scientific discipline that studies mental processes and behaviour in humans and other animals.

A brief treatment of the discipline of psychology follows. The subject matter of psychology is treated in a variety of other articles. For an overview of animal and human behaviour, see animal behaviour; human behaviour. For a treatment of the conscious experience of environment, see attention; perception; sensory reception (senses). For a treatment of internal states affecting behaviour and conscious experience, see dream; emotion; motivation; sexual behaviour, human; sleep. For a treatment of mental capacity and the processes of thought and learning, see animal learning; cognition; intelligence: Theories of intelligence (intelligence, human); intelligence: Development of intelligence (intelligence, human); learning theory; memory; thought. For a treatment of the integration and disintegration of the person's mental being as a whole, see mental disorder; personality. For a treatment of techniques for assessing human capacities and traits, see intelligence: Measuring intelligence (intelligence, human); personality assessment; psychological testing. For related philosophical and religious aspects, see epistemology; mind, philosophy of; religious experience.

Psychology is the science of individual or group behaviour. The word psychology literally means “study of the mind”; the issue of the relationship of mind and body is pervasive in psychology, owing to its derivation from the fields of philosophy and physiology. Psychology is intimately related to the biological and social sciences.

The broad reach of psychology sometimes gives it the appearance of disunity and promotes the lack of a universally accepted theoretical structure. Some of the divisions within psychology are applied fields, while others are more experimental in nature. The various applied fields include clinical; counseling; industrial, engineering, or personnel; consumer; and environmental. The most important of these specialties, clinical psychology, is concerned with the diagnosis and treatment of mental disorders. Industrial psychology is used in employee selection and related contexts in business and industry. The broad field known as experimental psychology includes specializations in child, educational, social, developmental, physiological, and comparative psychology. Of these, child psychology applies psychological theory and research methods to children; educational psychology is concerned with learning processes and problems associated with the teaching of students; social psychology is concerned with group dynamics and other aspects of human behaviour in its social and cultural setting; and comparative psychology deals with behaviour as it differs from one species of animal to another. The issues studied by psychologists cover a wide spectrum, comprising learning, cognition, intelligence, motivation, emotion, perception, personality, mental disorders, and the study of the extent to which individual differences are inherited or are shaped environmentally, known as behaviour genetics.

Experimental work using humans as subjects involves legal and ethical limitations. Therefore, a significant amount of research is done with animals, with the hope of transferring the knowledge gained concerning psychophysiological or behavioral functioning to humans.

The methods used in human research include observation (sometimes in nonlaboratory settings), interviews, psychological testing (also called psychometrics), laboratory experimentation, and statistical analysis. Psychometrics has in fact become a field in its own right, with psychometrists devising new tools for data collection, analysis, and new designs for experimental research.

Licensing requirements for American psychologists are regulated by their professional organization, the American Psychological Association (APA). The APA stipulates that in most divisions the bearer of the title “psychologist” must have a doctoral degree from an accredited institution. Principal employment settings include educational institutions, hospitals, prisons, business and industry, military establishments, and private practice. Many psychologists pursue a combination of private practice or consulting, research, and teaching.

The history of psychology is the history of thought about human consciousness and conduct. Psychological theory has its roots in ancient Greek philosophy and has been fed from streams such as epistemology (the philosophy of knowing), metaphysics, religion, and Oriental philosophy.

Over the centuries psychology and physiology became increasingly separated. A split developed between the essentially phenomenological (Phenomenology) (experiential) and mechanistic (physiological) conceptions of psychology. In general, through the end of the 19th century the British and German traditions were phenomenological, while the French and American were mechanistic. The history of psychology from the 19th century may be viewed as a debate between schools of systematic thought concerning the mind, such as associationism, structuralism, and functionalism; or alternatively, as a history of experimentation and research in various areas. Twentieth-century psychology began with structuralism, which employed the method of introspection to describe mental events. It then evolved into psychoanalysis, a derivative of psychiatric tradition, and produced behaviourism and Gestalt psychology, which were reactions against structuralism. humanistic psychology represented a rebellion against the reductionist and deterministic leanings of earlier schools.

By World War II, “schools” of psychology had largely faded away, leaving a common pool of psychological knowledge to which theoreticians, researchers, experimenters, and clinicians all contributed. biological psychology and psychobiology, fields of study combining psychology and physiology, grew in conjunction with these developments.