Senator Bernie Sanders (I-VT) at a campaign rally in Fairfield, Iowa, on April 6th, 2019. (Photo: Scott Olson/Getty Images)

Since the beginning of April, President Donald Trump has centered on a new message as he attempts to raise alarm about immigration on the country's southern border: "Our country is FULL!" he tweeted on Sunday, repeating claims he made to Border Patrol agents in in Calexico, California, that Friday.

Though Trump's new messaging is likely more of an emotional appeal than a demographic argument, reporters quickly fact-checked the president's claim: the New York Times ran a data-heavy article explaining how, in many parts of the country, shrinking towns and industries actually need more people. (Trump's claim "runs counter to the consensus among demographers and economists. ... They see ample evidence of a country that is not remotely 'full,'" the Times reported.)

The question of whether the country is indeed full aside, Trump's underlying assumption that the country could become "full" is a claim some of his most vociferous opponents on the left seem to also believe. On the same day Trump tweeted, Vermont Senator Bernie Sanders explained in an Iowa town hall why he's against open borders: Too many poor people would come to the United States, Sanders explained, and the country could not afford to pay for policies like universal health care or free college.

"If you open the borders, my God, there's a lot of poverty in this world, and you're going to have people from all over the world," Sanders said. "And I don't think that's something that we can do at this point. Can't do it."

The tension between increased immigration and the reforms Sanders advocates for is one that has troubled social democracies for generations. In countries that invest heavily in the social safety net, some leaders have warned that increased immigration could threaten existing social programs by essentially bankrupting the government. During elections in Sweden (one of the Scandinavian social democracies Sanders admires) in 2014, Jimmie Åkesson, a leader of the anti-immigrant far-right Swedish Democrats tweeted: "The election is a choice between mass immigration and welfare. You choose." During the French election in 2016, the fiery leftist candidate Jean-Luc Mélenchon claimed that immigrants "steal the bread from French workers." (In Europe, the emergence of anti-immigrant leftist leaders has led some to brand them "Left Nationalists.")

In contrast to these warnings, many experts believe that the fear of immigration crippling social programs is actually unfounded. "The zero sum mentality—as soon as we let people in, it's going to be more expensive—that's actually not at all what the social science has found," says Juan Pedroza, a professor of sociology at the University of California–Santa Cruz.

An expert in immigration, social inequalities, and public policy, Pedroza has used social demographic methods to study the impact of immigration on countries' and cities' existing social programs. In response to Sanders' remarks, Pedroza had some key takeaways:

1. More Immigration Does Not Necessarily Mean More Expenses

Pedroza says that Sanders, like other democratic socialists, sells his supporters on increased public spending by arguing that it is an investment in human capital and the economy, not an expense. If one accepts the belief that an increased social safety net can fund itself by empowering and relieving the country's working people (as Sanders believes), Pedroza argues that there's no reason to believe immigrants factor differently into the equation than native-born citizens.

Pedroza points to a wealth of research that reveals that increased immigration is more frequently a boon on countries' economies than a leach. Immigrants do not just arrive in a country and begin to use resources. They also start businesses, send their children to school, and spend money in their communities. In an overview of existing research on the economic impact of immigration to the U.S. published in 2017, researchers with the National Academies of Sciences, Engineering, and Medicine found that immigrants contribute significantly more in tax revenue than they take in government benefits. And they do so to a shocking degree: The average long-term fiscal impact of each new immigrant to U.S. was $259,000 in net flow to the government, the paper found.

Contrary to Sanders' argument, more immigration could actually increase the government's resources.

2. If You Actually Opened the Borders, Most Poor People Around the World Would Stay Where They Are

Another issue with Sanders' comments is the senator's belief that liberalized border policy would lead to a massive increase in poor people relocating to the U.S. Pedroza says that this argument reflects a misunderstanding about what drives emigration.

"Even if we wanted to invite the entire world's poor population, we know that most people would rather stay where they are and try to make life better for themselves," Pedroza explains. "It's only a select number of people who feel compelled to migrate."

The popular conception that people immigrate from poor countries to rich countries is unfounded, according to recent research. In 2010, Hein de Haas, a researcher out of the University of Amsterdam, wrote a broad study that challenged the idea that "push-pull factors" cause migration. Haas found that most of the world's migrants actually do not move from the poorest countries to the wealthiest countries. He also found that the world's most impoverished countries actually have lower emigration levels than mid-, middle-, and high-income counties.

A 2017 study of major emigration drivers argued: "The stereotype of the illiterate, poor and rural migrant reaching the borders of affluent countries has to be abandoned. The poorest people simply do not have the means to escape war and poverty and remain trapped in their country or in the neighboring one."

3. Politicians Have Historically Cut Funding to Social Programs Once Those Programs Start Serving Immigrants

There are historical examples of increased immigration threatening existing social spending programs. However, Pedroza argues that's not because the programs themselves have faced financing problems, but rather because politicians have threatened to cut such programs once they start serving immigrants.

"Turns out that most legislators, at least in the initial reaction to that change, end up defunding the social safety net," Pedroza says. "It's not necessarily an explicit reaction to that change, but [the historical research reveals] that trend."

Pedroza says that politicians and voters who feel willing to spend on "future generations" often balk at that same spending when the hypothetical future generation begins to look different, in terms of race or national origin. He argues that politicians like Sanders, who comes from a largely monolithically white state like Vermont, can often "take the bait" when it comes to fears of population change.

In 2018, a study by two researchers out the University of California–Berkeley and Stanford University found that white Americans were more likely to change their attitudes and favor welfare cuts if they came to believe that minorities were the likely beneficiaries of welfare. The title of the study put the finding bluntly: "Perceived Racial Status Threats Lead White Americans to Oppose Welfare Programs."

The study looked at numerous historical examples of how electorates in multiple U.S. states and regions began to sour on welfare spending when they began to perceive changing demographics. Pedroza says that the fear of demographic change can exist even when actual immigration rates are still low, and can trigger native-born populations to defund social spending.