Mexican immigrants heading home.

In August 2021, more than three decades after sneaking across the southern U.S. border as young adults to work and support their families in Mexico, Irma and Javier Hernandez checked in at La Guardia Airport for a one-way flight from New York to Oaxaca. They were leaving behind four U.S. children, stable jobs and a country they had grown to love.

But after years of living in the United States without legal status, the couple had decided it was time to return to their homeland. Ms. Hernandez’s mother was 91, and they feared she might die before they saw each other again. They had built a little house, where they could live, and had invested in a tortilleria, which they could run. Their children could fend for themselves.

“Only God knows how hard we worked day after day in New York,” said Ms. Hernandez, 57. “Ultimately we made the difficult choice to return.”

The Hernandezes are part of a wave of immigrants who have been leaving the United States and returning to their countries of origin in recent years, often after spending most of their lives toiling as undocumented workers. Some of them never intended to remain in the United States but said that the cost and danger of crossing the border kept them here once they had arrived — and they built lives. Now, middle-aged and still able-bodied, many are going back.

Mexicans started a gradual return more than a decade ago, with improvements in the Mexican economy and shrinking job opportunities in the United States during the last recession.

But departures have recently accelerated, beginning with crackdowns on immigrants under the Trump administration and continuing under President Joseph R. Biden Jr. as many older people decide they can afford to trade the often-grueling work for a slower pace in their home country.

Their departures are one of many factors that have helped keep the total number of undocumented immigrants in the country relatively stable, despite a flood of migrant apprehensions that reached two million last year. “It’s a myth that everyone comes here and nobody ever leaves,” said Robert Warren, a visiting fellow at the Center for Migration Studies, a think tank in New York.

The number of undocumented people from about a dozen countries, including Poland, the Philippines, Peru, South Korea and Uruguay, declined 30 percent or more from 2010 to 2020.

The undocumented population from Mexico, the principal source of immigrants to the United States, dropped to 4.4 million from 6.6 million during that period.

“Most of them never wanted to stay,” said Douglas S. Massey, an immigration scholar at Princeton University in New Jersey. “We gummed up the works when we militarized the border. They spent longer and longer time and had families.”

Now, he said, census data suggests that many of them are electing to go home.

“If they have savings and a house in Mexico, they can retire there,” he said. “Their kids born in the States are now old enough to take care of themselves and can go back and forth to visit.”

Ms. Hernandez left her Mexican pueblo in 1987 “por la necesidad,” she said.

In New York, she settled into nanny jobs with families in Manhattan. She fell in love with Javier, a fellow Oaxacan who was learning pizza making. They married and their first child, Jennifer, was born in 1992. Without legal status and with the border increasingly barricaded, the Hernandezes could not risk leaving the United States.

It was a heart-wrenching decision to make to now go back.

All four Hernandez children joined their parents on the plane to Oaxaca, and after settling into the house there they took their first-ever vacation as a family, a week on a Mexican beach. Then the children flew back to the United States.

“We cried all the way to New York,” Jennifer recalled. “It’s been a year and a half, and it’s still very hard,” she said, her voice cracking.

Older, and feeling the pull of their ancestral homes.