Behind the reversal: The policies that drove the region’s catch-up—relatively low taxes and low wages that attracted factories and blue-collar jobs—have proven inadequate in an expanding economy where the forces of globalization favor cities with concentrations of capital and educated workers.

“Those policies worked before, then they became fundamental constraints on the [South’s] long-term growth,” said Richard Florida, an urbanization expert at the University of Toronto.

Higher taxes and education spending aren’t a cure-all, as many northern states now suffering population loss have found. Nor is the South alone in its economic troubles: Automation and globalization have wiped out millions of good-paying factory jobs around the country, especially in the Rust Belt.

But these trends have fallen especially hard on the South, which is more rural than the rest of the country and has fewer big cities. In part because of its legacy of racial segregation the region has, relative to others, underinvested in human capital. Thus the South, the only region to have enjoyed such a dramatic rise in the postwar period, is the only one to experience such a retreat in the past decade.