Decades of research have linked the neurotransmitter serotonin (5-HT) to prosocial behavior. However, the precise mechanisms through which 5-HT influences social interactions remain unclear. A neuroeconomics approach may help clarify these mechanisms. In a recent study, lowering 5-HT levels in healthy volunteers enhanced their reactions to unfair treatment in the ultimatum game. Other studies investigating the neural mechanisms of social decision making have implicated brain regions that are modulated by 5-HT. Here, I review the evidence for the role of 5-HT in modulating social decision making and discuss the implications for understanding how prosocial behavior varies between individuals and across social contexts.