The U.S. border with Mexico is about 2,000 miles long, and a third of it (700 miles) has a wall or fence. Economists Treb Allen of Dartmouth College and Cauê Dobbin and Melanie Morten of Stanford University modeled what would happen if Trump were able to cover two-thirds of the border (1,300 miles) with some sort of wall.

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Their conclusion? It would hurt the U.S. economy because there would be fewer workers and lower output. It would also have a tiny impact on illegal immigration, and U.S. workers wouldn’t be better off.

“Our research shows that building a wall was an ineffective way of reducing migration,” Allen said after presenting the new research paper this weekend at the American Economic Association conference. “It was expensive to build, and it harmed U.S. workers.”

Expanding the wall would reduce the number of Mexican workers in the United States by 144,000 — about 1 percent — the economists found after extensive modeling and data analysis.

Trump has proposed spending at least $5 billion — about $15 per American — to extend the wall at the U.S.-Mexico border. The economists found that the incomes of low-skilled workers without college degrees would increase by only 58 cents a year, far less than the cost per American to build the wall. Incomes of higher-skilled workers would fall by $7.60.

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“This is a bad investment,” Allen said.

On top of the cost to build the wall, the U.S. economy would lose more than $4 billion a year, the economists calculated, meaning the country would forfeit nearly $30,000 in lost economic output for each Mexican migrant the wall stops.

It’s difficult to model the economic impact of a border wall. These researchers were able to do it because of two factors — special migration data they obtained from the Mexican government and the Secure Fence Act of 2006, which triggered the construction of walls and fences along 548 miles of the border between 2007 and 2010.

Mexicans living in the United States can apply for a “matrícula consular” ID card from the Mexican Embassy or a consulate. This government ID can be used to open bank accounts or obtain a cellphone. They are popular with Mexicans living without documentation in the United States and are considered the best indication of how many Mexicans are in the country without proper paperwork. The IDs also list the exact location in Mexico where a person was born, allowing researchers to get a sense of how they probably migrated.

The Secure Fence Act — which Sens. Barack Obama, Hillary Clinton and Charles E. Schumer voted for — was a large expansion of barriers and checkpoints at the border that cost $2.3 billion (or $7 per person in the United States).

The economists looked at the impact of that expansion on migration, jobs and the U.S. economy by comparing the data in 2006 and 2010. It was tricky because the Great Recession hit during that period, which played a large role in reducing illegal immigration into the United States because so few jobs were available, but the economists used a sophisticated model to split out the effects of the downturn from the wall.

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The Secure Fence Act resulted in a 0.6 percent decline in migration (about 83,000 fewer Mexican workers), the economists found.

Low-skilled U.S. workers without a college degree benefited a “tiny” amount — 36 cents a year in additional income — from having fewer low-skilled Mexican workers to compete with.

“This paltry effect turns out to be the largest positive benefit of the Secure Fence Act, as all other groups of workers were harmed,” the researchers wrote in their paper.

Perhaps not surprisingly, employees with a high school degree or less who live near the border in Arizona and California were the ones who benefited the most from walls and fencing, but even they saw their incomes improve by just over $7 — about the cost per American to erect the wall.

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Workers with college degrees lost $4.35, Allen, Dobbin and Morten calculated, and the overall economy declined by $2.5 billion.

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“This suggests that for each fewer migrant in the United States as a result of the Secure Fence Act, GDP declined by $30,000 (in addition to the direct costs of wall construction),” the economists wrote.

Trump insists a bigger wall is necessary not just to stop undocumented immigrants from taking U.S. jobs but for security reasons. He argues that many migrants are criminals and terrorists, which is not backed up by data.