Psychology is viewed as a relatively young discipline. The start of the modern concept is often attributed to Wilhelm Wundt (the "father of psychology"). Wundt attempted to quantify human thought into basic elements, he was inspired by the success of chemistry and physics in describing the world by component pieces such as atoms and elements. Wundt also founded the first modern laboratory for the study of psychology. This was done in 1879 at Leipzig University in Germany. Wundt's ideas were picked up by his student Edward B. Titchener who coined the term structuralism to describe it. Structuralism would come to dominate the study of psychology for most of the late 19th century and early 20th century.
Structuralism's goal was to describe all of Wundt's elements of thought. It used a technique called introspection, where volunteers were trained to breakdown their thoughts into basic elements and then shown various objects or exposed to various concepts and asked to describe the basic elements of their thoughts. For example, if shown a book a subject would attempt to describe the elements of what he is seeing by shape, and color but with out reference to higher order concepts such as "binding", "cover" or "pages."
The use of introspection and structuralism's seemingly inherent subjectivity created a backlash that developed into a the concept of functionalism. This idea is most famously put forth by William James. It attempted to understand mind and behavior not as a "how" but as a "why." It was heavily influenced by Charles Darwin and the Theory of evolution. It would later serve as the foundation for the behaviorist movement.
At the same time that structuralism and functionalism were being developed and debated Sigmund Freud was developing his ideas. Psychodynamics had an enormous influence on psychology and in many ways that influence continues to this day. While many of his ideas have been discredited Freud also was the first to introduce several core ideas into the field. These include: the concept of an unconscious mind being able to change and direct behavior, and that people go through developmental stages in which their mind and behavior change as they grow up and grow old.
Development of Behaviorism
At the turn of the century Ivan Pavlov started his work on the gastric reflex in dogs. He discovered that a salivary response could be activated by a cue such as a bell ringing if that cue was linked to the presentation of food. This concept was developed into the idea of classical conditioning. This work was absolutely pivotal in the history of psychology. It became combined with the concepts developed in functionalism and the field of behavioral psychology was born. Behavioral psychology was, in many ways, a reaction to the introspection, subjective based methods of structuralist and their goal of understanding the elements of the mind. Behaviorist abandoned the concept of mind as a scientific concept all together. Instead they attempted to describe all behavior as merely the product of inputs from the environment and outputs from the organism. The mind was viewed as a black box and never examined.
Pavlov, and later B.F. Skinner, as well as many others were able to formulate precise mathematical descriptions of training regimes and reactions. It was hoped that psychology could became a hard science like physics with mathematically rigorous theories. Skinner is most known for his work with operational conditioning or instrumental learning where small changes in behavior are rewarded in order to create complex behavior. This is perhaps most epitomized by the Skinner box in which he trained pigeons to perform many complex actions to get rewards.
Skinner also thought that all of human behavior was a function of conditioning and instrumental learning. He wrote several non-fiction books such as Beyond Freedom and Dignity (rumor has it that he enjoyed placing his initials B.F. at the start of his works) and several fiction books including Waldon 2 advocating the creation of Utopian society based on instrumental learning.
The Cognitive Revolution
During the 1960s several developments altered the course of psychological research. The first was the creation of the computer and the ideas of Allen Turing. People quickly saw the analogy between computers and the human brain and thought that perhaps a mathematically rigorous concept of the mind could be developed. On another front several experiments had begin turning up anomalous results. One of the most famous is the Garcia effect where an animal that is exposed to a novel food and then made sick instantly learns to no longer desire that food, however, if it is a food the animal has plenty of pre-exposure to it did not instantly acquire the conditioned dislike. This instantaneous acquisition in one condition and not in another could not be explained without cracking open the black box of the mind and allowing it to effect behavior.
Noam Chomsky is also credited with bringing down the behaviorist. Skinner was attempting to describe the acquisition of language as instrumental learning but Chomsky showed that language acquisition follow set rules that seemed innate in the person. A famous example of this is in errors that children make when learning grammar. Chomsky pointed out that children often will attempt to universally apply a grammatical rule and say something like "I runned to the store." Attempts to model language acquisition on computers that could make similar errors required certain innate rule sets existing at the beginning of language acquisition. A purely behaviorist approach was infeasible.
Thus cognitive psychology was born as a reaction against behaviorist. Cognitive psychologist attempted to understand the black box of the mind through computational analysis, modeling and rigorous experimentation. Also during this time clinical psychology was having its own revolution with psychoanalysis being questioned by existential and humanist approaches.
Developments to the present
In addition to cognitive psychology social psychology was also coming into its own. Stanley Milgram published his works on the Milgram experiment which demonstrated that normal individuals would obey an authority figure and endanger the health and life of other people. This was also linked to Philip Zimbardo's work on the Stanford prison experiment demonstrating the ability for college students to quickly turn into torturers. Other important concepts such as diffusion of responsibility and cognitive dissonance were being developed in experimental laboratories. In the 1970s E.O. Wilson published his book on sociobiology, this work as well as much of the work in social psychology got picked up by researchers with a strong biologic and evolutionary influence. It ultimately emerged as the field of Evolutionary psychology in the late 1980s and is currently one of the most hotly contested and productive areas of psychological research.
Technological developments during the last 30 years have also drastically altered the field of psychology. Computer processing power and memory increases have allowed researchers to develop models of many areas of cognition. More importantly imaging techniques that allowed psychologist to non-invasively peer into the mind of living people and see what areas are functionally active have revolutionized much of psychology.
Clinical work on psychotropic drugs for treatment of various psychosis such as schizophrenia, bi-polar and depression have also allowed researchers to start to understand the neurochemistry involved.