We investigate the relationship between ethnicity and culture, defined as a vector of traits reflecting norms, attitudes and preferences. Using surveys of cultural attitudes and values, we find that ethnic identity is a significant predictor of cultural attitudes, yet that cultural diversity is uncorrelated with ethnic diversity. The reason is that the degree of cultural heterogeneity across ethnic groups is small relative to total cultural heterogeneity. We propose new measures of the degree of overlap between culture and ethnicity, $chi^2$ and FST, that stem directly from a simple model of social antagonism. We study the cross-country correlates of cultural diversity and of the overlap between culture and ethnicity. Finally, we find that our proposed overlap measures are strong and robust predictors of civil conflict, that cultural diversity tends to reduce the incidence of conflict, and that ethnic fractionalization is not significantly related to conflict. Thus, civil conflict is more likely when culture and ethnicity reinforce each other.