Modern theories of crime and delinquency tend to be individualistic in their level of analysis and tend to focus on consensus crimes. It is virtually impossible to examine the phenomenon of ethnic genocide within such parameters. Recent histories of the Holocaust suggest that it was carried out by ordinary citizens who supported its objectives, not by dysfunctional psychopaths or individuals intimidated by powerful authority structures. This article proposes a theory of genocide based on xenophobia, a theory developed in recent accounts of evolutionary psychology. The tribal instinct which is at the core of human nature has a dark side of group prejudice and intolerance for out-groups; the evolution of sociability, altruism, and the instincts for coalitions goes together with hostility to outsiders. There are good grounds for criminologists to examine the potential of these new lines of evolutionary inquiry, particularly if they contain clues to the basis of collective ethnic violence and genocide, but there are also some strong caveats, including: (1) evolutionary theories of crime are still theories of action, the perspective remains voluntaristic; (2) the distinction between crimes that arise from nature (atavism) and crimes that arise from culture or society (crimes of passion) is a false distinction; (3) the adoption of an evolutionary perspective requires consideration of whether or not this perspective entails acceptance of heredity as fate. Note, references