An investigation into the effects of which “ethnicity” may be a cause is among the fastest-growing research agendas in comparative politics today. But causal claims about the effect of ethnicity in the discipline have so far been made without reference to concepts. As comparative political scientists, we talk about what ethnicity does while saying little about what ethnic identity is. We agree roughly on which identities we classify as ethnic. But we do not have a definition of ethnic identities that captures our classification and do not seem to believe that we need one. Many of us who theorize about the effect of ethnic identity proceed without a definition. Those who have provided definitions, including myself in previous work, propose theories and employ classifications that are disconnected from or inconsistent with them (Horowitz 1985; Fearon 2003; Chandra 2004). And, while we now have a large literature that criticizes existing data, measures, and theories about ethnicity, criticisms of the lack of concepts from research in this field are also largely absent.

This chapter places causal theorizing about ethnic identity on a conceptual foundation for the first time. It proposes a definition of ethnic identity that captures the classification of ethnic identities to which our causal claims refer and eliminates definitions that do not. It uses this definition to identify properties that can reasonably be associated with ethnic identity and those that cannot.