Many countries are ethnically diverse. However, despite the benefits of ethnic heterogeneity, ethnic-based political inequality and discrimination are pervasive. Why is this? This study suggests that part of the variation in ethnic-based political inequality depends on the relative size of ethnic groups within each country. Using group-level data for 569 ethnic groups in 175 countries from 1946 to 2017, I find evidence of an inverted-U-shaped relationship between an ethnic group's relative size and its access to power. This single-peaked relationship is robust to many alternative specifications, and a battery of robustness checks suggests that relative size influences access to power. Through a very simple model, I propose an explanation based on an initial high level of political inequality, and on the incentives that more powerful groups have to continue limiting other groups' access to power. This explanation incorporates essential elements of several existing theories on the relationship between group size and discrimination, and suggests a new empirical prediction: the single-peaked pattern should be weaker in countries where political institutions have historically been less open. This additional prediction is supported by the data.
