This book details the biology and sociology of ethnicity, showing how the two factors influence each other. The book first introduces the history of the concept of "ethnicity" and discusses recent uses of the term in social anthropology. The view that ethnic groups are biological entities, which pervades most academic disciplines, is contrasted to the current social anthropological idea that ethnic groups are self-defined social creations. Social decisions and actions by ethnic groups are shown to have significant biological effects, as exemplified by genetic and epidemiological data. The book includes an in-depth case study of the Basque people and shows the relevance of social and biological criteria for identifying ethnic membership. Researchers and students in sociobiology, human psychology, and social and biological anthropology will want to read this important new work.