NATCHEZ, Miss.—The American South spent much of the past century trying to overcome its position as the country’s poorest and least-developed region, with considerable success: By the 2009 recession it had nearly caught up economically with its northern and western neighbors.

That trend has now reversed. Since 2009, the South’s convergence has turned to divergence, as the region recorded the country’s slowest growth in output and wages, the lowest labor-force participation rate and the highest unemployment rate.

Behind...