Lessebo effect
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The lessebo effect is a phenomenon in psychology and medicine in which a person in a blinded clinical trial knows that they might receive placebo due to the existence of a placebo control group in the trial and this results in the person experiencing diminished placebo effects (positive expectations) and therapeutic improvement.[1][2] It has been described in several contexts including clinical trials of treatment for depression,[3][4][5] schizophrenia,[6] Parkinson's disease,[7][8][9] and rheumatoid arthritis.[10][1] The phenomenon was first named the "lessebo effect" by Mark Sinyor and colleagues in 2010.[1][2] They showed in a meta-analysis that antidepressant and placebo response rates are influenced by the presence of a placebo arm and by the number of treatment arms (and thus likelihood of receiving placebo) in trials.[2] A closely related but slightly distinct concept is the inverse placebo effect,[11][12][13] which the lessebo effect has sometimes been inappropriately confused and conflated with.[14][15]