Placebo Effects

Believing in something often helps to make it work. This is called the placebo effect. A placebo (pronounced pluh-SEE-bo) is literally a "pleasing thing." The word usually refers to a sugar pill or fake medicine. However, the phrase "placebo effect" has come to mean much more. To most psychology researchers, the phrase placebo effect now refers to any situation in which a person's belief in a treatment causes the treatment to work (Critelli & Neumann, 1984).

What are placebo effects? Are they imaginary?

Placebo effects, defined this way, are not imaginary. They are genuine changes produced by a person's knowledge or belief in a manipulation. For example, if you wore a certain cologne which made you feel more attractive, you might act more attractive and be more attractive. This would be a placebo effect if your belief affected your behavior, but the cologne by itself had no real effect. To determine whether that was the case, the placebo effect would have to be controlled.

How do you control a placebo effect?

When testing the effects of a new medicine, researchers must give the control group a placebo-a realistic-looking "fake" which contains no active ingredient. The experimental group gets the real medicine, the control group gets the placebo. Both groups think they are getting a real medicine. This way the researchers can tell if the medicine has any effect beyond the expected placebo effect, which should be present equally in both groups.

Students have no trouble understanding this idea, yet many still miss this question:

1) How do you control the placebo effect?

a) give the control group an experimental treatment

b) give the experimental group a "sugar pill"

c) create a placebo effect in the control group

d) make sure nobody gets a placebo

The correct answer is "c." To some students that sounds wrong. If it sounds wrong to you, study this concept until you understand it. A researcher tries to create a placebo effect in the control group to equalize the placebo effect in the two groups. That removes the placebo effect as a confounded variable, because it is not a difference between groups. (In other words, both groups must believe they are receiving a genuine treatment, to equalize the placebo effect and remove it as a confounding variable.)

What is a single-blind design?

An experimental design in which subjects do not know whether they are receiving a placebo treatment or a real treatment is called a single-blind design. The subjects are blind to which treatment they are receiving, although the experimenter may be aware of which they are getting.

Write to Dr. Dewey at

Don't see what you need? Psych Web has over 1,000 pages, so it may be elsewhere on the site. Do a site-specific Google search using the box below.

Custom Search

Copyright © 2007 Russ Dewey