Presenting a theory to explain how politics revolves around one axis of social cleavage instead of another, Daniel Posner examines Zambia, where people identify themselves either as members of one of the country’s seventy-three tribes or as members of one of its four principal language groups. Drawing on a simple model of identity choice, Posner demonstrates that the answer depends on whether the country is operating under single-party or multi-party rule, thus revealing how formal institutional rules determine the social cleavages that matter.