For centuries, philosophers and other scholars have argued over the nature of the mind and how it functions-the phenomena of mental life. But the relatively recent idea of using scientific methods to resolve such issues was what distinguished modern psychology.

In 1879 Wilhelm Wundt, a professor at the University of Leipzig in Germany, opened the first laboratory devoted exclusively to the study of psychology. Experiments were carried out under carefully controlled conditions to answer basic questions about the human mind.

Because most of the important first-generation psychologists studied under Wundt, the new discipline began as a basic science concerned mainly with laws of conscious experience. However, Hugo Munsterberg and Walter Dill Scott, two of Wundt's students who established themselves in the United States, began to explore the applications of psychological principles to industrial problems. Both wrote important books on these applications. Munsterberg was concerned primarily with industrial efficiency and Scott's (1908) with advertising. Between them they touched on many of the topics that have occupied I/O psychologists ever since.

At about the same time that Wundt was trying to describe the general laws that govern mental life, a British scholar and cousin of Charles Darwin, Sir Francis Galton, was studying how people differ from one another mentally. Coupled with the work of Alfred Binet, the pioneer in intelligence testing who gave us the concept of IQ (intelligence quotient), Galton's approach spawned a tradition in psychology that developed largely in parallel with Wundt's mental science (Hothersall, 1984). It has often been called the "mental testing movement".

Testing for individual differences and doing experiments to discover general mental principles have remained fairly distinct approaches in psychology despite periodic efforts to draw them together (Cronbach, 1957). Wundt's mental science approach dominated early psychology in the United States, but the practical possibilities of testing are what captured industry's interest. Hence, the individual differences approach became dominant in early industrial psychology and remained so for the next several decades.

During this period, Sigmund Freud's theories on the nature and treatment of mental disorders (and the role of unconscious events in mind and behavior) were also beginning to attract a great deal of attention, eventually helping to establish the health-care branch of psychology (Hothershell, 1984). Still, until nearly midcentury, psychology remained chiefly an academic discipline rooted in laboratory research. The industrial and clinical branches were considered peripheral, trivial and rather suspect offshoots, often lumped together as "applied psychology".

The evolution of I/O psychology from a minor offshoot to a recognized specialty within the field can be traced to a number of influences, but none more important than the two world wars. The demands of two massive war efforts, each representing dramatic changes in the doctrine and technology of warfare, called for radical changes in the management of human resources. Psychologists of all kinds were summoned to help, and in the course of doing so they discovered a great deal about the potential applications of their specialized knowledge and techniques. All branches of psychology made huge advances under the exigencies of war-an ironic twist of fate for a discipline pledged to the promotion of human welfare.

Both wars provided massive evidence of the value of testing for selection and assignment of people according to job requirements. Naturally, industry recognized the potential applications of these techniques, and the use of testing increased dramatically after both wars. In addition, World War II produced important advances in techniques used to train people, many of which found peacetime application. Educational, experimental, and industrial psychologists all contributed significantly to the design of these training programs. One notable example was simulation training, a technique in which the basic operations required in a job could be practiced on a replica of the job's environment without the risks and costs associated with learning in the actual situation.

In addition to selection, classification, and training developments, WWII fostered the creation of a whole new field based upon an entirely different strategy for improving human effectiveness. The essential idea behind it was that "human error" or poor performance is not always a matter of incapable or poorly trained personnel, but often reflects insufficient consideration of human characteristics in the design of the machines that people must operate. The field survived the war, and today constitutes a formally recognized as ergonomics.

In the three decades following WWII, all areas of professional psychology experienced tremendous growth, and the industrial field was transformed from a narrow applied specialty with a distinct emphasis on personnel functions to the multifaceted discipline that it is today. The official name was changed from industrial to industrial/organizational psychology in 1973 to reflect the growing influence of social psychology and other organizationally relevant social sciences.

One final trend in psychology has shaped the I/O field. Since the early 1900s, psychology has tended to be dominated by one or another theoretical perspective or kind of explanation (often called a "paradigm") during any given era. Early on, as we saw, the emphasis was on describing the general laws that govern conscious experience. Using introspection as a research tool, psychologists such as Wundt attempted to analyze and describe the contents of conscious experience (e.g. sensations, images, feelings) similar to the way a chemists might analyze the elements of matter. This paradigm was often referred to as structuralism because of its focus on the structure of mental content. Freud's work on the unconscious, however, coupled with a growing realization that much of what happens concsiously occurs for a purpose, shifted attention to the more dynamic functional properties of mind. From the late 1920s until the 1940s, therefore, the emphasis was on the mental and biological underpinnings of functions such as motivation, emotion, learning, and perception. The primary concern of functionalists was how humans and other living organisms adjust to their environment.

This school of thought, which constituted America's first unique paradigm, became known as American functionalism (Hothersall, 1984). It was a much less restrictive school of thought than structuralism, a feature that enabled both the individual differences approach and applied psychology to develop within it. William James, a Harvard professor who has often been called America's most influential psychologist and interestingly was the one who brought Hugo Munsternberg to the US.

Functionalism gave way to another paradigm, behaviorism, under the leadership first of John B. Watson, and later B.F. Skinner (Hottersall, 1984). Behaviorism held that both the content and functions of the mind are unsuitable subjects for scientific study because neither is open to public view. All that can be studied objectively are the behaviors (responses) that organisms exhibit and the environmental conditions (stimuli) that control them. In its most radical form, behaviorism denied the very existence of mind and maintained that all behavior could eventually be explained in terms of simple stimulus-response (S-R) laws.

From an applications standpoint, behaviorism focused on ways to shape or control behavior by manipulating stimulus conditions and consequences of behavior. Behavior modification ("B-mod") techniques became popular in clinical, counseling, developmental, school and even I/O applications, and in various forms remain so today. However, as a general philosophy, behaviorism never dominated the I/O specialty as it did the broader field of psychology during the 1950s and 1960s. That undoubtedly was attributable to I/O psychology's lack of any theoretical orientation during much of this period and its heritage in measurement and individual differences.

By the 1970s, the study of mental events, which behaviorism had all but eliminated from psychology began making a dramatic comeback thanks largely to the evolution of the computer. Here at last was a convenient model for how the human mind might work. To function effectively, the mind must perform many of the same operations as a computer, such as sensing and interpreting input information, storing it in various ways, performing logic tests, and selecting response. Mental functions could be inferred by analogy and cast into the form of hypotheses that could be tested rigorously in the laboratory.

The latest paradigm shift is still in full swing. It emphasizes cognitive processes (thinking) and has touched virtually every corner of modern psychology, form clinical practice to the basic science of conscious experience-Wundt's domain.