Report Credits Drop in Illegal Immigrants to Enforcement

"I call it the amnesty hump," Camarota said. He noted that the population increase during this period might not have been statistically significant, but "it seems that what was happening was that fewer illegal immigrants left than might otherwise have done so because they were hoping to qualify for legalization."

Also up for interpretation is the degree to which the drop in the number of less-educated Hispanic adults (and, by inference, illegal immigrants) was the result of fewer foreigners entering the country or more of them leaving. The U.S. Border Patrol reported a 20 percent decline in apprehensions along the southern border over fiscal 2007, a possible indication that fewer illegal immigrants attempted to enter the country.

Camarota and Jensenius note that census data do not answer the question. But the authors suggest that if less-educated Hispanic adults were not leaving in greater numbers than before, their total population would merely grow more slowly, not decline steeply.

Among those who are leaving, the vast majority are probably doing so on their own. Despite a surge in work site raids and other enforcement measures, as well as decisions by various state and local governments to train their police to identify illegal immigrants, only 285,000 immigrants were removed from the United States last year, and many of those were formerly legal immigrants who lost their status after committing a crime.

Camarota and Jensenius said they take this as possible evidence that tougher enforcement can have a multiplier effect, scaring many more illegal immigrants into leaving of their own accord than authorities can pick up. And the authors suggest that if the trends they identify are sustained, "it would cut the illegal population in half within just five years."

However, Randolph Capps, a researcher with the Urban Institute who has studied the number of U.S. children born to illegal immigrants, cautioned against such reasoning.

Even if all the findings in the study by Camarota and Jensenius prove correct, he said, it is probable that the first million illegal immigrants to leave were those who had arrived more recently and had the weakest ties to the United States.

The remainder, including the more than half of illegal immigrant adults who have children in the United States, Capps said, are less likely to leave unless they are removed by the government.

"Having a kid in school provides a really strong incentive to stay," he said. In addition, "People who are more settled in the United States have more options. They can move to another [state or county] where enforcement is not as strict. If they lose a job, they can find another. If one member of the family is arrested and deported, they can find other relatives to stay with."