The proportion of U.S. prison inmates who were black increased dramatically between 1940 and 2000. While about two-thirds of the increase occurred between 1940 and 1970, most recent research analyzes the period after 1970, focusing on explanations such as the war on drugs, law-and-order politics, discrimination, inequality, and racial threat. We analyze the growth in the racial difference in incarceration between 1940 and 1980, focusing on the role of demographic processes, particularly population growth, migration, and urbanization. We implement three analyses to assess the role of these demographic processes: (1) a simple accounting model that decomposes the national trend into population growth, changes in arrests, and changes in sentencing; (2) a model of state variation in incarceration that decomposes the racial difference in incarceration into population change, migration between states with different incarceration rates, and other processes; and (3) race-specific models of within-state variation in incarceration rates using state characteristics coupled with a decomposition of the role of changes in state characteristics.