Aggression in women, she notes, tends to take the form of verbal rather than physical violence, and is mostly one on one. Gang instincts may have evolved in women too, but to a much lesser extent, says John Tooby, an evolutionary psychologist at the University of California at Santa Barbara. This is partly because of our evolutionary history, in which men are often much stronger than women and therefore better suited for physical violence. This could explain why female gangs only tend to form in same-sex environments such as prison or high school. But women also have more to lose from aggression, Tooby points out, since they bear most of the effort of child-rearing.
Not surprisingly, McDermott, Van Vugt and their colleagues found that men are more aggressive than women when playing the leader of a fictitious country in a role-playing game. But Van Vugt's team observed more subtle responses in group bonding. For example, male undergraduates were more willing than women to contribute money towards a group effort - but only when competing against rival universities. If told instead that the experiment was to test their individual responses to group cooperation, men coughed up less cash than women did. In other words, men's cooperative behaviour only emerged in the context of intergroup competition.
Some of this behaviour could arguably be attributed to conscious mental strategies, but anthropologist Mark Flinn of the University of Missouri at Columbia has found that group-oriented responses occur on the hormonal level, too. He found that cricket players on the Caribbean island of Dominica experience a testosterone surge after winning against another village. But this hormonal surge, and presumably the dominant behaviour it prompts, was absent when the men beat a team from their own village, Flinn told the conference. "You're sort of sending the signal that it's play.
You're not asserting dominance over them," he says. Similarly, the testosterone surge a man often has in the presence of a potential mate is muted if the woman is in a relationship with his friend. Again, the effect is to reduce competition within the group, says Flinn. "We really are different from chimpanzees in our relative amount of respect for other males' mating relationships."
The net effect of all this is that groups of males take on their own special dynamic. Think soldiers in a platoon, or football fans out on the town: cohesive, confident, aggressive - just the traits a group of warriors needs.
Chimpanzees don't go to war in the way we do because they lack the abstract thought required to see themselves as part of a collective that expands beyond their immediate associates, says Wrangham. However, "the real story of our evolutionary past is not simply that warfare drove the evolution of social behaviour," says Samuel Bowles, an economist at the Santa Fe Institute in New Mexico and the University of Siena, Italy. The real driver, he says, was "some interplay between warfare and the alternative benefits of peace".
Though women seem to help broker harmony within groups, says Van Vugt, men may be better at peacekeeping between groups.
Our warlike past may have given us other gifts, as well. "The interesting thing about war is we're focused on the harm it does," says Tooby. "But it requires a super-high level of cooperation." And that seems to be a heritage worth hanging on to.