Posner's important new book is the best recent work on ethnic politics in Africa and should be read by anyone interested in ethnicity and contemporary Africa. In prose always clear and free of jargon, Posner lays out an extremely sophisticated theory of the formation and evolution of ethnic identities, with data from the southern African country of Zambia. He uses different methods and some neat natural experiments to demonstrate compellingly that since the beginning of the colonial era, Zambians have defined their ethnic identities almost entirely in response to the nature of the political institutions they have faced. Far from visceral and unchanging, these identities adapt very quickly as a self-interested response of individuals to changing institutional incentives. Identities presumably become less fluid in countries in which ethnic conflict crosses some threshold of communal violence, and in that sense one suspects that Posner's approach has most traction in peaceful countries such as Zambia. Nonetheless, this major advance in the study of ethnic politics has important implications for both scholars and policymakers.