Archer's review explores the extent to which human sex differences in aggression can be explained by (1) sexual selection theory versus (2) social role theory. From the perspective of a behavioral ecologist and evolutionary geneticist this seems like a highly unequal comparison. While sexual selection theory provides ultimate explanations (“Why has it evolved?”) based on principles that universally apply to the entire animal kingdom, social role theory provides a proximate explanation (“How does it come about?”) that is limited to at best a small range of higher taxa.
Social role theory argues that sex differences in aggressiveness are learnt rather than innate, a proposition that may be best explored using quantitative genetic methods. The extent to which individual differences in aggressiveness (within each sex) are genetically determined, as opposed to affected by the rearing environment, should also shed some light on the relative importance of innate versus social factors that could act on the between-sex difference in aggressiveness. A large meta-analysis of genetic versus environmental influences on human aggressive and criminal behavior (Rhee & Waldman Reference Rhee and Waldman2002) suggests that there indeed are some effects of the family rearing environment (explaining 16% of the variance). However, this is considerably less than the joint additive and non-additive genetic effects, which explain 41% of the observed variation. Hence, humans seem to show both – a certain level of genetic polymorphism with regard to aggressive behavior (within both sexes), as well as a certain amount of behavioral flexibility allowing humans to adjust their behavior in response to their environment (i.e., the perceived costs and benefits of aggressiveness). This flexibility may underlie the observation that male aggression against female partners declines with the Gender Empowerment Index (Archer's Fig. 2), which may reflect increasing reputational costs to male perpetrators and increasing risk of retaliation by the female partner (e.g., risk of being sued, risk of being divorced).
While behavioral flexibility often seems adaptive, sexual selection theory per se does not predict whether differences in behavior will result from genetically fixed (innate) strategies or individually flexible reactions to environmental cues. However, sexual selection theory does differentiate according to the ultimate goals of aggressive behavior. Here we take issue with the definition of aggression as a “behavior intended to harm another individual” (see target article, sect. 1, para. 2). We argue that this definition emphasizes a possible – but not necessary – consequence of the behavior, instead of focusing on its aim, which is to defend or obtain a resource. Ultimately, aggression serves to secure reproductive success (Darwinian fitness), which can be achieved by securing or defending resources. Therefore, in the context of Archer's review, it seems useful to differentiate among different types of conflicts over resources that may lead to variation in sex differences in aggression. Sexual selection theory can make predictions about the following:
1. Variation in male aggression against a female partner as a form of paternity protection. Indeed, if promiscuity occurs, males may risk losing paternity (i.e., the limited resource of fertilizable eggs).
2. Variation in male sexual aggression (against partner or non-mate), including forced copulations, to obtain paternity.
3. Variation in aggression towards conspecifics of the same sex to obtain or defend resources that give access to mates (e.g., food, territory) or to obtain or defend the mate(s) themselves.
4. Variation in aggression towards heterospecific individuals to obtain or defend resources (e.g., food, nest sites).
5. Variation in aggression towards conspecifics (not necessarily sex-dependent) or towards heterospecific individuals to defend offspring (e.g., protection against predation or infanticide).
Hence, aggression can be both a component of mating effort and of parental effort. Differentiating between them seems important because they might be caused by different proximate mechanisms, and because sex differences may have different underlying causes. Interestingly, the question whether aggressive behavior is genetically correlated among different contexts remains hugely underexplored.
As to the issue of male aggression directed towards the female mate, the important question to ask (in Archer's Table 4) would have been the percentage of women versus men who avoid having an extramarital affair due to fear their husband/wife could seriously injure them if they found out. It seems possible that the evolutionary origin of violence by males against their female partners has its roots in paternity insurance (Valera et al. Reference Valera, Hoi and Kristin2003), rather than being a byproduct (a genetic corollary) of greater male strength that evolved due to male-male competition. This could be tested by estimating the genetic correlation between male aggressiveness against their partner and aggressiveness against rival males. In any case, the fact that most women show strong preferences for tall and physically strong partners (e.g. Pawlowski & Koziel Reference Pawlowski and Koziel2002; Frederick & Haselton Reference Frederick and Haselton2007) suggests that over evolutionary times women benefited more from their partners' protection than they suffered from their men's physical superiority.
Further insights could be gained by considering species that show a partial sex-role reversal, as occurs in some birds or fish. In these species, male parental investment is larger than that of females, and females have a higher potential reproductive rate than males. Hence, males tend to be choosy while females compete among each other for access to males (Eens & Pinxten Reference Eens and Pinxten2000). Increased competition among females led to a stronger selective pressure on females to win in physical fights, and thereby to the evolution of larger and more aggressive females compared to the relatively small and peaceful males (Dale et al. Reference Dale, Dunn, Figuerola, Lislevand, Szekely and Whittingham2007). One would predict that these females show hardly any aggression against their male partners, since maternity (as opposed to paternity) is never uncertain. Support for this prediction would then suggest that the larger and more competitive sex does not principally dominate the smaller and less competitive sex, but rather, that within-pair aggression by males evolved as a specific adaptation to paternity insurance. In contrast, a strong positive genetic correlation between intra- and inter-sexual aggressiveness would suggest that male aggression against the partner could arise as a genetic corollary (i.e., a possibly even maladaptive byproduct) of intense male-male competition.
