In this paper, the evolutionary roots of gender differences in aggressive behavior are presented. Previous studies in the field of social psychology have shown that men are more aggressive than women not only in interpersonal, but also in intergroup relationships. From an evolutionary psychological view, it is predicted that outgroup aggression is triggered by the psychological mechanisms adapted to intergroup conflict specified for males. However, social psychologists demonstrated that ingroup cooperation, but not outgroup aggression, was dominant in intergroup conflict situations in a laboratory experiment. On the other hand, in these days, some evidence in the field of cultural anthropology, ethnography, and bioarcheology have clearly shown that hunter-gatherer and forager males frequently engaged in war. I discuss whether intergroup conflict influences selection pressure on male aggressive behavior as a reproductive strategy to enhance fitness.