We would like to applaud Archer for his target article, “Does sexual selection explain human sex differences in aggression?” The tone of the article title is reminiscent of the November 2004 National Geographic cover article that asked: “Was Darwin Wrong?” (With a resounding “NO!” answering the reader on the opening page). It's no surprise that Archer has confidently proclaimed a resounding “YES!” to his own question.
Archer effectively argues that social role theory fails to adequately explain the complexity of aggressive patterns found both within humans and across species by requiring that its adherents downplay the dramatic similarities that permeate the animal kingdom, underestimate cross-cultural similarities in human aggressive behavior, and disregard the impact of human sexual dimorphism. Here we would like to extend Archer's critique of social role theory by looking at differences both between animal species and the human sexes.
According to social role, or biosocial, theory (e.g., Eagly & Wood Reference Eagly and Wood1999; Wood & Eagly Reference Wood and Eagly2002), sex differences in social behavior result from the distribution of men and women into different social roles within society. These differences stem from sex differences in the physical attributes of the sexes as well as the changing local ecologies of humans. Despite acknowledging these dimorphisms as the starting point, this framework reduces the “role” of sexual selection in shaping these differences to little more than a historical footnote by failing to attend adequately to the variety of evolutionary forces integral in shaping the degree of sexual dimorphism within species.
Largely due to parental investment and mating patterns, which themselves are largely determined by the local ecological conditions, dimorphisms may take the shape of physical attributes such as body size, or behavioral attributes such as behavioral fitness display (see Sefcek et al. Reference Sefcek, Brumbach, Vásquez, Miller and Kauth2006, for review). Specifically, social role theory argues that hominid sexual dimorphism is small in magnitude compared to other primates and driven as much by fluctuations in female body size as it is by intrasexual competition (Plavcan & van Schaik Reference Plavcan and van Schaik1997b). We argue that the less-extreme dimorphism in humans is not evidence that the dimorphisms are not significant. Although the human sexes, which are approximately 15–20% different in body size, do not show the extreme dimorphisms of male and female gorillas or orangutans (e.g., these males being 50% larger in body size), they also do not show the remarkable similarity (monomorphism) in body size that gibbons and other socially monogamous primates do (Leutenegger Reference Leutenegger1982). That humans fall somewhere in-between suggests at least a moderate level of dimorphism as the result of a species-typical evolutionary history of moderate polygyny, cuckoldry, male parental investment, and male intrasexual competition, which leads to sex-typical differences in mating effort – that may manifest itself through aggression and other risky behaviors (Sefcek et al. Reference Sefcek, Brumbach, Vásquez, Miller and Kauth2006).
One important difference in mating effort is illustrated through sex differences in fitness indicating, whereby an individual “shows-off” in an effort to display behavioral energy or genetic quality (Miller Reference Miller, LeCroy and Moller2000). Such ornamental display is risky; it redirects metabolic resources away from survival and makes the individual conspicuous to intrasexual rivals and interspecies predators. Such advertisement may, however, yield a high-payoff. While humans do not display extreme physical ornaments like the peacocks tail, it is argued many products of the human mind and culture serve this fitness signaling function. Cultural artifacts produced through artistic expression (e.g., painting, musical production, poetic language, and humor) and scientific discovery are male-biased and are suggested to stem from intrasexual competition (Kanazawa Reference Kanazawa2000; Miller Reference Miller2001). These patterns tend to show male-biased sexual dimorphism in public display, yet monomorphism in both public appreciation and private display.
Furthermore, both men and women display cycle-specific dimorphic perceptual and behavioral changes related to aggression and self-protection that are in the service of enhancing reproductive fitness. For example, men perceive more dominance in male faces when their partner is ovulating, suggesting an evolved bias to identify the most likely intrasexual competitors in order to protect against potential cuckoldry (Burriss & Little Reference Burriss and Little2006). Fertile females who rate their partners as sexually unattractive (a phenotypic sign of high mutation load) report greater extrapair flirtation and more partner-enacted mate guarding behavior, itself an aggression-based mating tactic (Haselton & Gangestad Reference Haselton and Gangestad2006). Finally, men report more use of sexual coercion in intimate relationships when they perceive that their partner has engaged in sexual infidelity (Goetz & Shackelford Reference Goetz and Shackelford2006), as well as report deeper and more vigorous penile thrusting (Gallup et al. Reference Gallup, Burch, Zappieri, Parvez, Stockwell and Davis2003), and a higher sperm count per ejaculate (Baker & Bellis Reference Baker and Bellis1995) after periods of separation or alleged sexual infidelity. Conversely, fertile females become less aggressive and even though they report more attraction to male dominance, they engage in fewer risky behaviors – which suggests that women strategically avoid the types of activities that may increase their exposure to male aggression when fertile (Broder & Hohmann Reference Broder and Hohmann2003). Furthermore, in response to sexually coercive scenarios, female handgrip strength increases at peak ovulation, an adaptation that may have evolved to protect females against sexually coercive male tactics (Petralia & Gallup Reference Petralia and Gallup2002).
That so much variability in men's aggressive behaviors and female's self-defense behaviors is contingent on fluctuations in female fertility and potential infidelity, strongly suggests that human aggression in its various forms may be better explained as solutions to reproductive problems, shaped via sexual selection, rather than simply as consequences of social roles. Although it is certainly the case that men and women often respond to the same cues with aggression, the fact that aggression, fertility, and perceptions of infidelity are so intertwined suggests that sex differences in aggression are rooted in our evolutionary past. Because men and women, by dint of their biology have historically faced different adaptive challenges to reproductive success, those who inherited the most effective sexual strategies (e.g., Buss Reference Buss1998) would be better equipped to effectively reproduce. It seems that one strategy for differentially improving reproduction includes sex-specific risky-behavioral tendencies, with women focusing on low-risk forms of aggression and males focusing on high-risk forms of aggression.
