A new report finds that the number of Mexican immigrants in the United States has declined for the first time since the Great Depression. As the Supreme Court hears oral arguments today in Eric Holder’s lawsuit against Arizona, it’s worth considering what these findings might mean for immigration policy.

The report, from the Pew Hispanic Center, found that the total number of Mexican-born people living in the United States has dropped somewhat, from 12.6 million in 2007 to 12 million last year. This comes after 40 years of very rapid growth, rising from just 750,000 in 1970.

More important than the slight decline is the dynamic behind it: The number of Mexicans moving back doubled in the period from 2005 to 2010 compared with ten years earlier, while the number of new arrivals fell by more than half. (My colleague Steve Camarota noted both these developments several years ago; Pew had disputed the increase in return migration but seems to have come around.)

“The diminished flow appears largely to be a drop in unauthorized immigrants,” the report’s lead author told the Washington Post. That doesn’t mean illegal immigration from Mexico has stopped; the report estimated 1.4 million Mexicans, mostly illegal, moved north during 2005–2010, and, last year, the Border Patrol arrested about 340,000 people trying to sneak across the border. But the number of immigrants giving up and going home has indeed increased, and only between 5 and 35 percent of them are estimated to have been deported; the rest left on their own.

In other words, the policy of attrition through enforcement works. This is what Governor Romney meant when he mentioned “self-deportation.”

Does this mean that the whole immigration debate is now moot? Obviously, this has nothing to do with the Supreme Court’s deliberations today, which will center on how much authority Congress permits to states in the area of immigration enforcement. But the “intent” section at the beginning of S.B. 1070 puts the measure in its broader context: “The legislature declares that the intent of this act is to make attrition through enforcement the public policy of all state and local government agencies in Arizona.”

Is that still a good idea? The Los Angeles Times headline for this story suggests how the immigration expansionists view this development: “Report finds wave of Mexican immigration to U.S. has ended.” In other words, the challenge of immigrants flowing across the border is now past us, and as we watch it recede in the rear-view mirror, we can dismantle much of the enforcement infrastructure we built up, forego further tightening, and legalize the illegals still here. As blogger Mickey Kaus describes the “comprehensive immigration reform” supporters: “OK, the border’s secure. We want our amnesty now!”

Unfortunately, that’s nonsense. First of all, there’s no reason to think that pressure for illegal immigration from Mexico won’t increase again; in fact, the total illegal population has already stopped declining and may have begun to grow again. The U.S. economy is bound to pick up at some point and the newfound middle-class status of so many Mexicans, often cited as a reason for the drop in departures northward, is tenuous. As the Washington Post put it, the country’s new middle class is “fearful that recent gains could be lost in a financial crisis or social upheaval.”