Over the second half of the twentieth century, conflicts within national boundaries have become increasingly dominant. One third of all countries have experienced civil conflict. Many (if not most) such conflicts involve violence along ethnic lines. Based on recent theoretical and empirical research, this paper provides evidence that pre-existing ethnic divisions do influence social conflict. The analysis also points to particular channels of influence. Specifically, it is shown that two different measures of ethnic division — polarization and fractionalization — jointly influence conflict, the former more so when the winners enjoy a “public” prize (such as political power or religious hegemony), the latter more so when the prize is “private” (such as looted resources, government subsidies or infrastructures). The available data appear to stand in strong support of existing theories of inter-group conflict. Our argument also provides indirect evidence that ethnic conflicts are likely to be instrumental, rather than driven by primordial hatreds. Joint with Laura Mayoral and Debraj Ray.