Placebos and common factors in two decades of psychotherapy research.

Two decades of psychotherapy research conceptualizing placebos as variables common across psychotherapies are reviewed. Placebo treatments and placebo controls have been thought to be composed of factors common across psychotherapies by definition. Little empirical support, however, was found for this view. The components of placebo controls are not necessarily common with but are often very different from the components of designated therapies. A review was also conducted of the more obvious as well as the more subtle sources of noncontrolled variables that may differentially enter therapeutic conditions. The types of placebo control groups most often used in the literature are not adequate to control the effects of various so-called nonspecific variables or to test the active elements of psychotherapies. Two types of placebo groups were found to be the most appropriate to control for confounding variables in the study of psychological and active therapeutic components. (PsycINFO Database Record (c) 2010 APA, all rights reserved)