A METAANALYSIS OF OUTCOME STUDIES COMPARING BONA-FIDE PSYCHOTHERAPIES- EMPIRICALLY, ALL MUST HAVE PRIZES

This meta-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 bet ween treatments culled from 6 journals, it was found that the effect s izes were homogeneously distributed about 0, as was expected under the Dodo bird conjecture, and that under the most liberal assumptions, th e upper bound of the true effect was about .20. Moreover, the effect s izes (a) were not related positively to publication date, indicating t hat improving research methods were not detecting effects, and (b) wer e not related to the similarity of the treatments, indicating that mor e dissimilar treatments did not produce larger effects, as would be ex pected if the Dodo bird conjecture was false. The evidence from these analyses supports the conjecture that the efficacy of bona fide treatm ents are roughly equivalent.