Sex differences in aggression can be traced ultimately to sex differences in parental investment. Higher variance in reproductive success in men, resulting from lower parental investment, creates incentives for competition to achieve intrasexual dominance, while women's greater investment and role in caring for offspring creates costs for dangerous confrontations. Data suggest that, at a proximal psychological level, sex differences in fear, but not anger, mediate involvement in aggression. Although biparental care brings with it two-way sexual selection, female competition is chiefly conducted either intersexually (through the display of qualities attractive to men) or through indirect aggression (gossip and social exclusion), both of which are low-risk strategies. Under resource scarcity, competition between women can escalate to direct confrontation, but even then the severity of aggression is lower than that of men. Women and men are equal in the frequency of aggressive acts directed at intimate partners, which poses an explanatory problem for any theory of sex differences in aggression. We propose that the fear reduction in women necessary to permit sexual intimacy, possibly mediated by oxytocin, also diminishes women's normal restraint on aggressive behavior.