["Economics &Politics, EarlyView. ", "\nABSTRACT\nEthnic‐based political inequality is widespread, yet its underlying drivers remain poorly understood. This paper shows that an ethnic group's relative size is a key correlate of its access to central executive power. Using data on 575 groups across 181 countries from 1946 to 2021, and restricting attention to non‐dominant groups, I document a robust inverted‐U‐shaped relationship: intermediate‐sized groups receive greater access than both very small and very large groups. I interpret this pattern through a simple model in which incumbents trade off the conflict risk of exclusion against the rent cost of sharing power. The model further predicts that the inverted‐U should be pronounced where political institutions have historically been competitive, and weaker and closer to monotone where institutions have been biased toward incumbents. The evidence is consistent with this prediction. Together, the findings highlight how ethnic composition and institutional history jointly shape access to state power.\n"]