According to western observers, slavery was almost universal in Africa by the end of the slave trade era. I investigate the extent to which the international slave trades transformed the institutions of slavery in Africa. I use newly-developed data on travel time to estimate the inland reach of international slave demand. I find that societies in decentralized catchment zones adopted slavery to defend against further enslavement. More generally, I find that the international slave trades incentivized the evolution of aristocratic slave regimes characterized by slavery as a property system, polygyny as a family organization, inheritance of property within the nuclear family and hereditary succession in local politics. I discuss the implications for literatures on long-term legacies in African development.