This Open Access book examines how people often treat social group membership as inherent, immutable, informative and even intergenerationally inherited. Such essentialism remains one of the more puzzling folk intuitions, at odds with social science maintaining that people become culturally competent group members through enculturation, and that norms can change how group boundaries are defined quite substantially. Essentialism also features prominently in much rhetoric that justifies intergroup hostility and in researchers' attempts to explain it. Nonetheless, social scientists have not reached a consensus about essentialism's causal role in intergroup relations. In this Open Access book, contributors from a range of perspectives tackle fundamental questions in this field: Do humans share a tendency to essentialise groups? If so, where does this tendency come from? How is essentialism expressed in cognition, behavior, and institutions? What are its consequences for cooperation and social conflict? Psychologists, cultural anthropologists, and evolutionary biologists weigh in on these questions in this volume, often using specific cultural contexts as case studies to elucidate both the particularities and common patterns in the ways essentialism does, or does not, work in the real world.