During Tuesday night’s Democratic candidate debate, the CNN anchor Dana Bash asked a provocative question of Senator Bernie Sanders: “You want to provide undocumented immigrants free health care and free college. Why won’t this drive even more people to come to the U.S. illegally?”

“Because we’ll have strong border protections,” Sanders responded, later adding, “When I talk about health care as a human right, that applies to all people in this country. And under a Medicare for All single-payer system, we could afford to do that.” But Montana Governor Steve Bullock countered him. “This is the part of the discussion that shows how often these debates are detached from people’s lives,” he said. “We’ve got 100,000 people showing up at the border right now. If we decriminalize entry, if we give health care to everyone, we’ll have multiples of that.”

The back-and-forth was the continuation of an argument from the last round of debates—so many candidates, so much debate—and an encapsulation of something known as the “welfare magnet hypothesis.” The hypothesis is a deeply intuitive one: If you provide migrants with generous benefits, more of them will come. But it does not seem to be true. Migrants do not seem to be swayed by the generosity of benefits, or at least not heavily swayed by them; the prevalence of social networks and the availability of jobs are far more important factors.  […]