While psychological knowledge is typically applied to the assessment and treatment of mental health problems, it is also applied to understanding and solving problems in many different spheres of human activity. The vast majority of psychologists are involved in clinical, counseling, and school positions, some are employed in the industrial and organizational setting, and other areas such as human development and aging, sports, health, the media, legal, and forensics. Psychology incorporates research from the social sciences, natural sciences, and humanities.
The study of psychology in philosophical context dates back to the ancient civilizations of Egypt, Greece, China, India, and Persia. Historians point to the writings of ancient Greek philosophers, such as Thales, Plato, and Aristotle as the first significant work to be rich in psychology-related thought. In 1802, French physiologist Pierre Cabanis sketched out the beginnings of physiological psychology with his essay, Rapports du physique et du moral de l'homme (On the relations between the physical and moral aspects of man). Cabanis interpreted the mind in light of his previous studies of biology, arguing that sensibility and soul are properties of the nervous system.
German physician Wilhelm Wundt is known as the "father of experimental psychology," because he founded the first psychological laboratory, at Leipzig University in 1879.Wundt focused on breaking down mental processes into the most basic components, starting a school of psychology that is called structuralism. Edward Titchener was another major structuralist thinker.
Functionalism formed as a reaction to the theories of the structuralist school of thought and was heavily influenced by the work of the American philosopher and psychologist William James. In his seminal book, Principles of Psychology, published in 1890, he laid the foundations for many of the questions that psychologists would explore for years to come. Other major functionalist thinkers included John Dewey and Harvey Carr.
Other 19th-century contributors to the field include the German psychologist Hermann Ebbinghaus, a pioneer in the experimental study of memory who discovered the learning and forgetting curve at the University of Berlin; and the Russian-Soviet physiologist Ivan Pavlov, who discovered classical conditioning theory of learning whilst investigating the digestive system of dogs.
Starting in the 1950s, the experimental techniques set forth by Wundt, James, Ebbinghaus, and others would be reiterated as experimental psychology became increasingly cognitive—concerned with information and its processing—and, eventually, constituted a part of the wider cognitive science. In its early years, this development had been seen as a "revolution", as it both responded to and reacted against strains of thought—including psychodynamics and behaviorism—that had developed in the meantime.
From the 1890s until his death in 1939, the Austrian physician Sigmund Freud developed psychoanalysis, a method of investigation of the mind and the way one thinks; a systematized set of theories about human behavior; and a form of psychotherapy to treat psychological or emotional distress, especially unconscious conflict. Freud's psychoanalytic theory was largely based on interpretive methods, introspection and clinical observations. It became very well-known, largely because it tackled subjects such as sexuality, repression, and the unconscious mind as general aspects of psychological development. These were largely considered taboo subjects at the time, and Freud provided a catalyst for them to be openly discussed in polite society. Clinically, Freud helped to pioneer the method of free association and a therapeutic interest in dream interpretation.
Freud had a significant influence on Swiss psychiatrist Carl Jung, whose analytical psychology became an alternative form of depth psychology. Other well-known psychoanalytic thinkers of the mid-twentieth century included German-American psychologist Erik Erickson, Austrian-British psychoanalyst Melanie Klein, English psychoanalyst and physician D. W. Winnicott, German psychologist Karen Horney, German-born psychologist and philosopher Erich Fromm, English psychiatrist John Bowlby and Sigmund Freud's daughter, psychoanalyst Anna Freud. Throughout the 20th century, psychoanalysis evolved into diverse schools of thought, most of which may be classed as Neo-Freudian.
Psychoanalytic theory and therapy were criticized by psychologists and philosophers such as B. F. Skinner, Hans Eysenck, and Karl Popper. Skinner and other behaviorists believed that psychology should be more empirical and efficient than psychoanalysis—although they frequently agreed with Freud in ways that became overlooked as time passed. Popper, a philosopher of science, argued that Freud's, as well as Alfred Adler's, psychoanalytic theories included enough ad hoc safeguards against empirical contradiction to keep the theories outside the realm of scientific inquiry. By contrast, Eysenck maintained that although Freudian ideas could be subjected to experimental science, they had not withstood experimental tests. By the 21st century, psychology departments in American universities had become experimentally oriented, marginalizing Freudian theory and regarding it as a "desiccated and dead" historical artifact. Meanwhile, however, researchers in the emerging field of neuro-psychoanalysis defended some of Freud's ideas on scientific grounds, while scholars of the humanities maintained that Freud was not a "scientist at all, but ... an interpreter."
Behaviorism became the dominant school of thought during the 1950s. American behaviorism was founded in the early 20th century by John B. Watson, and embraced and extended by Edward Thorndike, Clark L. Hull, Edward C. Tolman, and later B. F. Skinner. Behaviorism is focused on observable behavior. It theorizes that all behavior can be explained by environmental causes, rather than by internal forces. Theories of learning including classical conditioning and operant conditioning were the focus of a great deal of research. Much research was done with laboratory-based animal experimentation, which was increasing in popularity as physiology grew more sophisticated.
Basic research in psychology includes perception, cognition, attention, emotion, motivation, brain functioning, personality, behavior, and interpersonal relationships. Some, especially depth psychologists, also consider the unconscious mind. Psychologists employ empirical methods to determine causal and correlational relationships between psychosocial variables. In addition, or in opposition, to employing empirical and deductive methods, clinical psychologists sometimes rely upon symbolic interpretation and other inductive techniques.
Skinner's behaviorism shared with its predecessors a philosophical inclination toward positivism and determinism. He believed that the contents of the mind were not open to scientific scrutiny and that scientific psychology should emphasize the study of observable behavior. He focused on behavior–environment relations and analyzed overt and covert (i.e., private) behavior as a function of the organism interacting with its environment. Behaviorists usually rejected or deemphasized dualistic explanations such as "mind" or "consciousness"; and, in lieu of probing an "unconscious mind" that underlies unawareness, they spoke of the "contingency-shaped behaviors" in which unawareness becomes outwardly manifest.
