Psilocybin and MDMA reduce costly punishment in the Ultimatum Game.
Disruptions in social decision-making are becoming evident in many psychiatric conditions. These are studied using paradigms investigating the psychological mechanisms underlying interpersonal interactions, such as the Ultimatum Game (UG). Rejection behaviour in the UG represents altruistic punishment - the costly punishment of norm violators - but the mechanisms underlying it require clarification. To investigate the psychopharmacology of UG behaviour, we carried out two studies with healthy participants, employing serotonergic agonists psilocybin (open-label, within-participant design, N 19) and 3,4-methylenedioxymethamphetamine (MDMA placebo-controlled, double-blind, crossover design, N 20). We found that both MDMA and psilocybin reduced rejection of unfair offers (odds ratio 0.57 and 0.42, respectively). The reduction in rejection rate following MDMA was associated with increased prosociality (R