Men have a built-in tendency to pull together in the face of a common enemy, which might explain why they are more interested in wars than women, a study has found.
A series of laboratory-controlled trials appear to support the belief that men are innately predisposed to close cooperation but only when there is a threat from outside their group.
Mark van Vugt, a professor of social psychology at the University of Kent, said war was almost an entirely male pursuit and this could not be explained simply on the ground that men were more aggressive or physically stronger than women.
"Men are more likely to support a war," Professor van Vugt told the British Association. "Men are more likely to sign up for the military. Men are more likely to lead in autocratic, militaristic ways than women."
The psychological tests involved asking individual men and women whether they would wish to contribute some of their earnings anonymously to members of the same group.
"They were strangers, in fact they did not have face-to-face contact as they were put in different cubicles," Professor van Vugt said. "It would be rational for each member of the group to keep what they earn and hope that other people would contribute to the group so that they can get an extra share."
It turned out that men were more likely to be altruistic to their own group if there was a perceived threat from an outside competing group. Women, however, showed no such response. "We all know that males are more aggressive than females but it's not necessarily always true," the professor said. "Men are more aggressive in inter-group competition but within that men also become more co-operative.
"In the laboratory it is the men who are more likely to sacrifice for the good of the community by throwing a lot of money into a group fund. Men respond more strongly to out-group threats. We've labelled this the male-warrior effect.
He added: "We believe these traits are very old and may have even been there in the common ancestor of humans because they are also seen in a very pristine, primordial form in chimpanzees who go out on raids to neighbouring communities.What distinguishes us from many other species is that males actually co-operate with one another."