The role of serotonin in aversive motivation and altruistic punishment

Counter to the predictions of influential economic models, humans often act against their own self-interest, incurring personal costs to help co-operators and punish cheaters. One potential mechanism governing the context-dependent variability of altruistic behaviour is neuromodulation by the neurotransmitter serotonin (5-HT), which responds to environmental stressors and shapes activity in brain regions implicated in social decision-making. Decades of research have linked 5-HT to prosocial behaviour across species; however, the specific motivational processes mediating this relationship have not yet been elucidated. I examine the influence of 5-HT on altruistic punishment by considering its function in basic motivational processes. Influential theories have implicated 5-HT in impulsivity, averse processing, and behavioural inhibition. Part I presents two experiments designed to arbitrate between these competing accounts of 5-HT function in aversive predictions, but not overall motor response inhibition or sensitivity to aversive outcomes. Study 2 replicates and extends these findings by showing that 5-HT selectively promotes Pavlovian rather than instrumental, withdrawal reactions to aversive predictions. Study 3 demonstrates that temporarily lowering 5-HT function increases the occurrence of altruistic punishment. Study 4 suggests that the effects of lowering 5-HT on altruistic punishment are partly mediated by concurrent increases in preference for immediate rewards, an effect that may depend on interactions between 5-HT and dopamine. In Study 5, I show that enhancing 5-HT function decreases the occurrence of altruistic punishment specifically through effects on harm aversion. Finally, Study 6 shows that the social context critically moderates the impact of 5-HT depletion on altruistic punishment. Findings provide strong evidence implicating 5-HT in human altruistic behaviour, and begin to clarify the influence of interacting neuromodulatory systems on social decision-making.EThOS - Electronic Theses Online ServiceGBUnited Kingdo