Comparative credibility of treatment rationales: Three tests of expectancy theory

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Abstract
Previous research has challenged the adequacy of conventional placebo controls, by demonstrating that active treatments, such as systematic desensitization, are more credible than supposedly inert control conditions. Such findings give support to the hypothesis that the arousal of positive expectancies is the key to therapeutic change. Three experiments are reported, in which Borkovec''s credibility rating method was applied to treatment comparisons modeled upon the findings of outcome research. In the 1st experiment, systematic desensitization was more credible than rational-emotive and client-centered therapy (P < 0.001), consistent with an expectancy-arousal explanation of Di Loreto''s comparative outcome study. In the 2nd experiment, Paul''s attention placebo condition was less credible than 3 active treatments (P < 0.001). In the 3rd experiment, in vivo desensitization was in some respects more credible than imaginal desensitization, which was in turn more credible in some respects than relaxation alone (P < 0.001). The results of the latter 2 experiments were broadly consistent with an expectancy-arousal interpretation of much outcome literature. These results emphasize the insufficiency of merely including a designated placebo control condition in outcome research. Credibility differences did not mirror precisely the results of the outcome research on which these studies were modeled. Methodological limitations are discussed, and further clinical research is advocated, in which the persuasiveness of treatment rationales is studied in its own right as a contribution to treatment efficacy.

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