Since the publication of Horowitz's Ethnic Groups in Conflict, comparative political scientists have increasingly converged on their classification of ethnic identities. But there is no agreement on the definition that justifies this classification— and the definitions that individual scholars propose do not match their classifications. I propose a definition that captures the conventional classification of ethnic identities in comparative political science to a greater degree than the alternatives. According to this definition, ethnic identities are a subset of identity categories in which membership is determined by attributes associated with, or believed to be associated with, descent (described here simply as descent-based attributes). I argue, on the basis of this definition, that ethnicity either does not matter or has not been shown to matter in explaining most outcomes to which it has been causally linked by comparative political scientists. These outcomes include violence, democratic stability, and patronage.