Researchers have concluded that men evolved to be aggressive towards outsiders, a tendency at the root of inter-tribal violence

It emerged through natural selection as a result of competition for mates, territory and status, and is seen in conflicts between nations as well as clashes involving rival gangs, football fans or religious groups, say researchers. They point out that in all cultures and throughout history, men have sought to get their way by initiating violence. Men prefer group-based hierarchies and are identified more strongly with their own groups than women. At a basic level, such tribal aggression helped men in a group to obtain more females, increasing their chances of reproduction.