Tryptophan, serotonin and human social behavior.

Animals research suggests that increasing serotonin can decrease aggression, increase affiliative behaviors and increase dominant behaviors. We tested the relevance of these data to humans by giving 100 healthy people tryptophan (1 g after each meal) and placebo, each for 12 days in a double-blind cross-over study. Social behaviors were studied using an event sampling method in which subjects filled in a one page questionnaire about their behaviors after each social interaction lasting at least 5 minutes. Tryptophan caused a significant decrease in quarrelsome behaviors and a significant increase in dominant behaviors.