A boy in Mexico looking at a section of the US-Mexico border barrier at Sunland Park, New Mexico, opposite the Mexican border city of Ciudad Juarez, on September 9. Reuters/Jose Luis Gonzalez There's a really good reason for President 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 US illegally now than in the past.

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 at 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 wrote. "Although in previous decades such labor 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 labor."

Economists widely agree that immigration is beneficial to growth and that Trump’s anti foreigner, antitrade stances will most 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. From 1990 to 2007, the paper said, "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, however, which made US economic prospects look a lot dimmer as 9 million jobs quickly evaporated, helped dent the attractiveness of the US 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 wrote.

"Because U.S. neighbors to the south are today experiencing much slower labor-supply growth, the future immigration of young low-skilled labor 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?