Sex differences in intergroup competition, aggression, and warfare: the male warrior hypothesis.

The social science literature abounds with examples of human tribalism, the tendency to categorize individuals on the basis of their group membership and treat in-group members benevolently and out-group members malevolently. I argue that this tribal inclination is an evolved response to the threat of intergroup violence and warfare that was endemic in ancestral human environments (and is still common today). Here I hypothesize that intergroup conflict has profoundly affected the social psychology of human males in particular--the male warrior hypothesis--and present evidence consistent with this claim. I also discuss implications of this hypothesis for managing intergroup relations in our society.