By Elli Leadbeater
Having a common enemy brings out the best in men, a new study has shown.
War it seems is a double-edged sword
Psychologists created an economics game, asking groups of volunteers to decide whether to keep money for themselves or invest in a group fund.
The men in the study were much kinder to groupmates if they thought that other groups were competing with them.
The findings, reported at the British Association's Science Festival, may help explain the evolutionary roots of men's interest and behaviour in war.
"One of the things that distances us from many other species is that males actually co-operate with each other," said Professor Mark van Vugt, of the University of Kent.
'Male warrior effect'
Some 300 participants in the games were each initially paid three pounds and divided into groups of six.
They could then choose whether to keep the money, or invest it in a group fund.
They were told that the group fund would later be doubled and divided equally amongst all group members.
The strategy that would make the most money in many situations would involve holding onto your own money, and hoping that others invested in the fund.
The researchers therefore used the amount of money that an individual gave to the fund as a measure of altruism, or kindness to other people.
The scientists found that when people thought that their group was competing against outsiders from other universities, the group dynamic became different to when everyone was competing for themselves.
The men in each group became less self-orientated, and were more altruistic than before, approximately doubling their donations.
"The men actually helped their group by becoming more altruistic towards them," said Professor van Vugt.
"We've labelled it the male warrior effect."
For the women, there was no difference in their behaviour between when they were playing for the group, or for themselves.
Professor Van Vugt believes that the findings may explain some elements of human warfare.
"We believe that men may have evolved a psychology which makes them particularly interested in war," he said.
"Men are more likely to support a country going to war. Men are more likely sign up for the military and men are more likely to lead groups in more autocratic, militaristic ways than women," he added.
"We all know that males are more aggressive than females, but is that always true?
"In situations in which you have inter-group encounters, yes, men start becoming more aggressive than women, but with that comes a lot of co-operation within their group."
Professor Van Vugt said is was likely that the traits observed in the experiment were present in the common ancestor of humans and chimps.
The latter display similar behaviour, albeit in a more primitive form, when they raid neighbouring chimpanzee groups.