Ginastera au bout de l'archet

Both MDMA and psilocybin reduce the rejection of unfair offers in the Ultimatum Game, a measure of costly punishment of norm violators. In two studies with healthy participants, psilocybin (open-label, within-participant, N=19) and MDMA (placebo-controlled, double-blind, crossover, N=20) lowered rejection rates (odds ratios 0.42 and 0.57, respectively). For MDMA, the reduction correlated with increased prosociality (R²=0.26). MDMA did not affect third-party rejection but increased amounts offered to others (Cohen's d=0.82). The findings suggest these compounds shift the concept of social reward toward direct relationship value, informing their use in drug-assisted psychotherapy.

Study at a glance

Abstract

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 (R2 = 0.26, p = 0.025). In the MDMA study, we investigated third-party decision-making and proposer behaviour. MDMA did not reduce rejection in the third-party condition, but produced an increase in the amount offered to others (Cohen's d = 0.82). We argue that these compounds altered participants' conceptualisation of 'social reward', placing more emphasis on the direct relationship with interacting partners. With these compounds showing efficacy in drug-assisted psychotherapy, these studies are an important step in the further characterisation of their psychological effects.