We have a problem with people crossing the southern border on foot without authorization. But who could have guessed that putting a wall in their way would stop them?

Well, pretty much every immigration restrictionist. But a new study from American Economic Journal: Applied Economics finds that, indeed, border fencing reduces illegal immigration.

The effects are not small. “Construction in a [Mexican] municipality reduces migration by 27 percent for municipality residents and 15 percent for residents of adjacent municipalities”; it also reduces migration from municipalities that are farther away but historically have relied on the fenced area as a crossing point. Border-wall construction disproportionately deters lower-skilled workers, though it does cause some to cross the border elsewhere instead.