Substantial and highly variable placebo response rates represent a major obstacle to antidepressant development in major depressive disorder (MDD). However, whether the likelihood of receiving active treatment or placebo, a proxy of the degree of expectation of improvement, may itself influence clinical trial outcome is unclear. The goal of this work was to examine whether the probability of receiving placebo influences clinical trial outcome antidepressant MDD trials. Medline/Pubmed publication databases were searched for randomized, double-blind, placebo-controlled trials of antidepressants for adults with MDD. 146 manuscripts involving 182 clinical trials were pooled (n = 36,385). Pooled response rates for drug and placebo were 53.8% and 37.3%. A meta-regression (random-effects) established that the probability of receiving placebo, year of publication, and baseline severity were independent predictors of the risk ratio of responding to antidepressants versus placebo. Specifically, a greater probability of receiving placebo, greater baseline severity and an earlier year of publication predicted greater antidepressant-placebo "efficacy separation". Fixed versus flexible dose design, trial duration and population age did not influence clinical trial outcome.