Placebo Effect

Mechanism

The placebo effect can be explained by two psychological theories: expectancy and classical conditioning. Research indicates that expectancy strongly influences the effect of a placebo response, and the response can be enhanced with verbal messages emphasizing that a positive change in symptoms should be anticipated. Classical conditioning theory has also been applied, and related research has found that placebos can effectively alleviate pain even after subjects are alerted to the fact that they are not receiving actual treatment, so long as the association between the placebo and analgesia has been well conditioned. The placebo effect, while psychologically mediated, has been found to affect neurobiological as well as [Page 1255]cognitive processes. For example, increases in striatal dopamine and endogenous opioids in Parkinson’s disease and pain patients, respectively, have been demonstrated following administration of a placebo.

Implications

Medical practitioners have been aware of the placebo effect for centuries, and knowledge of the phenomenon has become well incorporated in medical research. Because of the placebo effect, as well as the impacts of natural recovery and regression toward the mean, randomized controlled trials have become the gold standard for researching new drugs. In randomized controlled trials, participants are randomly assigned to either a treatment or a control group, with control participants often receiving a placebo. Ideally, assignment is double-blind, meaning that neither the participants nor the researchers are aware of who is receiving the placebo versus actual treatment. The treatment under investigation must then evidence incremental benefit above and beyond the placebo to demonstrate that any measured improvement is not an artifact of the placebo effect. Such a requirement poses a challenge for researchers investigating treatments for certain conditions such as depression, where the placebo effect is often quite high. Given the significance of the phenomenon, recent research has begun to investigate how lessons learned from the placebo effect can be harnessed by medical providers and clinically applied.

See also Classical Conditioning; Double-Blind Design; Hawthorne Effect; Random Assignment; Regression Toward the Mean