What you don't know can help you: An activating placebo effect in spider phobia.

Despite being an inert treatment, placebo has been repeatedly shown to induce the experience of automatic symptom relief, a therapeutic effect over which a person has no control. We tested whether a placebo that participants believed was an active drug would induce them to take action to overcome their symptomatic impairment, a self-efficacious role we term an activating placebo effect. Specifically, we tested whether a placebo presented to spider-phobic participants as a fear-reducing drug would induce them to approach a live tarantula. Sixty spider-phobic participants, identified by a fear questionnaire and assessing their approach behavior toward a live tarantula, were randomized to take a placebo, presented either as propranolol or a placebo, or to a no-treatment control group. Participants who believed the placebo was propranolol increased in their willingness to approach the tarantula, and actually moved physically closer to it, relative to the other two groups. They did so despite experiencing higher levels of fear, and subsequently improved in their self-efficacy beliefs about tolerating fear when encountering a spider. Changes in willingness to approach the tarantula mediated changes in approach behavior, which in turn mediated changes in self-efficacy. These results represent the first explicit demonstration of an activating placebo effect.