Although men and women both have incentives to aggress, women's use of aggression is consistently lower than men's except within intimate partnerships. The authors propose that women's aggression is best understood by considering the role of fear as an adaptive mechanism which reduces exposure to physical danger. The authors review evidence that men and women faced qualitatively different adaptive challenges over evolutionary time and that this resulted in a sex difference in direct aggression mediated by greater female fear. The authors suggest that the absence of a sex difference in intimate partner aggression results partly from a reduction in female fear mediated by oxytocin, which reduces stress responses to biologically necessary encroachments on women's bodies. The authors suggest that a more complete understanding of women's aggression requires: acknowledging that women's relative restraint with regard to aggression is itself an adaptation; researching in more depth the fear-reducing effects of oxytocin and how these might operate in intimate partnerships; and considering more fully how cultural and biological factors might interact. (Published Abstract)