Merida, Mexico - The flow of migrants crossing the Mexican border into the United States draws ire from the White House, but the surge of people heading in the opposite direction is less noticed.

This southern migration was described in detail in a recent Washington Post article, which talked to people in Mexico's original expat haven, San Miguel de Allende.

Mexico's statistics institute estimated this month that the U.S.-born population in this country has reached 799,000, a roughly fourfold increase since 1990. The U.S. Embassy here thinks it's actually 1.5 million or more.

It's not just retirees. There are also younger workers, such as tech workers who are location independent. Nearly 600,000 are American-born kids who returned with their Mexican parents. Put them all together, and the flow of migrants from the United States to Mexico is probably larger than the flow of Mexicans to the United States, writes the Post.

The American immigrants are pouring money into local economies, renovating historic homes and changing the dynamics of Mexican classrooms.

"It's beginning to become a very important cultural phenomenon," said Marcelo Ebrard, Mexico's foreign minister. "Like the Mexican community in the United States."

But Mexican authorities know little about the size or needs of their largest immigrant group. He has been tasked by President Andres Manuel Lopez Obrador with changing that.

U.S. and Canadian immigrants here have largely been welcomed. In San Miguel, 10 percent of the city's 100,000 residents are U.S. citizens. Mayor Luis Alberto Villareal delivers his annual State of the Municipality address in English and Spanish.

"Despite the fact that Donald Trump insults my country every day, here we receive the entire international community, beginning with Americans, with open arms and hearts," Villareal said.

Many of the Americans are probably undocumented, overstaying their six-month "tourist" visas. But the government has shown little concern.

"We have never pressured them to have their documents in order," Ebrard said. Typically, violators pay a small fine.

Technology and NAFTA have both paved the way to make expat life comfortable in Mexico. "For the things you can't find locally," a marketer named Bill Slusser said, "you just buy them off Amazon."

About 35,000 Americans live in the beach resort of Puerto Vallarta and about 20,000 reside near Lake Chapala, in central Mexico, according to the U.S. Embassy.

Americans are renovating homes in the historic center of Merida, the capital of Yucatan. Or savoring Pacific Ocean views from homes on Gringo Hill in Sayulita. There are so many Americans in Mexico City's trendy Condesa neighborhood that the guitarists who stroll outside the cafes ask for tips in English, the Post reports.

Around 75 percent of immigrants coming to Mexico are from the United States, outnumbering the migrants heading from Central America.