Psilocybin and MDMA reduce costly punishment in the Ultimatum Game
In two studies with healthy volunteers, the serotonergic agonists psilocybin and MDMA reduced costly punishment in the Ultimatum Game by lowering rejection of unfair offers; MDMA’s effect correlated with increased prosociality and also increased offers to others, though it did not reduce third‑party rejection.
Authors
- Robin Carhart-Harris
- David Nutt
- Mitul Mehta
Published
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’sd = 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.
Research Summary of 'Psilocybin and MDMA reduce costly punishment in the Ultimatum Game'
Introduction
Human social decision-making can be disrupted across psychiatric disorders and is often studied with paradigms that probe interpersonal mechanisms such as trust, norm enforcement and cooperation. The Ultimatum Game (UG) is a widely used social decision-making task in which a responder may accept or reject monetary splits proposed by another player; rejection of low, unequal offers is commonly interpreted as altruistic or costly punishment. Prior psychopharmacological work implicates the serotonin (5-HT) system in UG responder behaviour: lowering 5-HT via acute tryptophan depletion increased rejection of moderately unfair offers, while increasing 5-HT with SSRIs reduced such rejections. Psilocybin and MDMA both have prominent serotonergic actions and have shown promise in treating mood and trauma-related disorders, but their effects on high-level social decision-making have not been fully characterised. Gabay and colleagues report two experimental studies that test whether psilocybin and MDMA alter UG behaviour. They hypothesised that both compounds would reduce rejection rates of unfair offers in a first-person (FP) responder condition, that MDMA would not change rejection in a third-party (TP) condition (where equality considerations rather than direct personal harm dominate), and that MDMA would increase offers made when participants acted as proposers. Both studies included a non-social control (random-generated offers) and used repeated-measures designs to probe these effects.
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Study Details
- Study Typeindividual
- Journal
- Compounds
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- APA Citation
Gabay, A. S., Carhart-Harris, R. L., Mazibuko, N., Kempton, M. J., Morrison, P. D., Nutt, D. J., & Mehta, M. A. (2018). Psilocybin and MDMA reduce costly punishment in the Ultimatum Game. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-26656-2
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Carhart-Harris, R. L., Nutt, D. J. · Journal of Psychopharmacology (2017)
Studerus, E., Gamma, A., Kometer, M. et al. · PLOS ONE (2012)
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Ramaekers, J. G., Kuypers, K. P. C., Vollenweider, F. X. · Molecular Psychiatry (2026)
Sarmanlu, M., Kuypers, K. P. C., Vizeli, P. et al. · Progress in Neuro-Psychopharmacology and Biological Psychiatry (2023)
Balaet, M. · Frontiers in Neuroscience (2022)
Markopoulos, A., Inserra, A., De Gregorio, D. et al. · Frontiers in Pharmacology (2022)
Rucker, J. J., Marwood, L., Ajantaival, R. L. J. et al. · Journal of Psychopharmacology (2022)
Rodríguez Arce, J. M., Winkelman, M. J. · Frontiers in Psychology (2021)
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Preller, K. H., Vollenweider, F. X. · Frontiers in Psychiatry (2019)
Carhart-Harris, R. L., Friston, K. J. · Pharmacological Reviews (2019)
Dipasquale, O., Selvaggi, P., Veronese, M. et al. · NeuroImage (2019)
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