The new census data offers details about the populations of counties and metropolitan areas. Dr. Johnson calculated that deaths now exceed births in about 46 percent of counties in the country, far more than at the start of the decade, when the pattern held in just 29 percent of counties. Now large swaths of New England, Western Pennsylvania, Central Florida and much of Appalachia glow red on his map of counties that exhibit the pattern.
Once the pattern starts, it can be hard to stop. Dr. Johnson estimated that 90 percent of the counties that experienced the imbalance in one year saw it repeated during the decade. These tend to be places with little immigration and a dwindling number of young families. Their schools tend to close because there are not enough children to fill them.
All of this has had a dramatic effect on the populations in cities and towns. Large metro areas had the steepest decline over the course of the decade, Mr. Frey found in an analysis, with the growth rate down by nearly half. Rural areas, in contrast, grew slightly by the end of the decade, though that followed several years of declines.
Places that had once been popular destinations for young people — New York, Los Angeles and Chicago — ended the decade with some of the biggest declines. New York began losing population in 2017, and last year it registered a loss of more than 60,000 people, the biggest population decline of any American city, Mr. Frey found.
The housing market collapse of 2008 and the rising prices in suburbs prompted millennials to move to big coastal cities. But that pattern reversed by mid-decade, Mr. Frey said, as millennials fled rising rents and home prices.
Places with the biggest gains for the decade were Dallas, Houston, Phoenix, Atlanta and Washington, D.C. Medium-size metro areas, like Las Vegas, have also moved up the ranks of gainers, as have Charlotte, N.C.; Seattle; and Austin, Texas.