Establishing specificity in psychotherapy: a meta-analysis of structural equivalence of placebo controls.

Placebo treatments in psychotherapy cannot adequately control for all common factors, which thereby attenuates their effects vis-a-vis active treatments. In this study, the authors used meta-analytic procedures to test one possible factor contributing to the attenuation of effects: structural inequalities between placebo and active treatments. Structural aspects of the placebo included number and duration of sessions, training of therapist, format of therapy, and restriction of topics. Results indicate that comparisons between active treatments and structurally inequivalent placebos produced larger effects than comparisons between active treatments and structurally equivalent placebos: moreover, the latter comparison produced negligible effects, indicating that active treatments were not demonstrably superior to well-designed placebos.