In the current commentary, we discuss Stoltenberg and colleagues' finding (reported in this issue) that variation in the serotonin transporter gene (5-HTTLPR) is associated with prosocial behavior via effects on anxiety in social situations. We note how their results are consistent with evidence from the psychopharmacological literature and illustrate how a mediational framework can inform understanding of genetic and psychological associations, and we suggest that future studies that manipulate social context could further elucidate the relationship between genes involved in serotonin regulation and prosocial behavior.