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Placebo Effect

In research, a scientifically significant response that cannot be explained by physiological variables and is assumed to be psychological in origin.

Placebos are substances with no known pharmacological value that are given to members of a control group in an experiment. In studies determining the effectiveness of a particular drug, for example, the experimental group is given the drug being studied and the control group is given a placebo, which is made to look exactly like the actual drug. Neither group, nor the researchers, knows which received the drug and which the placebo. If the members of each group show similar responses, the placebo effect has been produced. For reasons not completely understood, the patients given the placebo have experienced the effects of the drug without actually taking it. In such cases, the drug itself is considered ineffective.

The placebo effect has been noted since ancient times, when animal parts or other naturally occurring substances were given as treatment for various human diseases and ailments. Throughout medical history, patients have recovered from illnesses after healers employed substances or methods that scientifically should have no effect. It is believed that patients' expectations that their condition will improve plays a major role in producing the placebo effect.

The use of placebos in psychotherapy is controversial, with some critics contending that it links therapists with "quack" treatments rather than legitimate, scientifically measurable methods. However, most researchers agree that the placebo effect, while not completely understood, plays a major and beneficial role in both physiological and psychological treatment.

Further Reading

Atkinson, Rita L.; Richard C. Atkinson; Edward E. Smith; and Ernest R. Hilgard. Introduction to Psychology. 9th ed. San Diego: Harcourt Brace Jovanovich, 1987.

Zimbardo, Philip G. Psychology and Life. 12th ed. Glenview, IL: Scott, Foresman, 1988.

Additional topics

Psychology EncyclopediaPsychological Dictionary: Perception: early Greek theories to Zombie