The role of cultural forces in either promoting or discouraging interpersonal violence is so obvious that it has been allowed to obscure the part played by biologic disorders in determining responses to endogenous and environmental challenges. Neuroscientists and clinicians have demonstrated, however, that aggression has a neuroanatomic and chemical basis, that developmental and acquired brain disorders contribute to recurrent interpersonal violence, that both biologic and sociologic factors are involved, and that to ignore either is to invite error.