National industrial concentration in the U.S. has risen sharply since the early 1980s, but there remains dispute over whether local geographic concentration has followed a similar trend. Using near population data from the Economic Censuses, we confirm and extend existing evidence on national U.S. industrial concentration while providing novel evidence on local concentration. We document that the Herfindhahl index of local employment concentration, measured at the county-by-NAICS six-digit-industry cell level, fell between 1992 and 2017 even as local sales concentration rose. The divergence between national and local employment concentration trends is attributable to the structural transformation of U.S. economic activity: both sales and employment concentration rose within industry-by-county cells; but reallocation of sales and employment from relatively concentrated Manufacturing industries (e.g., steel mills) towards relatively un-concentrated Service industries (e.g. hair salons) reduced local concentration. A stronger between-sector shift in employment relative to sales drove the net fall in local employment concentration. Holding industry employment shares at their 1992 level, average local employment concentration would have risen by about 9% by 2017. Instead, it fell by 5%. Falling local employment concentration may intensify competition for recent market entrants. Simultaneously, rising within industry-by-geography concentration may weaken competition for incumbent workers who have limited sectoral mobility. To facilitate analysis, we have made data on these trends available at concentration trends.
David Autor, Christina Patterson and John Van Reenen
19 April 2023 Paper Number POIDWP069
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