Definition: Expectancy effects refer to changes in behavior that occur simply because the participant in an experiment believes that their behavior should change. In other words, people sometimes change how they act in certain situations simply because they think that their behavior is supposed to change.
Researchers often utilize placebo control groups to determine if changes in behavior are due to the actual independent variable of interest or to expectancy effects.
These expectancy effects can take place in research as well as in psychotherapy situations. For example, imagine that a patient sees a doctor about an anxiety problem they are experiencing. The doctor offers a course of treatment and medication, assuring the patient that the treatment will be very effective.
Because of expectancy effects, the patient may be more likely to respond to the treatment prescribed. While improvement may only be the result of the placebo effect, because they believe that the treatment will have an impact on their symptoms, they may be more likely to actually experience improvements.