In the target article, Vigil argues compellingly for a socio-relational framework of expressive behaviours (SRFB) that integrates a vast array of reported sex differences. The main selection pressure proposed to underlie these sex differences is female exogamy and its social consequences. Female exogamy refers to a drive to outbreed, effectively analogous to male philopatry: the tendency to remain at or return to the natal territory. The former drive is presumably based on natural selection benefits in terms of decreasing the effects of defective genes and increasing genetic variation. This may have been very important during phases of evolution when small groups were geographically isolated, and to leave a group posed a considerable effort as well as a considerable risk. It is conceivable that the selection pressure for exogamy must for such periods have been very strong and must also have required very strong motivational mechanisms to overcome the individual's natural tendency to remain in the familiar environment. In species with sexual reproduction, it is of course sufficient that one sex outbreeds. Since it would probably inflict adaptive costs if both sexes outbred, the optimal solution is a sex-specific one, in which the non-migrating sex can develop adaptive benefits related to philopatry. It is conceivable that a sufficiently effective and sex-specific motivational mechanism is quite a difficult device to develop by means of natural selection, and that a selection pressure to reverse its sex-specificity is unlikely to emerge once its development has commenced.
It is therefore notable that, although female exogamy is a rare behavioural characteristic in the animal kingdom, it is nevertheless shared by humans and chimpanzees (Ember Reference Ember1978; Pusey Reference Pusey, Hamburg and McCown1979). It is estimated that around the time of sexual maturity, roughly one of every two female chimpanzees migrates to other territories (Pusey et al. Reference Pusey, Williams and Goodall1997). Female exogamy is consistent with the group sociality centred on male rather than female kinship observed among chimpanzees (Mitani & Watts Reference Mitani and Watts2005; Williams et al. 2004; Wilson & Wrangham Reference Wilson and Wrangham2003), whereas other primates such as macaques and baboons exhibit a female-based social system. Chimpanzee sociality features male kin forming territorial groups that typically engage in competition with other groups, a pattern with some parallels in human behaviour (Alexander Reference Alexander1990; Wrangham & Wilson Reference Wrangham and Wilson2004). These observations are consistent with the notion that female exogamy is a genetic behavioural trait in humans, because chimpanzees are considerably closer in the human lineage than are primates with female-centred sociality. For comparison, the current estimate of the human-chimpanzee split is some 5 million years old, whereas the split between Hominidae and Old World monkeys (Cercopithecoidea) such as baboons and macaques is on the order of 25 million years (see, e.g., Boyd & Silk Reference Boyd and Silk2006).
Any evolutionary scenario can be questioned on the causality of its relationships, because its hypotheses can rarely be subject to experimental tests. This is particularly true in the case of humans, who, among other complicating factors, suffer from a long life cycle and an extremely elaborate set of cultures that may propagate non-adaptive action and experience tendencies. Main approaches for assessing the validity of theories such as SRFB are therefore analyses of cross-cultural commonality, correlational studies, and observations unlikely to have been affected by culture, such as infant behaviour.
In addition to the many empirical data reviewed in the target article concerning emotional expression per se, such as crying (DeFruyt Reference DeFruyt1997; Kraemer & Hastrup Reference Kraemer and Hastrup1986), large sex differences have been reported for other behaviours that would also seem to be brought to bear on SRFB, such as sexual arousal (Chivers & Bailey Reference Chivers and Bailey2005; Chivers et al. Reference Chivers, Rieger, Latty and Bailey2004; Reference Chivers, Seto and Blanchard2007) and the prevalence of sexual assault (Elliott et al. Reference Elliott, Mok and Briere2004). Moderate sex differences are also found for emotional reactions to music both in humans (McCown et al. Reference McCown, Keiser, Mulhearn and Williamson1997; Nater et al. Reference Nater, Abbruzzese, Krebs and Ehlert2006) and in chimpanzees (Videan et al. Reference Videan, Fritz, Howell and Murphy2007). Female neonates display more interest for faces than for mechanical objects, whereas male neonates exhibit the opposite pattern (Connellan et al. Reference Connellan, Baron-Cohen, Wheelwright, Batki and Ahluvalia2001). Several studies have also reported on dose-response relationships between androgens and sex-typical behaviours of children, such as foetal testosterone in one-year-olds (Lutchmaya et al. Reference Lutchmaya, Baron-Cohen and Raggatt2002) and the severity of congenital adrenal hyperplasia (Nordenström et al. Reference Nordenström, Servin, Bohlin, Larsson and Wedell2002; Servin et al. Reference Servin, Nordenström, Larsson and Bohlin2003).
Finally, I note that the SRFB may have implications for societal policies, in particular those pertaining to equality and fairness between the sexes. Given that, according to the SRFB, females on average have a stronger tendency for submissive displays (ultimately intended to advertise trustworthiness) and males have a stronger tendency for aggression and dominance displays (ultimately intended to advertise capacity), these two tendencies would seem likely to conspire in between-sex interactions. A conflict of interest would in such interactions be likely to exaggerate the male dominance display, being his default approach strategy, which would in turn exaggerate the female submissive display, being her default withdrawal strategy, and so forth in a vicious cycle. In a situation where the male desires sexual activity and the female does not, it is conceivable that the proposed sex-specific display strategies might increase the risk for sexual coercion and rape as compared to sex-neutral displays (cf. McKibbin et al. Reference McKibbin, Shackelford, Goetz and Starratt2008; Thornhill & Palmer Reference Thornhill and Palmer2000).
Another aspect of this scenario is that sex-specific displays effectively may make men and women unequal in the eyes of the law. Indeed, it is common that rapists fail to be convicted because of insufficient evidence of violence. It is conceivable that the level of physical resistance and ensuing violence expected by the legal court as a display of refusal might be set by male standards, thereby seriously compromising the woman.
Sexual assault is but one example of instances where the recognition of sex differences might facilitate effective equality and fairness, in contrast to the typical denial from gender studies that biological sex differences exist. An important question, therefore, is: What empirical evidence and theoretical ground, such as the SRFB, might be required for recognising sex differences in societal policy?
