Before turning to the main point of my commentary I want to note that Archer's definition of sexual selection as involving “the choice of members of one sex by the other, and competition by members of one sex for access to the other” (target article, sect. 2.1.1) is not uncontested. Archer does not acknowledge, let alone resolve, the controversies surrounding historic and contemporary accounts of sexual selection (e.g., Andersson Reference Andersson1994; Cornwell & Perrett Reference Cornwell and Perrett2008; Cronin Reference Cronin1991; Darwin Reference Darwin1871/1901; Hubbard Reference Hubbard1990; Johnstone Reference Johnstone1995; Kirkpatrick & Ryan Reference Kirkpatrick and Ryan1991; Leonard Reference Leonard2005; Miller Reference Miller2006; Roughgarden et al. Reference Roughgarden, Oisha and Akcay2006; Stamos Reference Stamos2008; Wade & Shuster Reference Wade and Shuster2002; Wallace Reference Wallace1871; Williams Reference Williams1975). Darwin held that modifications acquired through sexual selection are strongly pronounced and fully developed only at maturity. He did not attribute all differences between the sexes to sexual selection but only those differences that could not be explained based on natural selection alone (Darwin Reference Darwin1871/1901). Since Archer links aggression to endocrine mechanisms that also underwrite larger size and musculature in males he needs to show that these traits are not fitness-enhancing outside of the realm of reproduction. If that were the case, they (and subsequently aggression) could be explained based on natural (not sexual) selection.

If we use Archer's definition of sexual selection, several issues need to be resolved. First, Archer focuses on the differences in (physical and verbal) aggression between sexes. He provides compelling evidence that these differences arise at around 24 months and are greatest in adolescence and early adulthood. Because sex differences arise so early, Archer concludes that they cannot be based solely on cultural learning throughout childhood (sect. 2.2.1). However, he has not shown that sex differences exist already when aggressive behaviour first emerges. “Rage reactions” occur in neonates; aggressive behaviour has been documented in 4- to 6-month-old infants of both sexes (Parens Reference Parens2008) and is well developed in 12-month-old infants (Holmberg Reference Holmberg1980; Lewis et al. Reference Lewis, Alessandri and Sullivan1990; Sroufe Reference Sroufe1995; Stenberg & Campos Reference Stenberg, Campos, Stein, Leventhal and Trabasso1990; Stenberg et al. Reference Stenberg, Campos and Emde1983). Alink et al. (Reference Alink, Mesman, van, Stolk, Juffer, Koot, Bakermans-Kranenburg and van IJzendoorn2006) did not find sex differences in aggression in 12-month-olds. For Archer's account it is important to uncover exactly when and why sex differences arise.

It is well documented that physical aggression peaks in early childhood and decreases continually. Hartup (Reference Hartup1974) reported that 4- to 6-year-olds are more aggressive than 6- to 7-year-olds. Cummings et al. (Reference Cummings, Iannotti and Zahn-Waxler1989) found that frequency of physical aggression, initiation of aggression, and average length of aggression periods decreased in both sexes between 2 and 5 years of age. Tremblay et al. (Reference Tremblay, Japel, Pérusse, McDuff, Boivin, Zoccolillo and Montplaisir1999, as cited in Fig. 1 of the target article; see sect. 2.5) and Tremblay et al. (Reference Tremblay, Nagin, Seguin, Zoccolillo, Zelazo, Boivin, Perusse and Japel2004) found that after a peak at age 2 to 3 years, aggression decreased steadily until age 11 in both sexes. This appears to be a problem for Archer's account, because it is difficult to see how a theory of sexual selection can explain that the amount of aggressive behaviour decreases steadily throughout childhood. If aggressive behaviour (a) results in greater success in inter-male competition for mates, and/or (b) is considered by females to be an attractive characteristic of potential mates, we should expect the highest values of aggression – not only the greatest difference between the sexes – at the time when mating occurs. Instead, the peak of aggressive behaviour occurs more than 10 years before the time of mate-choice. This development is very different from that of the other sexually dimorphic traits that are part of Archer's “adaptive complex” (see sects. 3.1. to 3.4). Sex differences in these traits (e.g., body height, absolute and fat-free body mass, strength, musculature, and “aggressive display features”) are usually most pronounced at the time of mating. However, we also find either a steady, “absolute” increase of these traits during childhood towards a maximum at late puberty/early adulthood (e.g., body height, body mass, strength), or a late onset (e.g., facial hair, lowering of the male voice). In no case do we have a decline in the absolute value of the trait similar to that observed in aggression. For these reasons, I suggest that it is implausible that aggression has been sexually selected for.

To maintain an evolutionary explanatory framework one might argue that females select mates that are best able to curb aggression. This move could still account for the early onset of aggression and, if we assume that aggression is underwritten by sexually dimorphic neuroendocrine mechanisms as suggested by Archer (sect. 2.1.2) and the sex differences in aggression. In addition, it could account for the decline in aggression throughout childhood. Tremblay et al. (Reference Tremblay, Japel, Pérusse, McDuff, Boivin, Zoccolillo and Montplaisir1999) suggest that we have to learn to inhibit aggression. It is not relevant here whether this learning is based on social or evolutionary mechanisms. Both processes could contribute to sex differences in aggression. Parental feedback can impact behaviour much earlier than learning based on explicit instruction (Alink et al. Reference Alink, Mesman, van, Stolk, Juffer, Koot, Bakermans-Kranenburg and van IJzendoorn2006; Côté et al. Reference Côté, Vaillancourt, LeBlanc, Nagin and Tremblay2006). Parents and caregivers react differently to aggressive behaviour in boys and girls, and it has been shown that this differential reaction can increase initially small differences in behaviour (Bergeron & Schneider Reference Bergeron and Schneider2005; Fagot Reference Fagot1984; Fagot & Hagan Reference Fagot and Hagan1985; Serbin et al. Reference Serbin, O'Leary, Kent and Tonick1973). Ostrov and Keating (Reference Ostrov and Keating2004) speculate that from an early age onwards, boys gravitate toward contexts in which overt aggressive tactics are more appropriate, while girls are drawn to contexts in which relational aggression is more effective. On the other hand, Chen et al. (Reference Chen, Chen, Liu and Wang2002) suggest that aggression can be seen as reliable indicator for poor ability to exert self-control. Hinshaw (Reference Hinshaw1992) suggests that aggression is associated with academic difficulties in school-age children. Arnold (Reference Arnold1997) and Stevenson et al. (Reference Stevenson, Richman and Graham1985) reported associations between aggression and language deficits in preschool children.

Finally, on Archer's account, women would face a conflict in mate choice. On the one hand, they would be attracted to aggressive males. On the other hand, they would be interested in a long-term relationship with a provider. It seems that females who resolve this conflict by choosing a mate who is not likely to harm them or their offspring will leave more offspring than those who choose an overly aggressive mate. This would lead to selection against aggression.

Much work remains to be done before we can understand the evolutionary history of aggression. As a longtime researcher of the field aptly put it: “We stand at the edge of the nest, then, regarding what we know about the development of aggression” (Hartup Reference Hartup, Tremblay, Hartup and Archer2005, p. 19).