Both Sens. Elizabeth Warren and Bernie Sanders, who are 2020 Democratic presidential candidates, have proposed plans to make public universities free for students.

Neither plan specifies whether it would make the schools free for international students coming to the US.

Even if international students were covered by a "free college" plan, the idea presents serious issues for students coming from abroad.

Neal McCluskey is the director of the Cato Institute's Center for Educational Freedom. Catherine Straus is a research associate at the center and a former international student from Taiwan.

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Higher education is a major US export, a reality that presidential candidates calling for "free" public college have overlooked.

Sens. Bernie Sanders and Elizabeth Warren in particular have placed the idea of offering "free college" among their signature issues, but when asked about the likely effect on international students the Warren campaign did not respond and the Sanders campaign did not provide comment in time for publication.

Unfortunately, both Democratic candidates' plans pose a risk to our international trade in education and offer a major reason why the US shouldn't rush into enacting a free college system.

For a little perspective, in the 2017-18 school year, there were 891,330 international students studying in US colleges.

At the undergraduate level, the focus of free college, 442,746 out of 16.8 million students were international, or 2.6% of the total.

At specific schools, international students are even more important. At the University of California at Berkeley, international students comprised about 3,600 of 30,600 undergraduates in 2018, nearly 12% of enrollment. And many fields of study, especially technical subjects such as engineering and economics, draw heavily from international pools.

Exempting international students from a free college plan risks losing those students

Ending tuition and fees for Americans would almost certainly lead to big revenue losses for institutions. Warren and Sanders would likely use the lure of federal matching funds to encourage states to boost their direct funding of colleges, and hence make them tuition- and fee-free.

In 2018, public colleges brought in more than $74 billion in tuition-and-fee revenue, minus state aid to students. With about 90% of public college students undergrads, free college would require $67 billion more annually. That's a ton of revenue to make up for, especially since most states are strapped by medical and pension costs.

Oh, and the national debt is approaching $23 trillion.

International students are already a favorite cash cow for US colleges, paying the same as out-of-state students — $26,290, on average. A need to make up for hemorrhaging domestic revenues would give public colleges powerful incentives to charge even higher sums.

But there's a good chance international students would balk at paying even more gargantuan prices for schools that are free to others. Were many to take their business elsewhere, that would be a big loss. Adjusting data for all international students compiled by the Association of International Educators, international undergrads contributed nearly $16 billion and 184,000 jobs to the American economy in just one academic year through tuition, fees, and room and board.

These students would have options. Attending a top school like Peking University, which has courses in English, would cost an international undergraduate student only about $4,200 per year. In Germany, universities such as the University of Berlin are free to international students. And many students might simply choose to stay home.

Couldn't international students shift to private US colleges? Maybe, but private institutions, except for those at the very top of the food chain — Harvard, MIT, Stanford — would struggle to survive against even cheaper public competitors, leaving even fewer, and less diverse, educational options for international students.

The loss of international students would not just deliver a direct financial hit. Interactions with students from abroad will be more and more important as Americans prepare for an economy that will increasingly transcend national boundaries and cultures. We need to know and understand other people to work successfully with them.

Problems if international students are covered

To avoid many of these problems, we could make public college free for international students. But it is difficult to imagine that ever gaining much popular support.

American taxpayers would be paying for others to accumulate valuable knowledge and skills that they would then take out of the country. Easier immigration might improve the situation — currently only 65,000 H1B visas are reserved for undergraduates desiring work in businesses — but many international students would likely leave regardless. After all, for many home would still be, well, home.

In the end, the effect that "free" college would have on international enrollment is not the most important to consider. But it is still one we ignore at our peril.

Neal McCluskey is the director of the Cato Institute's Center for Educational Freedom. McCluskey holds an undergraduate degree from Georgetown University, where he double-majored in government and English, has a master's degree in political science from Rutgers University, Newark, and holds a doctorate in public policy from George Mason University.

Catherine Straus is the research associate for the Cato Institute's Center for Educational Freedom. Straus holds an undergraduate degree from the University of California at Berkeley, where she majored in public policy and psychology.