What is psychology? The science of mind and behavior

Psychology is the scientific study of the mind and behavior: how people perceive, think, remember, feel, learn, develop and act, alone and with others. It is both a research science, which tests ideas about the mind with experiments and data, and a practice, which applies that knowledge to help people, most visibly through therapy. It became a separate science in the late 19th century and now spans fields from brain science to the workplace.

In short

• Psychology studies mental life and behavior with scientific methods, from lab experiments to large surveys, and applies what it finds in clinics, schools and workplaces.
• Its main branches include cognitive, developmental, social, biological and clinical psychology, each asking different questions about the same minds.
• A psychologist is usually trained in research and therapy, a psychiatrist is a medical doctor who can prescribe; and the field is still working through a replication crisis that showed many findings were weaker than reported.

Where did psychology come from?

Questions about the mind are as old as philosophy. Plato and Aristotle asked how anyone knows anything and what the soul is. Psychology became a science of its own when people started testing those questions by measurement. In 1879, Wilhelm Wundt set up the first laboratory dedicated to psychology at the University of Leipzig, timing how long people took to react to sounds and lights. In the United States, William James published The Principles of Psychology in 1890, which is still quoted.

The field then moved through schools, each reacting against the last.

Period	School	Its central idea
1880s onward	Structuralism and functionalism	Break conscious experience into parts, or ask what the mind is for
1890s onward	Psychoanalysis, Sigmund Freud	Unconscious drives and early experiences shape behavior
1913 to 1950s	Behaviorism, John Watson and B. F. Skinner	Study only observable behavior and how rewards and punishments shape it
1950s onward	Humanistic psychology, including [[what-is-maslows-hierarchy-of-needs	Abraham Maslow]]
1950s and 1960s onward	The cognitive revolution	The mind processes information; memory, attention and language can be studied
1990s onward	Cognitive neuroscience	Link mental processes to [[how-does-the-brain-work

Most psychologists today don't belong to a single school. They borrow from several, depending on the question.

What are the main branches of psychology?

Psychology divides into fields by the question they ask, not by the people they study.

• Cognitive psychology studies mental processes: perception, attention, memory, language, reasoning, decision-making.
• Developmental psychology studies how people change from infancy through old age.
• Social psychology studies how other people, groups and situations shape what people think and do, such as conformity, persuasion and prejudice.
• Biological psychology, or behavioral neuroscience, studies how the brain, hormones and genes relate to behavior.
• Personality psychology studies stable differences between people. Its best-supported model is the "Big Five" traits; popular type tests such as the Myers-Briggs have far weaker scientific support.
• Clinical and counseling psychology assess and treat mental health problems, from anxiety and depression to serious mental illness.
• Applied fields carry the science into particular settings: industrial-organizational psychology at work, school and educational psychology, health psychology, forensic psychology in the legal system, and sports psychology.

The US Bureau of Labor Statistics counted about 209,400 psychologist jobs in the country in 2025.

How do psychologists study the mind?

The mind can't be seen directly, so psychology infers it from what people do and say, and from the body. Its main methods each answer a different kind of question.

Take one question: does losing sleep hurt memory? An experiment assigns people at random to sleep normally or stay awake, then tests everyone's memory; because chance decided who lost sleep, a difference can be blamed on the sleep loss. A correlational study surveys thousands of people about their sleep and memory, which is cheaper and closer to real life, but can't show cause, since people who sleep badly might differ in other ways. A longitudinal study follows the same people for years to see whether poor sleep comes before memory decline. A case study examines one unusual person in depth, as the patient Henry Molaison, who lost the ability to form new memories after brain surgery in 1953, taught researchers what the hippocampus does. And neuroimaging shows which brain areas change activity in sleep-deprived people as they try to remember.

Good psychology uses several methods on the same question, because each covers the others' blind spots. Experiments show cause but in artificial settings; surveys reach real life but not cause; case studies find surprises that need testing in larger groups.

What's the difference between a psychologist and a psychiatrist?

People often mix them up, and the difference matters when choosing whom to see.

	Psychologist	Psychiatrist	Licensed counselor or therapist
Training	Usually a doctorate, a PhD or PsyD, plus supervised practice	Medical school, then a four-year psychiatry residency	Usually a master's degree plus supervised practice
Main tools	Psychotherapy, psychological testing, research	Diagnosis, medication, other medical treatments, and sometimes therapy	Psychotherapy and counseling
Can prescribe medicine	Generally no; a few US states allow specially trained psychologists to	Yes	No

The American Psychiatric Association describes a psychiatrist as a medical doctor who has completed medical school and residency, about 12 years of training after high school for a general adult psychiatrist. Rules on titles and licensing differ by country and by US state. This note explains the field; it isn't medical or mental health advice, and anyone in distress should contact a licensed professional or, in an emergency, local emergency services.

What is the replication crisis?

A finding is only trustworthy if it holds up when someone else repeats the study. In the 2010s, psychologists found that many did not. The landmark test came in 2015, when the Open Science Collaboration, about 270 researchers, repeated 100 studies published in three leading psychology journals in 2008.

The numbers tell the story. Of the 100 original studies, 97 had reported a statistically significant result. Of the 100 repeats, only 36 did. On average, the effects in the repeats were about half as large as in the originals. Some famous ideas, including some versions of "ego depletion", the claim that willpower runs out like a fuel, and "power posing", did badly in later large attempts. Older landmarks fared poorly too: the Stanford prison experiment turned out, on inspection of its archives, never to have been the controlled test it was taught as.

The causes were mostly ordinary. Small samples gave noisy results. Journals preferred surprising, positive findings. Researchers had many small choices in how to analyze data and, often without meaning to, picked the ones that gave a result. Psychology's response has been a model for other sciences: preregistering studies before collecting data, publishing data openly, running much larger samples, and journals agreeing to publish studies based on their design before the results are known.

Still argued

How bad the crisis was is still debated. In a 2016 comment in Science, Daniel Gilbert and colleagues argued that the 2015 project underestimated psychology's reliability, because many repeats differed from the originals in samples or procedures and because a single repeat with a modest sample will miss real effects by chance. The project's authors replied that their conclusions held. Most psychologists now accept that a substantial share of published findings were weaker than reported, while disagreeing about how large that share is and how much has been fixed.

The crisis is a reason for care, not for dismissing the field. Psychology's best-established findings, on learning, memory, perception, child development and effective therapies for conditions such as depression and anxiety, have been repeated many times. It is a science that has shown, unusually openly, how a science corrects itself.

Check yourself

• Without looking back: why can an experiment show cause when a survey of thousands of people can't?
• What is the difference in training and treatment between a psychologist and a psychiatrist?
• What did the 2015 replication project find, and what changes did psychology make in response?

Sources

• Psychology — Wikipedia
• Psychologists, Occupational Outlook Handbook — US Bureau of Labor Statistics
• What is Psychiatry? — American Psychiatric Association
• Estimating the reproducibility of psychological science — Open Science Collaboration, Science, 2015