The scientific investigation of basic behavioral processes including sensation, emotion, and motivation, as well as such cognitive processes as perception, memory, decision making, learning, problem-solving, and language.
Experimental psychologists work to understand the underlying causes of behavior by studying humans and animals. Animals are studied within and outside of laboratory settings for a variety of reasons. A researcher may wish to learn more about a particular species, to study how different species are interrelated, to investigate the evolutionary significance of certain behaviors, or to learn more about human behavior.
Experimental psychology flourished in the second half of the nineteenth century with the work of such figures as G. T. Fechner (1801–1887), whose Elements of Psychology (1860) is considered the first study in the field, and Wilhelm Wundt (1832–1920), who established the first psychological laboratory in 1879. Others, including Hermann Ebbinghaus (1850–1909) and E.B. Titchener (1867–1927), used laboratory methods to investigate such areas as sensation, memory, reaction time, and rudimentary levels of learning. While controlled laboratory studies continue to make major contributions to the field of psychology, experimental methods have also been used in such diverse areas as child development, clinical diagnosis, and social problems. Thus, the concept of experimentation can no longer be limited to the laboratory, and “experimental psychology” is now defined by method and by the kinds of processes being investigated, rather than its setting.
An experiment in any setting tests a hypothesis, a tentative explanation for an observed phenomenon or a prediction about the outcome of a specific event based on theoretical assumptions. All experiments consist of an independent variable, which is manipulated by the researcher, and a dependent variable, whose outcome will be linked to the independent variable. For example, in an experiment to test the sleepinducing properties of the hormone melatonin, the administration of the hormone would be the independent variable, and the resulting amount of sleep would be the dependent variable.
In simplest terms, the effects of the independent variable are determined by comparing two groups that are as similar to each other as possible, with the exception that only one group has been exposed to the independent variable being tested. That group is called the experimental group; the other group, which provides a baseline for measurement, is called the control group.
Although ideally the experimental and control groups will be as similar as possible, in practice, most psychological research is complicated by a variety of factors. For example, some random variables— differences in both the subjects themselves and in the testing conditions—are unavoidable and have the potential to disrupt the experiment. In addition, many experiments include more than one group of subjects, and establishing a true control group is not possible. One method of offsetting these problems is to randomly assign subjects to each group, thus distributing the effect of uncontrollable variables as evenly as possible.
In experiments utilizing a placebo, experimenter bias may be prevented by a double-blind design, in which not only the subjects but also the persons administering the experiment are unaware of which is the control group and what results are expected. In general, experimenters can minimize bias by making a vigilant attempt to recognize it when it appears, as well as resisting the temptation to intentionally influence the outcome of any experiment. However, research has shown that researchers can often not be aware of their own bias which may influence the study. The results of experiments are generally presented in a report or article that follows a standard format of introduction, method, results, and conclusion.
Experimental research can also be conducted through quasi-experiments, studies that lack the control of a true experiment because one or more of its requirements cannot be met, such as the deliberate use of an independent variable or the random assignment of subjects to different groups. Studies of the effects of drugs on pregnant women, for instance, are based on data about women who have already been pregnant and either taken or not taken drugs. Thus, the researcher has no control over the assignment of subjects or the choices with which they are presented (because it would be unethical to give potentially harmful drugs to pregnant women), but he or she can still measure differences between the two populations and obtain significant findings. These findings gain validity when they are based on data obtained from large numbers of subjects and when their results can be replicated a number of times. Such studies provide a basis for investigations that would otherwise be impossible.
See also Experimental design ; Research methodology .
Bausell, Barker. The Design and Conduct of Meaningful Experiments Involving Human Participants: 25 Principles. New York: Oxford University Press, 2015.
Christensen, Larry, Burke Johnson, and Lisa Turner. Research Methods, Design, and Analysis, 12th edition. Boston: Pearson, 2014.
Crano, William, Marilynn Brewer, and Andrew Lac. Prin-ciples and Methods of Social Research, 3rd edition. New York: Routledge, 2014.
Howitt, Dennis. Introduction to Research Methods in Psychology, 4th edition. New York: Pearson, 2014.
Schneider, Sandra. Experimental Design in the Behavioral and Social Sciences. Los Angeles: SAGE, 2013.