What causes genocide? Why do some stand by, doing nothing, while others risk their lives to help the persecuted? This book analyzes riveting interviews with bystanders, Nazi supporters, and rescuers of Jews during the Holocaust to lay bare critical psychological forces operating during genocide. The book's examination of these moving—and disturbing—interviews underscores the significance of identity for moral choice. The book finds that self-image and identity—especially the sense of self in relation to others—determine and delineate our choice options, not just morally but cognitively. It introduces the concept of moral salience to explain how we establish a critical psychological relationship with others, classifying individuals in need as “people just like us” or reducing them to strangers perceived as different, threatening, or even beyond the boundaries of our concern. The book explicates the psychological dehumanization that is a prerequisite for genocide and uses knowledge of human behavior during the Holocaust to develop a broader theory of moral choice, one applicable to other forms of ethnic, religious, racial, and sectarian prejudice, aggression, and violence. It suggests that identity is more fundamental than reasoning in our treatment of others.