Experimental psychology is an approach to psychology that treats it as one of the natural sciences, and therefore assumes that it is susceptible to the experimental method. Many experimental psychologists have gone further, and have assumed that all methods of investigation other than experimentation are suspect. In particular, experimental psychologists have been inclined to discount the case study and interview methods as they have been used in clinical and developmental psychology.
Since it is a methodological rather than a substantive category, experimental psychology embraces a disparate collection of areas of study. It is usually taken to include the study of perception, cognitive psychology, comparative psychology, the experimental analysis of behavior, and some aspects of physiological psychology.
The earliest experimental psychologists, including Hermann Ebbinghaus and Edward Titchener included introspection among their experimental methods. However, in the first half of the twentieth century, experimental psychology became closely allied with behaviourism, especially in the United States, and this led to some neglect of mental phenomena. In Europe including the United Kingdom this was less so, and under the influence of psychologists such as Sir Frederic Bartlett, Kenneth Craik, W. E. Hick and Donald Broadbent, experimental psychologists focused on topics such as thinking, memory and attention, laying the foundations for the subsequent development of cognitive psychology.
With the expansion of psychology as a discipline in the latter half of the twentieth century, and the growth in the size and number of its subfields, the phrase "experimental psychology" has come to cover too broad an area to be much used. Most psychologists would now identify with a smaller field such as cognitive or comparative psychology. Furthermore most are happy to use a range of methods rather than confining themselves to a strictly experimental approach, and developments in the philosophy of science have lessened the exclusive prestige of experimentation. Conversely, the experimental method is now widely used in fields such as developmental and social psychology that were not part of experimental psychology as it was originally conceived. The phrase continues in use, however, in the titles of a number of well established, high prestige learned societies and scientific journals, as well as some university courses of study in psychology.
Recently, experimental psychologists such as Ulf-Dietrich Reips have begun using the Internet as a convenient medium for Internet-based experimenting. Examples for such studies can be found in the Web Experimental Psychology Lab and the web experiment list.