A person's behavior during social interaction is due not only to the person's dispositional characteristics but is also determined by his or her social partner. If a person consistently elicits the same behavior from others, the person has apartner effect. Partner effects in affect, cognition, and behavior are examined. Partner effects in behavior are presumed to be caused by partner effects in affect and cognition. A social relations analysis of data from six studies which meet rather strict methodological criteria suggests that partner effects are present in affect and cognition but are fairly weak. Further, partner effects are hardly evident at all on behavioral observations of friendliness and dominance, but are more evident in nonverbal behavior, especially gaze. The low level of partner effects is hypothesized to be due to the context in which the research was done, the degree of acquaintance between interaction partners, and the moderating effect of individual differences.