Abstract: We provide new measures of ethnic, linguistic, and religious fractionalization for about 190 countries. These measures are more comprehensive than those previously used in the economics literature and we compare our new variables with those previously used. We also revisit the question of the effects of ethnic, linguistic, and religious heterogeneity on the quality of institutions and growth. We partly confirm and partly modify previous results. The patterns of cross-correlations between potential explanatory variables and their different degree of endogeneity makes it hard to make unqualified statements about competing explanations for economic growth and the quality of government. Our new data, which features the underlying group structure of ethnicities, religions and languages, also allows the computation of alternative measures of heterogeneity, and we turn to measures of polarization as an alternative to the commonly used index of fractionalization. Copyright 2003 by Kluwer Academic Publishers