The flip side of all of this is that moving to the densest urban areas remains a good bet for college-educated workers. Cities offer them very different kinds of jobs than small towns do. They can enjoy much higher wages for their skills there (in addition to all the amenities big cities provide).

Other research Mr. Autor is conducting with Juliette Fournier, an M.I.T. doctoral student, suggests that the densest urban counties have become so appealing to prime-age workers that they’re now less likely to move away at life stages when previous generations have retreated to the suburbs, like when children arrive.

Policymakers have suggested that low-skilled workers head to the same places where college-educated workers are growing wealthy, like New York and the Bay Area (although many have argued that high housing costs and strict land-use regulation in these places block lower-income workers from opportunity).

The Harvard economist Ed Glaeser, whose work has long championed the benefits of cities, argues that they could still offer advantages to low-skilled workers because of high unemployment in many rural communities. Perhaps the kinds of low-skilled jobs that major metros offer are the same as those in smaller towns — but such jobs are a lot easier to find in big cities.

Low-skilled workers may also find opportunities in cities that don’t come in the form of higher wages. They could come from the availability of nonprofits and social services, or of training programs, or from better access to health care and public transit. And there are other ways to measure opportunity in a community, like whether it enables poor children to get ahead.

The wage pattern Mr. Autor describes looks startling to many economists in part because he has taken a well-recognized divergence in the labor market — between the boom in highly paid jobs for college graduates and the growth of low-paid service-sector work — and mapped it onto the country, by population density.

But other scholars have been studying pieces of this picture for some time. The sociologist William Julius Wilson has documented the disappearance of precisely the kinds of urban jobs Mr. Autor describes.