Common Factors Model

The Dodo Bird

Seventy years ago, Saul Rosenzweig introduced the concept of common factors in what has become a classic article, noting that apparently diverse forms of psychotherapy appear to be similarly successful. Attempting to capture the peculiarity of this phenomenon, Rosenzweig invoked Lewis Carroll's Alice in Wonderland, noting the dodo bird's proclamation that “Everyone has won, and all must have prizes.” The finding that most psychotherapies tend to have similar rates of efficacy has subsequently been labeled the dodo bird conjecture.

Ironically, the empirical basis of common factors research was spurred by Hans J. Eysenck's controversial outcome study, in which he found minimal, custodial treatment to be more effective than psychotherapy. In response to Eysneck's work, researchers set about rigorously demonstrating the beneficial effects of psychotherapy. Mary L. Smith and Gene V. Glass used meta-analysis, a statistical method that allows for the findings of many research studies to be combined, to establish the therapeutic effect of psychotherapy. They combined the results of more than 375 studies and provided compelling evidence in support of the efficacy of psychotherapy. Subsequent meta-analyses have confirmed the efficacy of psychotherapy over no treatment. Most striking, however, is that meta-analytic findings are consistent with Rosenzweig's original proposition that different psychotherapies have approximately similar effects. In the most extensive meta-analysis of psychotherapy treatment studies to date, Bruce E. Wampold and colleagues found no evidence of differential treatment effects.

This finding leads to a paradox. Treatments with different theorized mechanisms of change yield quite similar results. Thus, psychotherapies could be like radiation and chemotherapy, completely different, yet similarly effective treatments for the same illness.

Alternatively, they could be quite similar in their active ingredients, like two aspirin-containing pain relievers marketed under different brand names. Common factors researchers investigate hypotheses related to the latter.

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