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The number of undocumented Mexican immigrants in the U.S. is declining, even without a big, beautiful border wall.

A new Pew study released on Tuesday showed that the unauthorized Mexican population, estimated at 5.8 million, decreased by about half a million since 2009, a significant departure from immigration trends over the last three decades that was likely brought on by the Great Recession. Overall, many more undocumented Mexican immigrants have moved out in recent years than those who have newly arrived on U.S. soil.

The data would appear to undercut the centerpiece of Donald Trump’s immigration policy: a 2,000-mile wall that he wants Mexico to pay for, extending the 702-mile fence already in existence. The “great wall” may strike a chord with Trump’s base, but the country’s immigration problem is more about figuring out what to do with the people who are already here. (Trump’s other big proposal is to deport all 11 million unauthorized immigrants in the U.S., which is considered equally unfeasible.)

Trump also tends to conveniently overlook President Barack Obama’s tough stance on illegal immigration, referring to his “deadly non-enforcement policies that allow thousands of criminal aliens to freely roam our streets” at a rally earlier this month. Obama has, in fact, deported more than 2.5 million people since he took office, according to Customs and Border Patrol—more than any U.S. president before him and almost more than every other president combined from the 20th century.

Interestingly, Trump has distanced himself somewhat from his border wall in recent weeks. Instead, he has focused on “extreme immigrant vetting” as a means of weeding out terrorists, particularly after last weekend’s bombings in New York and New Jersey and the stabbing in Minnesota.