BA Science Festival: War is an inevitable consequence of male psychology, according to new research to be published in the journal Psychological Science.
Dr Mark Van Vugt, of the University of Kent, described his "male warrior hypothesis" yesterday at the BA Festival of Science.
He said that evolutionary scientists believe that altruism and co-operation are the result of our uniquely human history of warfare against different groups.
His findings indicated that this was particularly true for men, because "men respond more strongly to threat".
"For men, it is important to have an enemy to bond people together," Dr Van Vugt said.
Women, on the other hand, strongly identify with their group even in the absence of conflict.
"The only way to evolve out of war is to have more women leaders," he suggested.
He also indicated that democratic societies have the best opportunity to promote female leaders.
To test the male warrior hypothesis, Dr Van Vugt's team recruited more than 300 male and female university students for a "group investment" experiment.
In the experiment, volunteers were assigned to groups of six, but each worked alone and did not meet other members of the group.
Each member of the group received a small amount of money, which they could keep entirely for themselves or they could invest in their team.
If, on average, the group members invested more than they kept for themselves, everybody doubled their money.
However, investors only gained if the majority of the group chose to co-operate and invest their money also.
The twist in the experiment was that half of the volunteers were told that they were competing against groups from other universities, while the other half believed they were just competing for themselves.
When volunteers believed they were competing against outside groups, men invested more in their own group.
Women behaved similarly in the two experimental conditions and invested more overall than their male counterparts.
"The results show that men start to identify and co-operate more with their group under conditions of intergroup threat," Dr Van Vugt said.
Dr Van Vugt went on to say that "men needed a threat from another group in order to start pulling their weight for their own group".
This co-operation may have had many evolutionary benefits for our ancestors.
For example, it would have been useful in protecting food sources from predators and in developing democratic societies.
But as male altruism was tightly linked to male competition, Dr Van Vugt warned that "male co-operation is a double-edged sword".