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Background Although placebo-control clinical trials that withhold effective treatments can be permissible, how best to inform participants of the placebo design has received little attention. Aims To determine the effect of disclosing quantitative outcome estimates of individual treatment v. entering placebo-control randomised control trial (RCT) on willingness to enrol in such an RCT. Method We randomised 278 adult patients at a depression clinic to receive standard disclosure (n = 129) or enhanced (n = 149) quantitative outcome estimates (based on decision analysis) of individual treatment v. RCT, and assessed their willingness to enrol in the RCT. Results A greater proportion of those in the standard arm preferred enrolling in RCT (41.3% v. 23.8%, P = 0.002). Those in the standard arm preferred RCT more for direct benefit than altruism reasons, whereas the opposite was true in the enhanced arm. Conclusions Disclosing the quantitative outcome implications of placebos may select for fewer but more altruistic participants.