This paper reviews the literature on sex and cultural differences in physical aggression and argues that only through understanding the interactions among evolutionary predispositions. hormonal influences, and social/situational factors can we possibly make sense of the patterns of human aggression that we see around us. Specifically, it is proposed that the process of natural selection has shaped hormonal responses in males that are sensitive to situations involving challenges to status and/or competition with other males. and that these hormonal changes are essential ingredients of the aggressiveness that occurs in these situations. Models of aggression that focus only on situational and cognitive/emotional triggers of aggressive behavior and attempt to understand human aggression without any reference to biology are destined to be incomplete at best. (C) 2009 Elsevier Ltd. All rights reserved.