Inequalities matter. While work on inequality is often focused on individuals and households, inequalities between groups in society can also be stark. Inequality between ethno-communal groups in particular is linked in the research literature to a variety of major socio-economic outcomes, including civil conflict, the quality of democratic governance, and electoral behavior. Yet, our basic empirical knowledge about variation in such ethno-communal inequalities – how they manifest across countries and over time – remains weak. This hinders our ability to build and test major theories. In this context, this article undertakes an important brush-clearing exercise. First, it shows that several largely distinct bodies of research in the social sciences – on horizontal inequality, ethnic inequality, cross-cuttingness, and ranked/unranked societies – each speak to this sort of inequality and offer measures that can be understood largely as different operationalizations of the same broader concept: group-based inequality (GBI). We focus in particular on GBI with reference to ethno-communal groups (EGBI). Second, we thus argue, analyzing data from multiple EGBI measurement projects collectively using various methods, including factor analysis, provides new traction on building the empirical foundations necessary for further theory development. We illustrate with discussion of two relatively straightforward hypotheses drawn from Horowitz (1985) concerning a declining trend in EGBI over the past century and for regional variation in EGBI. In a concluding section, we highlight data gaps and explore promising areas for future research.
