To investigate interpersonal theory, the authors examined the social behavior of 112 previously unacquainted male-female pairs collaborating on a joint task. Structural equation modeling analyses provided clear evidence of complementarity: In addition to trait affiliation and dominance both predicting situational behavior, each interaction partner's behavior influenced the other's in accordance with the theory, eliciting similar levels of affiliation and opposite levels of dominance. There were also clear biases related to participants' interpersonal traits: When behavior, as indexed by observers, was held constant, more trait affiliative participants tended to view themselves and their partner as warmer during the interaction; likewise, more trait dominant participants perceived themselves to act more dominantly than they actually did. Throughout, there were no sex differences.