Archer makes a compelling case – conceptually and empirically – that many patterns of human sex differences in aggression, and adaptations that contributed to an aggressive strategy, have arisen by the evolutionary process of sexual selection. Alternative theories that assume a sexually monomorphic evolved mind, such as social role theory, are implausible on conceptual grounds and have so much empirical evidence against them that they can safely be consigned to a footnote in the history of psychology. I suggest that the case for sexual selection in explaining sex-differentiated patterns of aggression is even stronger than that presented in Archer's target article, and that progress in understanding human aggression will be advanced by a more detailed consideration of the adaptive problems solved by implementation of an aggressive strategy.

Hypotheses about adaptive problems solved by aggression lead to predictions about psychological and behavioral “design features” of adaptations for aggression. Archer alludes to some of the broad classes when he suggests that contingent use of aggression evolved to solve adaptive problems of competing for mates, resources, and status; and he details some of the attendant anatomical and physiological features that accompany adaptations for aggression. The specific ways in which competitions occur provide a deeper understanding of aggression, as well as powerful evidence supporting Archer's overarching claim.

Many theorists have proposed that aggression evolved as a context-contingent solution to a host of adaptive problems, including: appropriating the resources of others; reacquiring resources previously appropriated by competitors; preemptively defending against attack; establishing a reputation that deters aggression from others; inflicting costs on intrasexual rivals that damages their ability to retaliate; ascending dominance hierarchies; dissuading romantic partners from infidelity; aggressive stalking to acquire new mates or regain former mates; eliminating fitness-draining offspring; and obtaining sexual access to the otherwise inaccessible (Buss Reference Buss2005; Buss & Duntley Reference Buss, Duntley, Schaller, Kenrick and Simpson2006; Reference Buss and Duntley2008; Buss & Shackelford Reference Buss and Shackelford1997b; Campbell Reference Campbell1995; Reference Campbell2002; Duntley & Shackelford Reference Duntley and Shackelford2008; Smith Reference Smith2007; van der Dennen Reference van der Dennen1995). Examination of a few of these reveals details about adaptations for aggression, and renders the case for sexual selection origins stronger.

Consider war, a particularly dramatic a form of aggression. Mythology aside, there exists not a single case in which women formed same-sex coalitions to kill other female coalitions for the purpose of purloining resources, territory, and mates. Yet history is replete with evidence that men routinely wage war for precisely these purposes (Buss Reference Buss2005; Chagnon Reference Chagnon1983; Tooby & Cosmides Reference Tooby and Cosmides1988; Smith Reference Smith2007; van der Dennen Reference van der Dennen1995). Identifying adaptations for this specific form of aggression reveals additional evidence in support of the sexual selection origins. Among the Yanomamo of Venezuela, unokai men (those who have killed) have more wives and children than those who have not killed (Chagnon Reference Chagnon1988). The paleontological evidence brims with findings of male skulls and skeletons, with injuries corresponding in size and shape to weaponry existing at the time, and an otherwise inexplicable dearth of female skulls and skeletons (Grauer & Stuart-Macadam Reference Grauer and Stuart-Macadam1998). DNA studies of genetic signatures suggest that warriors who vanquished other groups of men sired many progeny (Zerjal et al. Reference Zerjal, Xue, Bertorelle, Wells, Bao, Zhu, Qamar, Ayub, Mohyuddin, Fu, Li, Yuldasheva, Ruzibakiev, Xu, Shu, Du, Yang, Hurles, Robinson, Gerelsaikhan, Dashnyam, Mehdi and Tyler-Smith2003), pointing directly to the sexually selective benefits of aggression.

Ascending status hierarchies is almost certainly one evolved function of physical aggression, as is using aggression to maintain positions attained. History is replete with influential leaders such as Joseph Stalin of Russia, Pol Pot of Cambodia, Saddam Hussein of Iraq, Idi Amin of Uganda, François Duvalier of Haiti, Benito Mussolini of Italy, Ion Antonescue of Romania, Mao Zedong of China, Kim Il Sung of North Korea, Ferdinand Marcos of the Philippines, Slobodan Milosevic of the former Serbia, Ne Win of Burma, and Pablo Escobar of Colombia – all of whom murdered to get to the top, and continued to murder to quash competitors in order to maintain their status positions (Buss Reference Buss2005). Most turned their positions of power into mating opportunities, again supporting a sexual selection explanation.

Although these examples might imply that men have a monopoly on aggression, examination of other adaptive problems and their evolved solutions suggest a more nuanced depiction. Evidence points to differently designed infanticidal adaptations in women and men. Women are more inclined to kill their infant when they have many years of future reproduction ahead of them, or when the infant lacks an investing father or is congenitally deformed (Daly & Wilson Reference Daly and Wilson1988). Men are more inclined to kill infants when there is suspicion of knowledge of a lack of paternity (Daly & Wilson Reference Daly and Wilson1988).

Less gruesome forms of aggression, such as derogation of competitors, show sex-differentiated design features. Women derogate rivals along the dimensions of sexual promiscuity, sexual fidelity, and physical appearance (Buss & Dedden Reference Buss and Dedden1990; Campbell Reference Campbell2002). Men are more likely to derogate rivals along the dimensions of resources, future resource trajectories, physical strength, and athletic prowess (Buss & Dedden Reference Buss and Dedden1990; Schmitt & Buss Reference Schmitt and Buss1996). These findings suggest a complex way in which sexual selection has influenced human aggression. The domains of intrasexual competition (one component of sexual selection) are dictated by the mate preferences of the opposite sex (the second component of sexual selection) (Buss Reference Buss1988b).

Sexual selection, in short, provides a powerful overarching theory of the origins of human aggression, but not solely in explaining the broadly based sex differences in physical aggression. Sexual selection theory also explains many forms of female aggression. Perhaps most important, identifying with greater specificity the adaptive problems for which aggression evolved as one context-contingent solution provides a key to future scientific advances. In this sense, aggression is not singular in nature, but rather is an overarching term for a collection of context-specific cost-inflicting strategies.