There’s a really good reason for Donald Trump to rethink his hardline anti-immigration promise of building a costly “Great Wall” along the US-Mexico border: Many fewer people are actually trying to cross into the United States illegally.

Those are the findings of a new study released as part of the Brookings Institution’s Papers on Economic Activity conference, and authored by University of California, San Diego economists Gordon Hanson, Chen Liu and Craig McIntosh.

“From the rhetoric during and since the 2016 presidential election, one would think that the United States continues to experience a surge of low-skilled immigration,” the authors write. “Although in previous decades such labour inflows certainly occurred, since the Great Recession U.S. borders have become a far less active place when it comes to the net arrival of foreign labour.”

Economists widely agree that immigration is beneficial to growth and that Trump’s anti-foreigner, anti-trade stances will likely be detrimental to the very workers he promised to help during the campaign.

But beyond this, the Brookings paper suggests the border wall idea is counterproductive on its face.

The statistics are striking. Between 1990 and 2007, “the number of working-age immigrants with 12 or fewer years of schooling more than doubled, rising from 8.5 million to 17.8 million individuals.”

The Great Recession, which made US economic prospects look a lot dimmer as 9 million jobs quickly evaporated, helped dent the attractiveness of the United States as a destination for workers.

Moreover, Trump’s focus on Mexico is misguided, the authors suggest, and speaks more to xenophobic populism than any fundamental economic concern.

“The undocumented population declined in absolute terms between 2007 and 2014, falling on net by an annual average of 160,000 individuals, while the overall population of low-skilled immigrants of working age remained stable,” the authors write.

“Because U.S. neighbours to the south are today experiencing much slower labour-supply growth, the future immigration of young low-skilled labour looks set to decline rapidly, whether or not more-draconian policies to control U.S. immigration are implemented.”

Put another way, if they’re not coming, why build it?

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