A meta-analysis of outcome studies comparing bona fide psychotherapies: Empiricially, "all must have prizes."

This recta-analysis tested the Dodo bird conjecture, which states that when psychotherapies intended to be therapeutic are compared, the true differences among all such treatments are 0. Based on comparisons between treatments culled from 6 journals, it was found that the effect sizes were homogeneously distributed about 0, as was expected under the Dodo bird conjecture, and that under the most liberal assumptions, the upper bound of the true effect was about .20. Moreover, the effect sizes (a) were not related positively to publication date, indicating that improving research methods were not detecting effects, and (b) were not related to the similarity of the treatments, indicating that more dissimilar treatments did not produce larger effects, as would be expected if the Dodo bird conjecture was false. The evidence from these analyses supports the conjecture that the efficacy of bona fide treatments are roughly equivalent. In 1936, Rosenzweig proposed that common factors were responsible for the efficacy of psychotherapy and used the conclusion of the Dodo bird from Alice in Wonderland (Carroll, 1865/1962) to emphasize this point: At last the Dodo said, 'Everybody has won, and all must have prizes' (p. 412). Later, Luborsky, Singer, and Luborsky ( 1975 ) reviewed the psychotherapy outcome literature, found that the psychotherapies reviewed were generally equivalent in terms of their outcomes, and decreed that the Dodo bird was correct. Since Luborsky et al.'s seminal review, the equivalence of outcome in psychotherapy has been called the Dodo bird effect. To many interested in the technical aspects of particular psychotherapies, the Dodo bird effect was distasteful and, on the face of it, unbelievable: If the indiscriminate distribution of prizes argument carried true conviction . . . we end up with the same advice for everyoneRegardless of the nature of your problem seek any form of psychotherapy. This is absurd. We doubt whether even the strongest advocates of the Dodo bird argument dispense this advice. (Rachman & Wilson, 1980, p. 167)