Weighing the Evidence for Psychotherapy Equivalence: Implications for Research and Practice.

In the past two decades, numerous meta-analyses have been published that examine the question of psychotherapy equivalence. Hunsley and Di Giulio (2002) critically reviewed this literature and concluded that there was abundant evidence that the Dodo bird verdict of equivalence across psychotherapies is false. In this article, we summarize and update Hunsley and Di Giulio's (2002) review of recent meta-analyses and comparative treatment studies relevant to this question. Taken together, the empirical evidence clearly indicates that psychotherapy nonequivalence is the rule, not the exception. We discuss these findings and their implications for psychological research and practice. Keywords: psychotherapy equivalence, Dodo bird, comparative treatment studies, evidence-based treatments, effectiveness research ********** Since Rosenzweig (1936) asserted that all psychotherapy produced equivalent outcomes (and quoted the Dodo bird from Alice in Wonderland saying, "Everyone has won, and all must have prizes"), psychotherapy equivalence has been referred to as the Dodo bird verdict, and frequent claims have been made about the general equivalence of all forms of psychotherapy. Proponents of this perspective have argued that psychotherapy, in general, is effective and that there is no compelling evidence to suggest that some treatments are better than others for clinical problems (e.g., Bohart, O'Hara, & Leitner, 1998; Zinbarg, 2000). Accordingly, the various theoretical orientations are merely variations on a single theme and, although their distinctions may be important to clinicians and psychotherapy researchers, they are essentially meaningless with respect to actual treatment outcome. Claiming all psychotherapies are equivalent is like suggesting that, for example, because applied behavioral analysis is useful for treating autistic disorder, any treatment provided for this disorder, be it thought field therapy or play therapy, is likely to be equally effective. Indeed, Luborsky et al. (2003) recently suggested that psychoanalysis, despite a lack of empirical comparisons with other treatments, may plausibly be assumed to be equivalent to other efficacious psychotherapies in light of the typical research finding of psychotherapy equivalence. Given the ubiquity of the claims for psychotherapy equivalence and the limited attention typically given to the actual research purporting to support the claim, there is the real possibility that practitioners and students in mental health fields accept the Dodo bird verdict simply because it appears to be generally and uncritically accepted by others. In the past two decades, numerous meta-analyses have been published that examine this question of psychotherapy equivalence. Hunsley and Di Giulio (2002) critically reviewed this literature and concluded that there was abundant evidence that the Dodo bird verdict is false. In this article, we begin by summarizing Hunsley and Di Giulio's (2002) review and then provide an updated review of relevant meta-analyses and comparative treatment studies published since Hunsley and Di Guilio's review. We consider evidence from (a) treatment outcome studies that compare the treated group with a control group to whom no services are provided (typically, a wait-list control group) and (b) comparative treatment studies that compare at least two active treatments (with a no-treatment control group sometimes, but not always, included). Clearly, comparative treatment studies are most relevant to the Dodo bird verdict as they provide a "head-to-head" comparison of treatments drawing on the same sample of clients randomly assigned to each condition. For the most part, the results we present use the d statistic for estimating the effect size of treatments (i.e., the difference between treatments or between treatment versus no treatment is expressed in standard deviation units); in some instances, when useful for interpretative purposes, we also provide information on other types of effect sizes. …