Over the weekend, the New York Times had a fascinating article, based on this Brookings report, about cities that are slowly losing their middle-class residents. For instance, 43 percent of those in New York City are considered high-income and 41 percent low-income, leaving only 16 percent in the middle. Housing costs, as well as the low supply of middle-class jobs, are pushing people out to the suburbs.
This looks like a complex phenomenon with a variety of causes and consequences, but one rather striking effect is that many poor urban dwellers will have a harder time moving up the economic ladder. As a San Francisco Chronicle article noted a few weeks ago, people living in the poorer parts of this city often have nowhere to move if they want to escape. Meanwhile, many middle-income workers who have jobs in the city have to finding affordable housing in the suburbs or exurbs, but that forces them into long commutes and less time at home (not to mention all the carbon emissions given off by those four-hour drives).
At any rate, economists don't seem to be too worried about any of this, but it seems like the sort of trend that's drastically understudied and could cause problems down the road. Either way, the article's worth a read.