A meta-analysis 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 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.