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 doubleThe quality and nature of social interactions are defining features of both mental health and psychiatric disorders, but the tools used to study social interaction in psychopharmacological research generally examine global functioning rather than specific aspects of social interaction. Social science research in this area has advanced greatly over the past decade, and it is possible to conceptualize major components of social interaction and how these components are interrelated. While details vary, typically interpersonal behaviors can be organized in a circle defined by two major axes (Carson 1969;Foa 1961;Kiesler 1983;Leary 1957;Wiggins 1995;Wiggins and Broughton 1985). One axis encompasses dominant and submissive behaviors. This axis has been called by various names including agency, power, and status. Dominant behaviors include setting goals for others or telling others to do something while submissive behaviors include not voicing an opinion or waiting for others to act (Moskowitz 1994). The second axis encompasses agreeable and quarrelsome behaviors. Sample agreeable behaviors include listening attentively to others and expressing reassurance while quarrelsome behaviors include showing impatience and making sarcastic comments (Moskowitz 1994). Among many labels, this axis has been referred to as communion or affiliation.The two axes of human social interaction can be studied using an event sampling method. This method assesses individuals' social behaviors and affect in individuals' natural environments rather than in the laboratory. The individual completes a brief questionnaire after each significant social interaction, throughout the 25 , NO . 2 day. Abundant evidence has accumulated demonstrating the reliability and validity of methods for sampling interpersonal behavior and affect in everyday life (Csikszentmihalyi and Larson 1987;Diener and Emmons 1984;McAdams and Constantian 1983;Moskowitz 1994). While behavior along the axes varies greatly from one social interaction to the next, aggregation of behaviors in events across several days provides measures with substantial temporal stability (Brown and Moskowitz 1998). The event-contingent recording procedure (Wheeler and Reis 1991) used in the present study has several advantages. Interpersonal situations are sampled throughout the day, providing reports of behavior and affect at home and at work. Reports of behavior and affect are recorded close in time to their occurrence, so retrospective biases are minimized. It is possible to construct behavior and affect scores for time periods of specified lengths, such as morning, afternoon, and evening, or a day, or a week. Previous research using this method has demonstrated the concurrent relation between interper...