The number of illegal immigrants living in the United States dropped by 1 million in two years, according to new estimates by the Department of Homeland Security. The government thinks that 10.8 million illegal immigrants lived in the country in January 2009, down from a peak of nearly 12 million in 2007. If the official estimates are correct, not since 2005 has the population of illegal immigrants been as low as it was last year.

Some private researchers, however, are questioning the magnitude of the drop. “It’s very clear the undocumented population basically stopped growing after 2006,” said Jeffrey Passel, a demographer with the nonpartisan Pew Hispanic Center. “It’s plausible that the numbers have gotten smaller. But the way that they’re measuring it, if you compare this estimate with the one two years ago, it overstates the degree of decline.”

Twice over the past two years, Passel said, the U.S. Census Bureau has changed the way it measures immigration in its annual population surveys. Since nearly all estimates of the unauthorized immigrant population rely on census survey numbers, these changes might have distorted the results. The authors of the government estimates could not be reached for comment Tuesday.

The report cautioned that changes made to the census survey could have affected the results. The report, produced annually since 2005, is the government’s official tabulation of immigrants living here illegally.

Most researchers agree that no matter the size of the population, which is notoriously hard to measure, the rate of illegal immigration dropped sharply during the recession.

They disagree, however, on the causes. “The number of new undocumented immigrants coming in has plummeted,” Passel said. Other researchers conclude that the drop is not because fewer illegal immigrants are coming in, but because more are leaving. “The illegal population is falling significantly,” said Steve Camarota of the Center for Immigration Studies, which advocates reducing immigration. “The only way for that to happen is for a lot more people to be going home.”

The government demographers reached the same exact estimate — 10.8 million illegal immigrants nationwide — that Camarota concluded in his demographic study last year. But while most researchers cite the recession as the cause for the decline, Camarota said border enforcement plays an important part. “The decline in the population begins before the economy turns down,” he said. “That suggests that, at least initially, it’s because of the stepped-up enforcement that increased during the end of the Bush administration.”

Other analysts disagree, saying that slower migration flows are a worldwide trend associated with the economic downturn. “It’s really driven by fewer people coming in,” said Jeanne Batalova, a researcher with the Migration Policy Institute based in Washington, D.C. “People are staying put wherever they are. They are less likely to migrate, but if they’re already in the country, the preferred destination, they will do their best to stay and weather the storm. That’s particularly the case for undocumented immigrants.”

There are no estimates of how the population of unauthorized immigrants has changed in the first year of the Obama administration, but demographers are likely to begin guessing later this year when the Census Bureau releases new population estimates for 2009.