Serotonin modulates striatal responses to fairness and retaliation in humans

Humans are willing to incur personal costs to punish others who violate social norms. Such "costly punishment" is an important force for sustaining human cooperation, but the causal neurobiological determinants of punishment decisions remain unclear. Using a combination of behavioral, pharmacological, and neuroimaging techniques, we show that manipulating the serotonin system in humans alters costly punishment decisions by modulating responses to fairness and retaliation in the striatum. Following dietary depletion of the serotonin precursor tryptophan, participants were more likely to punish those who treated them unfairly, and were slower to accept fair exchanges. Neuroimaging data revealed activations in the ventral and dorsal striatum that were associated with fairness and punishment, respectively. Depletion simultaneously reduced ventral striatal responses to fairness and increased dorsal striatal responses during punishment, an effect that predicted its influence on punishment behavior. Finally, we provide behavioral evidence that serotonin modulates specific retaliation, rather than general norm enforcement: depleted participants were more likely to punish unfair behavior directed toward themselves, but not unfair behavior directed toward others. Our findings demonstrate that serotonin modulates social value processing in the striatum, producing context-dependent effects on social behavior.