first? - Psychology

Once the study is complete and the observations have been made and recorded the researchers need to analyze the data and draw their conclusions. Typically, data are analyzed using both descriptive and inferential statistics. Descriptive statistics are used to summarize the data and inferential statistics are used to generalize the results from the sample to the population. In turn, inferential statistics are used to make conclusions about whether or not a theory has been supported, refuted, or requires modification.


Baron and Kenny (1986) Α] laid out several requirements that must be met before one can speak of a mediation relationship. They are outlined below using a real world example. See the diagram above for a visual representation of the overal mediating relationship to be explained.

Regress the dependent variable on the independent variable. In other words, confirm that the independent variable is a significant predictor of the dependent variable.

Independent Variable Dependent Variable

Regress the mediator on the independent variable. In other words, confirm that the independent variable is a significant predictor of the mediator. If the mediator is not associated with the independent variable, then it couldn’t possibly mediate anything.

Independent Variable Mediator

Regress the dependent variable on both the mediator and independent variable. In other words, confirm that the mediator is a significant predictor of the dependent variable, while controlling for the independent variable.

This step involves demonstrating that when the mediator and the independent variable are used simultaneously to predict the dependent variable, the previously significant path between the independent and dependent variable (Step #1) is now greatly reduced, if not nonsignificant. In other words, if the mediator were to be removed from the relationship, the relationship between the independent and dependent variables would be noticeably reduced.

  • β32 is significant
  • β31 should be smaller in absolute value than the original mediation effect (β11 above)

The following example, drawn from Howell (2009), Β] explains each step of Baron and Kenny’s requirements to further understand how a mediation effect is characterized. Step 1 and step 2 use a regression analysis, whereas step 3 uses a multiple regression analysis.

How you were parented (i.e., independent variable) predicts how confident you feel about parenting your own children (i.e., dependent variable).

How you were parented Confidence in own parenting abilities.

How you were parented (i.e., independent variable) predicts your feelings of competence and self-esteem (i.e., mediator).

How you were parented Feelings of competence and self-esteem.

Your feelings of competence and self-esteem (i.e., mediator) predict how confident you feel about parenting your own children (i.e., dependent variable), while controlling for how you were parented (i.e., independent variable).

Such findings would lead to the conclusion implying that your feelings of competence and self-esteem mediate the relationship between how you were parented and how confident you feel about parenting your own children.

Note: If step 1 does not yield a significant result, one may still have grounds to move onto step 2. Sometimes there is actually a significant relationship between the independent and dependent variables but because of small sample sizes, or other extraneous factors, there could not be enough power to predict the effect that actually exists (See Shrout & Bolger, 2002 Γ] for more info).

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