Warfare in humans is part of our evolutionary history

New Scientist - How Warfare Shaped Human Evolution

An article reviewing different theories about how human warfare might have evolved, and perhaps how it influences our in-group/out-group behavior. Warfare has been with us for tens of thousands of years; it is estimated that 10% of all male deaths in hunter-gatherer societies was from warfare. It was probably similar to chimpanzee warfare, which involves making well-planned raids on neighboring out-groups with overwhelming force. Greater in-group bonding and out-group competition has shown in psychological studies of men, who become more motivated to compete with out-groups than within a group. On a hormonal level, men react to a greater degree when talking to a woman who is paired with an out-group member than when she is paired with an in-group member. Aggression from women, the studies claim, tends to be more one-on-one, rather than men being more in-group bonding and out-group aggression.