We examine urbanization using new data that allows us to track the evolution of population across both rural and urban areas in the United States from 1880-2000. We find an upward-sloping relationship between initial population density and subsequent population growth for a range of intermediate densities, which induces polarization in the population distribution. We provide theory and evidence that these and our other findings about the population distribution are driven by structural

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... ation: an aggregate reallocation of employment away from agriculture and an agricultural employment share that decreases with initial population density.