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. Psychotherapy treatment outcome studies have used the doubleblind randomized placebo control design to rule out the effects of various common factors (Goldfried & Wolfe, 1996). This design was originally developed in the United States and the United Kingdom in the 1930s (Gehan & Lemak, 1994; Shapiro & Shapiro, 1997; Wampold, 2001a) for the purpose of holding constant all factors except the medication’s active ingredient. Scientific medicine researchers sought to adapt the concept of randomized clinical trials to establish that the benefits of medications were due to physiochemical properties rather than to patients’ expectations, hopes, or other psychological processes. The placebo pill, used in the medical double-blind randomized placebo control design as the typical way of controlling all factors incidental to the treatment, is designed to be indistinguishable from the active medication—in appearance, taste, and smell. In this design, it is necessary that the patient, the administrator of the treatment, and the evaluator be unaware of the patient’s treatment condition because the design is intended to rule out psychological factors that are incidental to the purported active ingredient. Clearly, for instance, if the patient were aware that he or she was receiving a pill with no active ingredients, the expectation for improvement would be attenuated. As noted by Shapiro and Shapiro (1997): Gold [who developed the design in the United States] advocated a comparison between “an allegedly potent agent and a blank of such physical properties as to render a distinction between the two impossible except through some pharmacologic potency which may exist ... [the recommended] double-blind procedure which calls for an investigation in which neither the patient nor the doctor is aware of the identity of the two agents until the results are in and analyzed. This is imperative to avoid the influence of subconscious bia s... ”(Gold, 1954, p. 724). The statement by Gold culminated twenty years of pioneering study of methods with which to reliably and validly evaluate the effectiveness of new drugs. (p. 148) Shortly after the randomized double-blind placebo control group design was adopted in medicine, Rosenthal and Frank (1956) suggested that the design be used in psychotherapy research to rule out factors that are incidental to ingredients specified by the treatment protocol (i.e., to control for the common factors in therapy):