I aim to explicate the pattern of differences and relations between the sexes in two groups of C capucinus, in terms of phylogenetic, social and ecological predictors. I use three lines of evidence to develop predictions as to how male and female C. capucinus interact and how the sexes differ in behavior: (I) phylogenetic similarities to other species of Cebus; (2) a general model of sex differences in ''female-bonded'' social systems; and (3) ecological analogy of Old World monkeys. First, T conclude that phylogenetic affinity is a good predictor in that C. capucinus are similar to other species of Cebus in many patterns of sex-differentiated behavior. An exception is that unlike C. apella and C. olivaceus, in which a single breeding male is reported to be highly conspicuous in each group, C. capucinus live in a decidedly multimale system. Secondly, the general model of sex differences in female philopatric, male-dispersal societies, which was originally developed for Old World species, also accurately predicts several aspects of social behavior in C. capucinus. Thirdly, a proposed ecological analogy between Cercopithecus ascanius and species of Cebus is not substantiated in this study of C. capucinus, though the analogy is apparently well suited to the social dynamics of other species in the genus Cebus.