The Effects of Positive and Negative Placebos on Human Memory Performances

Two experiments investigated the possible effects of memory-enhancing and memory-impairing placebo capsules (which participants believed to contain active drugs) on participants' performance in a delayed free recall task. In both experiments participants were randomly assigned to either control, positive, or negative placebo conditions, and their memory performance was tested prior to (baseline trial) and after (test trial) the administration of the placebo. Different patterns of results emerged for positive and negative placebos for actual memory performance measures. Whereas negative placebo produces standard placebo effects by impairing both free recall and accuracy scores on test trial, positive placebo does not affect either of these measures (null placebo effect). On the other hand, both positive and negative placebos produce standard placebo effects with respect to participants' self-reports of perceived changes in memory performance: those in the positive placebo group tend to report that the “drug” improved their performance, and those in the negative group tend to report that it impaired it.