Declining Numbers of Undocumented Immigrants Reported

The number of illegal immigrants in the country has dropped by as much as 1.3 million in the past year, an 11 percent decline since a historic peak last August, according to the Center for Immigration Studies, a restriction immigration group based in Washington. The groups findings add to a growing body of studies indicating that the population of illegal immigrants in the United States is dropping significantly.

The more controversial issue is whether this decline can be attributed to a step up in enforcement policies or the declining economy. "The decline can easily be explained by changes in the economy," said Steve Levy, senior economist at the Center for Continuing Study of the California Economy in Palo Alto. He said California had lost 134,000 construction jobs since the summer of 2006.

The arguments provoked by the study reflected the difficulties of discussing options for halting illegal immigration, when researchers cannot agree on how many illegal immigrants there are and how they are affected by immigration enforcement.

The Center based their findings on census figures for foreign-born Hispanics ages 18 to 40 with a high school degree or less. They estimated that three-quarters of those Hispanics were illegal immigrants, and that they made up about two-thirds of the illegal immigrants in the United States.

Using those estimates, they concluded that the illegal immigrant population had dropped to 11.2 million, from a historic high of 12.5 million in August 2007.

Jeffrey Passel, a demographer at the Pew Hispanic Center in Washington who studies illegal immigration, said his research also showed a decline in immigrants. But because of recent changes in the census's data reporting, he said it was too soon to make precise calculations.

Wayne Cornelius, director of the Center for Comparative Immigration Studies at the University of California, San Diego, said Mr. Camarota and Ms. Jensenius had applied "highly arguable assumptions" to their data.

"They offer no direct evidence that fewer undocumented immigrants are attempting to come to the United States, or that fewer of them are getting in, or that more of those already here are leaving the United States as a result of enforcement efforts," said Mr. Cornelius, who has studied the impact of border enforcement over the past 15 years.