I use newly-developed data on Africa to estimate the effects of the international slave trades (circa 1500–1850) on the institutional structures of African economies and societies (circa 1900). I find that: (1) societies in slave catchment zones adopted slavery to defend against further enslavement; (2) slave trades spread slavery and polygyny together; (3) politically centralized aristocratic slave regimes emerge in West Africa and family-based accumulations of slave wealth in East Africa. I discuss implications for literatures on long-term legacies in African political and economic development.