Control theory and multiple placebo effects

The importance of the placebo in medicine is well documented, yet our understanding of placebo effects remains limited. One obstacle to a greater understanding is the fact that the phrase 'placebo effect' has been used to refer to a number of qualitatively distinct behavioral events. Thus, different theories have been promulgated, each one tending to explain a selective portion of the placebo phenomenon. To help clarify matters, three kinds of placebo effects are described, and theories that have been used to explain them are reviewed. Limitations of these theories are then discussed, and a general model of placebo effects, based on a control theory perspective, is presented. Some implications and limitations of this model are outlined.