Of course, it wasn’t simply “moving” that mattered—it was that they moved to specific areas that were growing. When farming jobs were plentiful in the Midwest, for example, people moved there—in 1900, states including Iowa and Missouri were more populous than California. Black men who moved from to the North from the South earned at least 100 percent more than those who stayed, according to work by Leah Platt Boustan, an economist at Princeton. Additionally, for most of the 20th century, both janitors and lawyers could earn a lot more living in the tri-state area of New York, New Jersey, and Connecticut than they could living in the Deep South, so many people moved, according to Peter Ganong, an economist at the University of Chicago. With less labor supply in the regions that they left, wages would then increase there, and fall in the regions they were moving to, as the supply of workers increased. As a result, for more than 100 years, the average incomes of different regions were getting closer and closer together, something economists call regional income convergence. Wages in poorer cities were growing 1.4 percent faster than wages in richer cities for much of the 20th century, according to Elisa Giannnone, a post-doctoral fellow at Princeton.

But over the past 30 years, that regional income convergence has slowed. Economists say that is happening because net migration—the tendency of large numbers of people to move to a specific place—is waning, meaning that the supply of workers isn’t increasing fast enough in the rich areas to bring wages down, and isn’t falling fast enough in the poor areas to bring wages up. Why is this? Why have people stopped moving? The reason, economists believe, is that while there are good wages in economically vibrant cities like New York and San Francisco, housing prices are so high that they outweigh any gains people stand to make in earnings. As a result, high-income cities are still appealing to many workers, but only highly skilled workers who can command salaries high enough to make it worthwhile to move. Low-income workers will end up spending much of their incomes on housing if they move, and so stay put.

This is the conclusion of Ganong and Daniel Shoag, a professor at Harvard and Case Western Reserve University, in a recent working paper. They find that though janitors still earn more in the tri-state area than in the Deep South, the move no longer presents an obvious opportunity because the costs of living in New York have gotten so high. Janitors in the New York area now spend on average 52 percent of their incomes on housing, the authors find, compared to lawyers, who spend just 21 percent of their incomes on rent. Their research finds that because of these factors, the migration patterns for low-income households are beginning to diverge from the migration patterns from high-income households for the first time in American history. High-skill workers are still moving to places that offer them high incomes, but now, low-skill workers are moving away from places where average wages are high.