In their recent book "Why Not Kill Them All? The Logic and Prevention of Mass Political Murder," Daniel Chirot and Clark McCauley go beyond exploring the psychological motives for genocidal killings and offer a historical and comparative examination of genocide. Princeton University Press writes: "Rather than suggesting that such horrors are the product of abnormal or criminal minds, the authors emphasize the normality of these horrors: killing by category has occurred on every continent and in every century. But genocide is much less common than the imbalance of power that makes it possible. Throughout history human societies have developed techniques aimed at limiting intergroup violence. Incorporating ethnographic, historical, and current political evidence, this book examines the mechanisms of constraint that human societies have employed to temper partisan passions and reduce carnage."