Between 1880 and 1980, people generally moved from poor states to rich states, seeking the best jobs. “The creation of a single automobile plant—Ford’s River Rouge complex, completed in 1928—boosted Michigan’s population by creating more than 100,000 workers,” as Tim Noah reported. Migration promoted geographical equality.

But today, not only are families moving less, but also they’re moving in the opposite direction, from rich areas to poorer areas, mainly because of housing costs.

Whereas the middle class used to move toward productivity and jobs, like the River Rouge complex, they’re now barricaded from the most productive places by housing costs. So they’re moving toward cheap housing, instead. Tighter land-use regulations in rich metros pushed up housing values, according to the Harvard economists Peter Ganong and Daniel Shoag. Expensive housing in productive metros priced out the middle class and created “segregation along economic dimensions, with limited access for most workers to America’s most productive cities,” they wrote.

In the larger picture, high migration rates used to be a force for national equality. Now, low migration rates, and the new direction of migration from rich to poorer states, are a force for geographical inequality.