Is the Dodo bird endangered in the 21st century? A meta-analysis of treatment comparison studies

The Dodo bird hypothesis asserts that when bona fide treatments are compared they yield similar outcomes and this hypothesis is consistent with a common factors or contextual model of psychotherapy. Wampold et al. (1997), the most recent comprehensive meta-analysis to test the Dodo bird hypothesis, yielded consistent evidence of treatment equivalence. However, some of Wampold et al.'s analytic strategies, such as using multiple effect sizes from the same study and prioritizing long-term follow-up, may have obscured treatment differences. The current meta-analysis updated Wampold et al. by analyzing studies published in the subsequent 16 years (k = 51). Separate effect sizes were calculated for primary outcomes versus secondary outcomes, at termination and follow-up. Contrary to the Dodo bird hypothesis, there was evidence of treatment differences for primary outcomes at termination. Furthermore, cognitive-behavioral treatments may be incrementally more effective than alternative treatments for primary outcomes. Consistent with the Dodo bird hypothesis, there was little evidence of treatment differences for the secondary outcomes at termination and follow-up. There are small, statistically significant differences between bona-fide treatments when the specific targets of those treatments are assessed, but not when secondary outcomes are assessed, providing mixed support for both specific factors and contextual models of psychotherapy.