A heterosexual group of nonreproductive rhesus (Macaca mulatto) containing vasectomized males was compared with a matched control group containing intact males. Comparisons were made on data collected before the birth of the first infant in the control group. Three Darwinian a priori hypotheses were used to predict differences between groups. The first hypothesis correctly predicted more affiliative and sexual behavior among experimental heterosexual dyads. The results did not support the second hypothesis that predicted less affiliation between experimental males. The third hypothesis correctly predicted that heterosexual affiliation and sexual behavior would occur between more of the possible heterosexual dyads in the experimental group. Two-tailed tests showed the females in the reproductive group engaged in significantly less intrasexual affiliation. The results suggest failure to reproduce has a causal influence on the affiliative and sexual interaction patterns of rhesus macaques.Studies of differences between contracepting and noncontracepting groups of nonhuman primates may help us to understand differences between our social and sexual behavior and that of our recent ancestors. The effects of contraception on humans may be so great that studies of natural groups of nonhuman primates may not shed much light on the behavior people in developed nations (Rasmussen,