Researchers have found that in the heat of battle they have the same chemicals running through their bloodstreams as protective mothers, meaning they develop incredibly strong bonds with each other but become extremely aggressive to outsiders.
The effect resolves around the hormone oxytocin which is released at times of stress and when people socialise with each other.
But the scientists have found that this chemical, often referred to as the love or bonding hormone, also makes them – like mothers – incredibly aggressive to outsiders.
Using a computer simulation game they found that volunteers given a spray of the hormone bonded more quickly and deeply with their own group but became much more hostile to outsiders.
Dr Carsten De Dreu, of the University of Amsterdam, said that the phenomenon was known as "parochial altruism" or "tend and defend".
This meant that boosted levels of oxytocin produced "in-group love" and "out-group aggression", he said.
Dr De Dreu, who published the findings in Science, said: "Oxytocin is a double edged sword. It makes you kinder to your group but more aggressive to those outside."