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A ‘demographic perfect storm’: Suffolk County had one of the steepest population declines in the US, census data show

Suffolk County — which is anchored by Boston and its roughly 675,000 residents — lost 28,850 people who moved elsewhere between July 2020 and July 2021.David L. Ryan/Globe Staff/file

Suffolk County saw one of the steepest population declines in the country last year, losing 3.3 percent of its population in just 15 months after tens of thousands of people moved out of Boston and its surrounding communities, new census data show.

Overall, the United States experienced its lowest annual rate of population increase in history between July 2020 and July 2021 and, at 393,000 people, its smallest population gain in more than a century, said Kenneth Johnson, a demographer and professor of sociology at the University of New Hampshire.

The nationwide trends owe to a number of factors. Nearly three-quarters of all counties saw more deaths than births last year, with four states — including Maine, New Hampshire, and Rhode Island in New England — experiencing what’s called natural decrease in every single one of their counties.

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With fertility rates already in decline and an aging population, the COVID-19 pandemic helped accelerate the losses, creating what Johnson called a “demographic perfect storm.”

But in Suffolk County and other counties that encompass major cities, the drops are also explained by large swaths of people moving out to other places, including potentially more suburban or rural settings.

Suffolk County — which is anchored by Boston and its roughly 675,000 residents but also includes Chelsea, Revere, and Winthrop — lost 28,850 people who moved elsewhere between July 2020 and July 2021, data show. It’s the largest loss to domestic migration it has experienced since at least 2000, said Susan Strate, a demographer and senior population program manager at the UMass Donahue Institute.

Overall, the county saw a 3.3 percent decline in population between April 2020 and July 2021, the seventh-largest in the United States, according to the Census Bureau, dropping Suffolk’s population to 771,245 people.

“This is not just Boston. It’s happening, in general, to the very large cities,” Strate said.

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Los Angeles County, for example, lost 179,757 people to domestic migration, while New York County lost 113,642. The latter, which covers Manhattan, saw the overall steepest decline in its population at 6.9 percent, followed closely by San Francisco County, which covers the city of San Francisco, at 6.7 percent.

Meanwhile, Arizona’s Maricopa County, which includes Phoenix, gained 58,246 people, the largest increase for a county in the country. At 46,866, it also gained the most residents through domestic migration.

Other factors are also likely contributing to Suffolk County’s decline. International migration, which has typically helped drive people into Boston, slowed considerably last year. The county gained fewer than 3,000 residents who moved from another country. That number was 11,678 in 2017, Strate said, before the Trump administration’s more rigid immigration policies and the pandemic helped stanch the flow of people into the United States.

Strate said high housing costs could also be driving people out of Boston, as well as the flexibility many white-collar workers gained during the pandemic to work remotely. In some cases, she said, the millennials who once flocked to Boston a decade ago could now be “entering a different life phase,” in which they’re starting families and seeking the extra room and cheaper housing suburban and rural communities offer.

There’s also what Strate called the potential “head for the hills effect” brought on by the pandemic.

Cape Cod, Nantucket, and Martha’s Vineyard — popular destinations for people’s second seasonal or vacation homes — saw the biggest percentage gains in population in Massachusetts. And while New Hampshire saw more deaths than births, every county around the state saw more people move in than out, helping grow the state’s population by more than 11,000 people.

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Massachusetts, on the other hand, lost 37,497 from its population.

Whether New Hampshire’s gain was specifically its neighbor’s loss is not clear from the data. But given the traditional flow of people between the two states, at least part of New Hampshire’s population growth is “probably an exodus from Massachusetts,” said Johnson, the UNH demographer. Nearly one-quarter of the people who already live in New Hampshire, for example, were born in Massachusetts, he said.

“We’ve been hearing a lot of anecdotal evidence that when businesses started to allow people to work from home more, there were migration trends into the less urbanized areas. This new census data suggest that there is some truth to that,” he said. “How long that will go on is still unclear.”

The population losses in Massachusetts stand in stark contrast to what were the unexpected gains it enjoyed during the last decennial census. Massachusetts’ population grew to slightly more than 7 million people in 2020, the largest increase of any New England state over the previous decade at 7.4 percent.

The 7,029,917 people counted as Massachusetts residents also exceeded estimates for the state by more than 100,000 people.

That growth helped ensure that Massachusetts retained each of its nine congressional seats, even as states like New York and California were slated to lose representation in Congress.

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Matt Stout can be reached at matt.stout@globe.com. Follow him on Twitter @mattpstout. Martin Finucane can be reached at martin.finucane@globe.com.