The Role of Motivation and Expectancy in the Placebo Effect

Abstract

Placebo has been found to be an important component of treatments including psychological and pharmacological treatment of depression, transplant surgery for Parkinson’s, acupuncture, smoking cessation interventions, and analgesic treatment of pain. Although the placebo effect has been observed across a wide range of disciplines, the effect sizes vary widely and it is not well understood how placebo effects are produced. The current study draws upon research in perception and motivation to propose a more comprehensive model of the placebo effect. Specifically, the model proposes that more motivated persons pay greater attention to bodily sensations and other stimuli, which are then interpreted according to expectations, producing a placebo response. In the current study, both motivation and outcome expectancy were manipulated, creating a 2x2 study design. College students (N=152) were asked to evaluate a series of placebo pheromone substances (slightly scented water) and attention/task diligence was assessed as the amount of time spent on the rating task and the number of evaluations made. The placebo response was assessed as the attractiveness rating of the chosen sample and the variability in ratings, with greater variability and higher attractiveness ratings indicating greater placebo response. It was predicted that those in the high motivation group would have greater diligence on the rating task, which would, in turn, lead to greater placebo response. It was further predicted that there would be a main effect for expectancy on placebo response. Consistent with hypothesized effects, more highly motivated students had greater placebo responses, and the relationship was mediated by task diligence. Thus, as students spent greater time on the evaluation task, they found the scent of their chosen sample to be more pleasing and perceived greater differences among samples. No effect was found for expectancy. These findings are important because they suggest possible mechanisms for maximizing treatment effects in medical and psychological settings, where factors such as nonspecific treatment effects and placebo are believed to influence outcomes. Future research should seek to further clarify the relationship of expectancy and motivation to placebo outcomes by examining mediating factors such as attention and carefully manipulating both variables to ensure maximum effects.