Despite a large body of research in non-human primates, the relationship between naturalistic patterns of social behaviour and basal cortisol levels has been understudied in humans. The present study examined the relationship between patterns of interpersonal functioning and cortisol levels in 23 offspring of parents with bipolar disorder (BD), at high risk for the development of an affective disorder, and 22 offspring of parents with no affective disorder (controls) in late adolescence and young adulthood. Using event-contingent recording, participants rated their dominance, submissiveness, quarrelsomeness, and agreeableness in naturally occurring social interactions over 14 consecutive days and provided salivary cortisol twice daily in the afternoon over the same period. In the full sample, multilevel modelling analyses revealed that dominance was a significant positive predictor of afternoon basal cortisol levels, t(35) = 2.58, p < 0.05. Moreover, risk group (having a parent with BD or parents with no affective disorder) significantly interacted with mean levels of quarrelsomeness to predict afternoon cortisol levels, t(29) = 2.06, p < 0.05. Offspring of parents with BD who reported more frequent quarrelsome behaviours exhibited lower levels of afternoon cortisol relative to high-risk offspring reporting few quarrelsome behaviours and control offspring. The results are consistent with evidence that dominance is associated with high cortisol levels in an unstable environment, and suggest that quarrelsomeness among high risk youth contributes to altered hypothalamic–pituitary–adrenal activity.