Sex differences in aggression: what does evolutionary theory predict?

The target article claims that evolutionary theory predicts the emergence of sex differences in aggression in early childhood, and that there will be no sex difference in anger. It also finds an absence of sex differences in spousal abuse in Western societies. All three are puzzling from an evolutionary perspective and warrant further discussion.

Commentary/Archer: Sex differences in aggression Sex differences in aggression: What does evolutionary theory predict? doi:10.1017/S0140525X0999031S Elizabeth Cashdan Department of Anthropology, University of Utah, Salt Lake City, UT 84112-0060. [e-mail] http://vmww.anthro.utah.edu/peopIe/facuIty/eIizabeth-cashdan.htmI Abstract: Tlie target article claims llial evolutionary theory predicts the emergence of sex differences in aggression in early childhood, and that there will be 110 sex difference in anger. It also linds an absence of sex differences in spousal abuse in Western societies. All three are puzzling from an evolutionary perspective and warrant further discussion. I agree with Archer that "Social roles ... have their ultimate origins in evolutionary history" (target article, sect. 5, last para.) and think that the difference in levels of explanation between evolutionary theorv and social role theorv is responsible for some of the confusion surrounding this debate. Archer's review of these theories as tliev applv to sex differences in same-sex aggression is verv helpful, but a few of the evolutionary predic­tions raise additional questions. Most of Archer's evolutionary predictions regarding same-sex conflict are clear, but two are puzzling. One is the claim that evol­utionary theorv predicts the earlv emergence of sex differences in direct aggression. If this is a sexually selected trait, whv should it appear before it is needed in mating competition? Most sexually selected traits appear at puberty, so an additional argument is required to support this prediction. Also puzzling is Archer's assertion that evolutionary theorv would not predict a sex difference in anger. Emotions motivate behavior and are affected bv selection onlv if tliev affect behavior. If theorv predicts a sex difference in aggressive behavior, whv would it not also predict a sex difference in the emotion that motivates it? The answer mav help us understand the evolutionary reasons for greater male same-sex aggression. As Archer notes, two argu­ments have been proposed: (1) greater benefit to males because of greater variance in male reproductive success (the usual argu­ment), and (2) greater cost to females, due to their greater par­ental investment (Campbell 1999). If the first of these is driving sex differences in aggression, we should expect reduced anger in women, to motivate their less intense aggressive compe­tition. The second argument, in contrast, would predict equally intense competition but would temper anger with fear, thereby leading to less costlv, but not less emotionallv-intense, forms of aggression. This seems more consistent with the data showing that women and men experience similar degrees of anger, although tliev mav express it differently. Mv chief concern with this otherwise valuable target article lies in its treatment of partner violence. Evolutionary theorv provides a robust explanation for the finding that males are more likelv to control sexual access to females than the converse, and often use aggression to enforce it. Tn view' of this, Archer's claim that there is no sex difference in spousal abuse in Western nations is surpris­ing and deserves another look. The claim of sexual symmetry ignores much contradictory evidence, ignores sex differences in motive, and relies heavily on studies using the problematic (Dobosh et al. 1992) Conflict Tactics Scale (CTS). Johnson (2006) has shown that violence involving proprietariness and control ("intimate terrorism") is heavily male-biased, unlike the disputes picked up bv the CTS, w'hich arise chieflv from conflicts of dailv life ("situational couple violence"). The former also causes far more harm, both phvsicallv and psychologically, than more sexually symmetrical altercations (Johnson & Leone 2005). Evolutionary theorv that addresses male sexual proprie­tariness and concern over cuckoldrv provides a phvlogeneticallv broad explanation for this more serious type of male-biased BEHAVIORAL AND BRAIN SCIENCES (2009) 32:3/4 2 73 Commentary/Archer: Sex differences in aggression violence, and leads to predictions that distinguish it from violence arising from conflicts of interest in other domains (Daly & Wilson 19SS; Wilson & Daly 1996).