Urbanization and Structural Transformation
利用1880-2000年美国城乡人口数据，发现中等密度地区的人口增长更快，导致人口密度分布随时间更加分散，这源于农业就业份额的初始差异和结构转型。
We examine urbanization using new data that allow us to track the evolution of population in rural and urban areas in the United States from 1880 to 2000. We find a positive correlation between initial population density and subsequent population growth for intermediate densities, which increases the dispersion of the population density distribution over time. We use theory and empirical evidence to show this pattern of population growth is the result of differences in agriculture's initial share of employment across population densities, combined with structural transformation that shifts employment away from agriculture. Copyright 2012, Oxford University Press.