Rosarito, Mexico - In 2017, for the first time in twenty years, Mexico topped the list of International Living's annual ranking of the best places for U.S. citizens to retire. The population of Americans in Mexico is rising, in size as well as in age.

Yet most of them may be there illegally. South of the border, it's relatively easy for U.S. citizens to live without legal documentation. In fact, some official reports indicate that illegal Americans seem to be the rule, not the exception.

One 2015 study from Mexico's National Institute of Statistics and Geography reveals that a stunning 91.2 percent of Americans in the country don't have their papers in order. That figure includes typos and other minor irregularities, and doesn't appear to account for dual nationals.

The welcome many American immigrants feel in Mexico stands in stark contrasts to the way their Mexican counterparts are treated by Uncle Sam.

Mexicans generally embrace Americans and the influx of U.S. dollars that accompany them, and the Mexican government rarely deports Americans – typically just for very serious crime. Fines related to immigration paperwork can range from $50 to a few hundred dollars.

Conversely, under the Trump administration, even those people who grew up in the United States, brought here as babies or children, are now being deported for lacking formal documentation.

"There is a great deal of irony there," says Sheila Croucher, author of the book, The Other Side of the Fence: American Migrants in Mexico. She adds, "I believe the Mexican people are remarkably adept at separating the actions and attitudes of the U.S. government from that of the American people."

According to the U.S. State Department, around a million Americans currently reside in Mexico. But that number is only an estimate, since citizens aren't closely tracked leaving the United States.

"I think it's safe to say that over the past few years there has been a marked increase," Croucher says. "And there's been no indication of a reverse flow of people coming back from the towns in Mexico where large numbers of Americans have settled."

Statistics from the U.S. Social Security Administration show that distributions to beneficiaries in Mexico increased 7.2 percent between 2012 and 2016. Yet those numbers, too, aren't exactly reliable: many American seniors move south while keeping their old bank accounts back home.

Some of the 15,000 Americans skirt formal residency requirements by obtaining tourist visas, and renewing them every 180 days. Otherwise, a one-year temporary visa costs about $194 USD.

Cheap living, proximity and established enclaves of Americans are three top reasons why Americans are drawn to retiring in Mexico, says Jen Stevens, the executive editor of International Living.

Hot spots for American immigrants, young and old alike, include cities and towns like San Miguel de Allende, Rosarito, the Valle de Guadalupe, Lake Chapala and Ajijic.