In monkeys increasing serotonin function enhances affiliative interactions and promotes the acquisition of dominance. To examine whether similar effects occur in humans, we treated 98 subjects for 12 days with the serotonin precursor tryptophan (1g TID) and for 12 days with placebo in a double-blind, cross over study. Agreeableness/quarrelsomeness and dominance/submission were measured using an event-contingent method, in which subjects reported on various behaviors during important social interactions throughout their day. Tryptophan decreased quarrelsome behavior, but only when placebo was given first, suggesting that a decrease in quarrelsomeness when tryptophan was given first may have carried over into the subsequent placebo period. Tryptophan increased dominant behavior, an effect that was independent of the order of treatment, the broad social context, and the subject's and partner's sex. Our results suggest that serotonin may enhance dominance in humans, as in monkeys, and illustrate the advantages of the event contingent methodology in studying the associations between biology and human social interaction.