1. ¹University Department of Psychiatry, University of Oxford, Oxford, UK
2. ²Departments of Anthropology, Psychiatry and Behavioral Sciences, Emory University, Atlanta, GA, USA
3. ³Department of Psychiatry, Yale University, New Haven, CT, USA

Correspondence: Dr RD Rogers, Department of Psychiatry, University of Oxford, Warneford Hospital, Oxford, Oxfordshire OX3 7JX, UK. Tel: +44 1865 226 399; Fax: +44 1865 793 101; E-mail: robert.rogers@psych.ox.ac.uk

Received 23 December 2004; Revised 21 July 2005; Accepted 22 August 2005; Published online 11 January 2006.

Abstract

Adaptive social behavior often necessitates choosing to cooperate with others for long-term gains at the expense of noncooperative behaviors giving larger immediate gains. Although little is know about the neural substrates that support cooperative over noncooperative behaviors, recent research has shown that mutually cooperative behavior in the context of a mixed-motive game, the Prisoner's Dilemma (PD), is associated with increased neural activity within reinforcement circuitry. Other research attests to a role for serotonin in the modulation of social behavior and in reward processing. In this study, we used a within-subject, crossover, double-blind design to investigate performance of an iterated, sequential PD game for monetary reward by healthy human adult participants following ingestion of an amino-acid drink that either did (T+) or did not (T-) contain l-tryptophan. Tryptophan depletion produced significant reductions in the level of cooperation shown by participants when playing the game on the first, but not the second, study days. This effect was accompanied by a significantly diminished probability of cooperative responding given previous mutually cooperative behavior. These data suggest that serotonin plays a significant role in the acquisition of socially cooperative behavior in human adult participants, and suggest novel hypotheses concerning the serotonergic modulation of reward information in socially cooperative behavior in both health and psychiatric illness.