President Trump’s senior policy adviser Stephen Miller had a tense exchange with reporters at Wednesday’s press briefing as he defended the administration’s new proposal to dramatically curtail legal immigration. The plan prioritizes highly skilled workers over family members for green cards.

The administration reasons that too many low-skilled immigrant workers are entering the country, costing Americans jobs and wages.

Miller said the U.S. has, in recent years, issued green cards without regard to whether an immigrant can "pay their own way or be reliant on welfare, or whether they’ll displace or take a job from an American worker."

Calling a green card the "golden ticket" of immigration, Miller said Trump’s proposal "puts the needs of the working class ahead of the investor class."

"We’re protecting blue-collar workers," he said.

The claim:

"We’ve seen significant reductions in wages for blue-collar workers, massive displacement of African-American and Hispanic workers, as well as the displacement of immigrant workers from previous years who oftentimes compete directly against new arrivals who are being paid even less," Stephen Miller said.

Miller went on to say the "numbers of low-skilled [immigrant] workers in particular is a major detriment to U.S. workers."

The question:

Have immigrants taken jobs from and lowered wages for American blue-collar workers?

The short answer:

Economists disagree whether or how much an influx of immigrants depresses wages. Some have found that new immigrants depress wages for certain groups, such as teenagers or workers with a high school diploma or less. Others say the overall effect on the economy is tiny, and an influx of immigrant workers vitalizes the economy overall.

Either way, the forces driving wage reductions for blue-collar workers go far beyond immigration.

The long answer:

It is true that wages for low-wage workers have declined — they fell 5 percent from 1979 to 2013. That may not seem like a huge drop, but during that same period, the hourly wages of high-wage workers rose 41 percent,according to the Economic Policy Institute.

However, economists disagree over whether an influx of immigrant labor caused or contributed to declining blue-collar jobs and wages.

Asked to provide a study that supported the administration’s assertion, Stephen Miller cited work from George Borjas, a Harvard labor economist, on how the Mariel boatlift affected blue-collar wages in Miami. In 1980, 125,000 Cuban immigrants came to the U.S., mostly Miami, from the town of Mariel.

Borjas, Miller said, "went back and re-examined and opened up the old data, and talked about how it actually did reduce wages for workers who were living there at the time."

Borjas’ new analysis found that the wages of high school dropouts in Miami dropped between 10-30 percent after the refugee influx (the analysis looked at 1977 to 1993).

But an earlier study on the boatlift, from 1990 by Princeton economist David Card, looked at wages of "less-skilled" workers overall (as opposed to just high school dropouts) and found "virtually no effect on the wages or unemployment rates of less-skilled workers, even among Cubans who had immigrated earlier."

The debate remains unsettled, and it’s impossible to extrapolate the effect of the boatlift on Miami to the whole country.

A recent analysis commissioned and published by the National Academies of Sciences, Engineering, and Medicinefound "the literature on employment impacts finds little evidence that immigration significantly affects the overall employment levels of native-born workers."

Overall, the analysis called the inflow of foreign-born people "a relatively minor factor in the $18 trillion U.S. economy." However, the analysis does cite recent research that immigration could reduce the number of hours worked by teenagers and some evidence that recent immigrants reduce the employment rate of prior immigrants.

There are, of course, other forces that have depressed blue-collar wages: increased automation, globalization, declining unionization and government policies on overtime. The Trump administration recently said it would not defend an Obama-era rule that made workers who make less than $47,000 per year (up from about $24,000) eligible for overtime.

Not just about economics

The politics of the bill reach beyond economics. At Wednesday’s briefing, Miller was also asked to defend the fact that the bill prioritizes English-speaking immigrants. Miller, along with adviser Steve Bannon, have led an ethnonationalist wing at the White House. Bannon has complained about the number of Asian CEOs in Silicon Valley, implying that they are adversarial to America’s "civic society."

"Are we just going to bring in people from Great Britain and Australia?" CNN’s Jim Acosta asked.

"The notion that you think this is a racist bill is so wrong and so insulting," Miller answered, adding that the country’s foreign-born population into the U.S. "has quadrupled since 1970."

While Miller is correct that the foreign-born population has increased, in 1970 the number of foreign-born residents in the U.S. was the lowest in a century. The share of the population that’s foreign born today is about the same as the late 1800s and early 1900s, according to the U.S. Census.

Now what?

It is important to note that Trump’s plan would have a steep uphill battle to get through this Congress. Still, not surprisingly, there is also debate around how Trump’s drastic proposal will affect blue-collar wages moving forward. The Trump administration hopes the plan will free up future jobs for American low-wage workers. But Mark Zandi, Moody’s chief economist, who has advised John McCain and donated to McCain and Hillary Clinton’s campaigns, told Politico the plan is a "mistake" that will cause the labor force to come to a "standstill" in the next decade. "It is hard to imagine a policy that would do more damage to long-term economic growth," he said.

As NPR’s Brian Naylor noted, economists believe the country’s low unemployment rate (4.4 percent) coupled with retiring baby boomers will result in a labor shortage in the coming years.

Additionally, an open letter signed by 1,470 economists argued that "the benefits that immigration brings to society far outweigh their costs, and smart immigration policy could better maximize the benefits of immigration while reducing the costs."

On the campaign trail, President Trump praised highly skilled workers, which his plan now prioritizes — but studies are also mixed on whether those workers positively impact wages or also depress the wages of American-born workers.

Scott Horsley and Shaheen Ainpour contributed to this report.

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