Toward Understanding the Placebo Effect: Investigating a Possible Retrocausal Factor

Objective: Conventional models of placebo effects assume that all mind–body responses associated with expectation can be explained by ordinary causal processes. This experiment tested whether some placebo effects may also involve retrocausal, or time-reversed, influences. Design: Slow cortical potentials in the brain were monitored while adult volunteers anticipated either a flash of light or no flash, selected with equal probability by a noise-based random number generator. Data were collected in individual sessions of 100 trials, contributed by 13 female and 7 male adult participants. Outcome measures: Ensemble median slow cortical potentials 1 second prior to a light flash were compared with the same measures prior to no flash. A nonparametric randomized permutation technique was used to statistically assess the observed difference. Electroencephalographic data were analyzed separately by gender. Results: Females' slow cortical potentials significantly differentiated before stimulus onset (z  2.72, p  0.007, two-tailed); males showed a suggestive effect in the opposite direction (z  1.64, p  0.10, two-tailed). Examination of alternative explanations indicated that the significant effect in females was not caused by anticipatory strategies, equipment or environmental artifacts, or violation of statistical assumptions. Conclusions: This experiment, in accordance with previous studies showing similar, unconscious "presentiment" effects in humans, suggests that comprehensive models seeking to explain placebo effects, and in general how expectation affects the mind and body, may require consideration of retrocausal influences.