The subtractive expectancy placebo procedure: A measure of non-specific factors in behavioural interventions

Placebo procedures currently used in clinical psychology and health psychology are criticized on methodological and ethical grounds. The approach of adding expectation of therapeutic efficacy to an actually inert intervention (Additive Expectancy) is of dubious validity unless the inertness of the ‘treatment’ can be established, which is difficult if not impossible to do with behavioural methods. Withholding treatment from clients is a serious ethical problem. The Subtractive Expectancy Procedure, described in this paper, administers a known active treatment but leads the client to expect no therapeutic effect. This avoids both of the serious difficulties mentioned above, although problems of credibility and deception need to be dealt with in both procedures.