This article investigates the conditions that are conducive to extreme political violence in ethnically heterogeneous nations. Theories of resource mobilization, ethnic competition, and split labor market propose that democratization and economic modernization encourage ethnic competition, increasing the likelihood of extreme political violence within nations experiencing political and economic change. In the light of these theories, the conditions that possibly foster conflict in multi-ethnic nations are identified with respect to levels of democracy, political change (or democratization), and levels of economic development. The effects of these variables on extreme political violence are examined with several logit regression analyses on a pooled time- series sample of 126 nations from 1948 to 1982. The findings show that ethnic heterogeneity is not associated with higher levels of violence within nations, except under certain political conditions. Both democracy and economic development relate to political violence in a curvilinear inverted U-shape form. For ethnically heterogeneous societies, however, the inverted U-curve for democracy is asymmetric, with democracy's pacifying impact relative to semi-democracies only about half as potent as in ethnically homogeneous societies and less than that of strict autocracy.