Millions of Americans live outside the United States, though as a group they remain an enigma, according to a new report.

Between 2.2 million to 6.8 million Americans live in another country, either on a temporary basis or permanently, said a report called "Counting the Uncountable: Overseas Americans," published on the Migration Policy Institute website.

Americans are the most broadly dispersed people in the world, living in at least 100 countries. Mexico and Canada, however, are home to the largest population of U.S. citizens.

“Americans leave the United States for a variety of reasons, most commonly for marriage or partnership, study or research, or employment,” the report said. “Many originally intended to return to the United States after a limited period overseas, but prolonged their stay when an employment contract was extended, when they met and remained with a partner, or stumbled upon an unexpected work opportunity.”

The report was authored by Amanda Klekowski von Koppenfels of the University of Kent at Brussels and Joe Costanzo of George Mason University.

American expatriates are involved in a variety of jobs and activities overseas. About one-fifth of nearly 900 Americans the authors surveyed for their report were working in education, many of them teaching English. Another fifth were working in IT or communications.

Others are veterans of the U.S. armed services, the report said, "who have remained overseas after retirement, or after the conclusion of a conflict or a tour of duty, often marrying local residents."

The authors conceded that coming up with a number of Americans overseas "is a significant challenge."

"Unlike many countries, the United States does not require its citizens to register a place of residence, either in the United States or abroad," the report said. "Further, there is no longer a central database against which to calculate U.S. citizen departures. Nevertheless most estimates do show an upward trend over time."

Estimates of how many Americans become expatriates each year varies widely, with ranges from 18,000 to 45,000, the report said.

“The United States is not the only country to struggle with counting its population living abroad,” the report noted. “Most countries do not enumerate those leaving as carefully as they do those arriving; the United States is no exception.”

But countries should track how many of their citizens live in other countries, the report said, because their residence elsewhere has an impact in both their adopted homeland as well as the one they left behind.

Expatriates, for instance, often remain involved in their homeland’s politics.

“One reason for the interest in knowing how many citizens live overseas is the role of external citizens in home-country elections,” the report said. “Their impact, in countries across the globe, has grown in recent years.”

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