Over all, 80 percent of American counties encompassing 149 million people experienced a decline in the number of residents ages 25 to 54 between 2007 and 2017, according to the paper, which was written by Adam Ozimek of Moody’s Analytics and Kenan Fikri and John Lettieri of the Economic Innovation Group.

They project that the trends will continue, and that by 2037, two-thirds of American counties will have fewer adults of prime working age than they did in 1997, despite overall population growth in that period. (Their projections tried to take into account undocumented immigrants.)

Policies to encourage American families to have more children would help over the long run by increasing the supply of potential workers in the future. So could efforts to ensure that even struggling cities have the kinds of amenities young families desire, particularly good schools.

The population of different places is always fluctuating, and economists have traditionally viewed that as a mostly healthy process. Workers make their way to where they will be the most productive, enabling the overall economy to adapt and grow.

But people who study regional economies are increasingly concerned that some aspects of this wave of demographic change make the pain more severe for places left behind — which can get stuck in a vicious cycle.

“There’s a possibility that once local areas start on this downward spiral, it’s self-reproducing,” said Timothy Bartik, a senior economist at the W.E. Upjohn Institute for Employment Research.

A shrinking supply of working-age people can prompt employers to look elsewhere to expand, making it harder for local governments to raise enough taxes to pay for infrastructure and education, and encouraging those younger people who remain to head elsewhere for more opportunity.