The Male-Warrior Hypothesis

Evolutionary scientists argue that human cooperation is the product of a long history of competition among rival groups. There are various reasons to believe that this logic applies particularly to men. In three experiments, using a step-level public-goods task, we found that men contributed more to their group if their group was competing with other groups than if there was no intergroup competition. Female cooperation was relatively unaffected by intergroup competition. These findings suggest that men respond more strongly than women to intergroup threats. We speculate about the evolutionary origins of this gender difference and note some implications. A tribe including many members who, from possessing in high degree the spirit of patriotism, fidelity, obedience, courage, and sympathy, were always ready to aid one another, and to sacrifice themselves for the common good, would be victorious over most other tribes, and this would be natural selection. (Darwin, 1871, p. 132) Evolutionarily minded social scientists assert that human altruism and cooperation are the result of the species’ unique history of intergroup conflict and warfare (Alexander, 1987; Buss, 1999; Campbell, 1975; Tooby & Cosmides, 1988). Social psychological research is consistent with this idea. Humans spontaneously make ‘‘us versus them’’ categorizations and quickly develop deep emotional attachments to groups even when membership is based on trivial criteria, like the flip of a coin (Brewer, 1979; Ostrom & Sedikides, 1992; Tajfel & Turner, 1979). Humans also readily discriminate against members of out-groups (Fiske, 2002) and engage in costly altruistic actions to defend their group (De Cremer & Van Vugt, 1999; Sherif, 1966). We hypothesize that an ancestral history of frequent and violent intergroup conflict has shaped the social psychology and behavior of men in particular. Compared with women, men are more likely to engage in intergroup rivalry because for them the (reproductive) benefits, for example, in access to mates and prestige gains, sometimes outweigh the costs (Buss, 1999; Tooby & Cosmides, 1988). Indeed, research on traditional societies shows that tribal warfare is almost exclusively the domain of men, and that male warriors have more sexual partners and greater status within their community than other men do (Chagnon, 1988). A U.S. study on male street gangs revealed that gang members have above-average mating opportunities (Palmer & Tilley, 1995). Finally, recent experiments in social psychology have shown that whereas women are more interpersonally oriented, men are more group oriented (Baumeister & Sommer, 1997); men also recall group events better than women (Gabriel & Gardner, 1999), and men engage more frequently in competitive between-group interactions than women do (Pemberton, Insko, & Schopler, 1996). Thus, there is some theoretical and empirical support for the idea that men’s behaviors and cognitions are more intergroup oriented than women’s. We refer to this idea as the male-warrior hypothesis. This general hypothesis leads to the prediction that men, more than women, increase their altruistic group contributions during intergroup competition. In this article, we report three experiments in which we tested the male-warrior hypothesis using a social-dilemma task.