Arizona's undocumented immigrant population dropped by more than 100,000 in 2008, the largest decrease of any state, but the reasons for the decline aren't clear.
Experts agree the flow of new illegal immigrants entering the United States is decreasing, but they don't agree on whether the number of immigrants leaving has risen.
According to newly released estimates from the Department of Homeland Security, Arizona's undocumented immigrant population fell from 560,000 in January 2008 to 460,000 in January 2009 — a drop of 18 percent. The agency's estimates used immigration, visa and other data on foreign-born residents.
Experts cite a poor economy and tougher enforcement against illegal immigration for Arizona's decrease.
"It's the economy dropping off more than anything else, but I think enforcement also played a role," said Erik Lee, associate director of the North American Center for Transborder Studies, an Arizona State University think tank.
Nationally, the undocumented immigrant population dropped 7 percent over the year, from 11.6 million to 10.8 million.
The decrease followed a surge that saw Arizona's undocumented population grow by 70 percent and the national undocumented population grow by 37 percent between January 2000 and January 2008.
Arizona lost 261,000 jobs — 9.8 percent of the state's work force — during the recession starting in December 2007, according to the U.S. Bureau of Labor Statistics.
Construction, manufacturing and hospitality were especially hard hit. Those also are the sectors where jobs for immigrants, both legal and illegal, are concentrated. The Arizona Department of Commerce said the construction industry lost more than 78,000 jobs, a 37 percent drop.
Arizona also cracked down on illegal immigration, including an employer sanctions law that requires employers to use a federal program to verify whether new workers are legally eligible to work in the U.S.
In addition, the federal government put thousands of additional Border Patrol agents along the U.S.-Mexico border, built hundreds of miles of fences and barriers and deported thousands of illegal immigrants.
Jeffrey Passel, a senior demographer at the Washington, D.C.-based Pew Hispanic Center, said there's no evidence the economic downturn or more enforcement caused more Mexicans to go home. Mexicans make up about 60 percent of the nation's undocumented population and most of that population in Arizona.
Passel said, however, there's strong evidence to suggest the number of immigrants coming from Mexico has dropped.
"The current recession has had a harsh impact on employment of Latino immigrants, raising the question of whether an increased number of Mexican-born residents are choosing to return home. This new Hispanic Center analysis finds no support for that hypothesis in government data from the United States or Mexico," a July Pew Hispanic Center report said.
The research director at the Washington, D.C., Center for Immigration Studies, Steven Camarota, agrees that the number of illegal immigrants from Mexico has plummeted, but he also believes a growing number of undocumented immigrants are going home because of the economy and immigration enforcement.