A Pew Hispanic Center report released today confirms what has been widely known, that the number of illegal immigrants in the United States has dropped sharply since 2007. The real argument is over what’s behind the decline.

According to Pew’s Jeffrey Passel and D’Vera Cohn, the annual inflow of unauthorized immigrants dropped by two-thirds during 2007-09 compared to 2000-05. That plunge has contributed to an overall decline in the total number of illegal immigrants in the United States from a peak of 12 million in March 2007 to 11.1 million in March 2009. Pew calls this “the first significant reversal in the growth of this population over the past two decades.”

Advocates of more restrictive immigration policies have been quick to credit increased enforcement for the decline, but that thesis doesn’t hold up to scrutiny. While enforcement efforts have indeed been ramped up in the past couple of years, the change has not been dramatic. Resources devoted to border and interior enforcement have been increasing pretty steadily since the early 1990s.

It seems implausible that more recent, incremental increases would have such a visible effect when years of increased enforcement efforts before now so visibly failed. In fact, the same restrictionists who constantly complain that nothing has been done to enforce our immigration laws are among those now praising that supposedly non-existent enforcement for the drop in illegal immigrants. They can’t have it both ways.

The more obvious explanation is the steep economic recession that began to bite in 2008. The downturn has been especially brutal in the housing and construction industries where many illegal immigrants found employment during the previous boom. As evidence, the decline in the number of illegal immigrants has been steepest in those states, such as Nevada, California, and Florida, where the housing downturn has been the most severe.

When the economy revives, I predict the inflow and population of illegal immigrants will begin expanding again, too. This problem will not be solved until Congress and the president work together to enact comprehensive immigration reform that widens opportunities for legal immigration.