Experimental Psychology

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.


Yale psychologist Stanley Milgram (1933–84) conducted a series of controversial experiments to test people's obedience to authority in which volunteers were asked to inflict what they thought was a series of increasingly severe electric shocks on innocent individuals. The experiments raised ethical questions because of the psychological suffering it caused some of the participants.

Milgram was interested in the conditions that led to the extreme anti-Semitism and the Holocaust in Europe during WWII. His model for the experiment was Adolf Eichmann, the Nazi war criminal whose defense during histrial was that he was “justfollowing orders.” Eichmann was a model for Milgram's theory that most people blindly obey authority, and that they do so because it is what is expected of them, rather than as a result of a strong conviction about the morality of what is being asked of them. He sought to demonstrate that given the right conditions, any person could be transformed into a murderous Nazi or mass murderer without much difficulty.

In order to attract participants, the experiments were advertised as “Learning Studies” in newspaper ads and mail solicitations, seeking both “teacher” and “learner” volunteers. The subjects consisted of 40 males between ages 20 and 50, representing a wide range of occupations and education levels. Participants in the study were paid $4.50.

The subjects drew slips of paper from a hat in order to be assigned roles. The drawing was rigged so that all of the volunteers were assigned “teacher,” with the “learner” roles being played by actors.

Volunteer subjects were presented with realistic-looking simulated shock generators, which were marked with voltage levels ranging from 15 to 450 volts and corresponding designations ranging from Slight Shock to Danger: Severe Shock. They were told that the study was an examination of the effects of electroshock on learning.

The teacher and the researcher (the “authoritative role” ) were in one room, while the learner was strapped into a device resembling an electric chair in an adjoining room. The learner was then presented with a series of learning tasks and the teacher was instructed to give the learner a shock whenever they gave an incorrect response. They were then instructed to increase the voltage with each incorrect answer. The actors who were posing as learners were given instructions on how many answers to get correctly, and when to begin communicating increasing discomfort and pain. When the apparent voltage level reached 300 volts, the learners would pound on the walls, apparently in pain. After several times banging on the wall, the learner will cease responding at all. Milgram reported that teachers usually looked to the researcher for guidance periodically as the voltages increased. If a subject expressed a desire to stop, the researchers would give the following series of responses, in sequence:

Many of the subjects expressed disapproval with shocking a human despite their cries and objections, but still the majority complied with the commands of the researchers. 26 of the participants obeyed the commands fully and 14 stopped participating at some point due to the apparent suffering of the victims.

Milgram reported that some participants exhibited visible signs of anxiety—especially when administering the highest levels of electroshock— including sweating, shaking, stuttering and nervous laughter, which even led to uncontrollable seizures in three of the individuals. The ethical questions raised by this experiment helped develop future policies and guidelines for treatment of subjects in experimental studies.

The results of the experiment supported Milgram's theory that most people will obey blindly in the presence of legitimate authority even if it is in breach of personal moral beliefs.

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 .


Control group—
The group of subjects without the manipulation or experimental intervention, used as a baseline to measure the effects of the intervention.
Dependent variable—
The characteristic or variable measured by the experimenter that is expected to be affected by the independent variable.
Experiment group—
The group of subjects with the manipulation or experimental intervention.
Independent variable—
The characteristic or variable manipulated by the experimenter.



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.