In this article, the assertion that different psychological therapies are of broadly similar efficacy—often called the ‘Dodo Bird Verdict’—is contrasted with the alternative view that there are specific therapies that are more effective than others for particular diagnoses. We note that, despite thirty years of meta-analytic reviews tending to support the finding of therapy equivalence, this view is still controversial and has not been accepted by many within the psychological therapy community; we explore this from a theory of science perspective. It is further argued that the equivalence of ostensibly different therapies is an inevitable consequence of the methodology that has dominated this field of investigation; namely, randomised controlled trials [RCTs]. The implicit assumptions of RCTs are analysed and it is argued that what we know about psychological therapy indicates that it is not appropriate to treat ‘type of therapy’ and ‘diagnosis’ as if they were independent variables in an experimental design. It is noted that one logical consequence of this is that we would not expect RCTs to be capable of isolating effects that are specific to ‘type of therapy’ and ‘diagnosis’. Rather, RCTs would only be expected to be capable of identifying the non-specific effects of covariates, such as those of therapist allegiance. It is further suggested that those non-specific effects that have been identified via meta-analysis are not trivial findings, but rather characterise important features of psychological therapy. Copyright © 2009 John Wiley & Sons, Ltd.