Expectancy Effects, Failure of Blinding Integrity, and Placebo Response in Trials of Treatments for Psychiatric Disorders: A Narrative Review.
JAMA psychiatry May 1, 2025 Nathan T M Huneke, Guilherme Fusetto Veronesi, Matthew Garner et al. 50 citations
Expectancy effects—participants' beliefs about treatment—can bias the results of psychiatric randomized clinical trials by compromising blinding integrity and inflating effect sizes. This narrative review is the first to examine the interplay between expectancy, unblinding, and treatment outcomes in such trials. Evidence from experimental and clinical studies shows that expectation shapes placebo and active treatment responses. Meta-analytic data from psychedelic and anxiety disorder research indicate that unblinding due to perceived efficacy or side effects can alter effect sizes. The authors recommend collecting expectancy data and monitoring blinding integrity, and they propose developing objective outcome measures less susceptible to expectancy effects to improve trial reliability.