Further evidence for individual differences in placebo responding: An interactionist perspective

Objective: A prior investigation found that individuals low in optimism are more likely to follow a negative placebo (nocebo) expectation. The present study tested the hypothesis that individuals high in optimism are more likely to follow a positive placebo expectation. Methods: Individuals (N = 56) varying in their level of optimism were randomly assigned to one of three conditions. In the first condition, participants were given the expectation that a placebo sleep treatment would improve their sleep quality (placebo expectation condition). In the second condition, participants engaged in the same sleep treatment activity but were not given the positive placebo expectation (treatment control condition). Finally, a third group did not receive the positive placebo expectation and also did not engage in the placebo sleep treatment (no-placebo control condition). Results: Optimism was positively associated with better sleep quality in the placebo expectation condition (r = .48, P < .05). Optimism scores were not associated with better sleep quality in either the treatment control condition (r = -.17, P = .46) or the no-placebo control condition (r = -.24, P = 35). Conclusion: Dispositional optimism relates to placebo responding. This relationship, however, is not manifested in a simple increase or decrease in all types of placebo responding. Rather, it appears that, as optimism increases, response to the positive placebo expectation increases, whereas response to nocebo expectation decreases. It is recommended that future research on personality and placebo effects consider the interaction between situational and dispositional variables. (C) 2007 Elsevier Inc. All rights reserved.