We use population data from the U.S. Census to track regional patterns of growth from 1790 through 1990. At the county level, we find that an initial general tendency towards population convergence lasting roughly through the 1800s becomes reversed: particularly in the post-WWII period, regional populations have diverged. The finding of divergence hinges on two factors: the exclusion of transition dynamics and the level of aggregation. Regarding the former, state-level populations exhibit consistent patterns of transitional population growth over roughly two- to six-decade periods surrounding the admission of states to the union, followed by long periods of relatively steady growth. When transitional periods are included in our county-level analysis, divergent steady state patterns of growth become masked. Regarding the latter, when we aggregate to the state level, divergent county-level patterns of growth are again masked: even when transitional periods are excluded, state-level populations exhibit tendencies towards convergence.