Immigrants have helped generate some of America’s most beloved inventions. Alexander Graham Bell, born in Scotland, helped develop the telephone. David Lindquist, a Swede, was the chief engineer at Otis, and pioneered the electric elevator. Herman Frasch, born in Germany, worked in America on a process that would become fracking.

Countries don’t only welcome immigrants because they are good inventors; the best argument for allowing people to migrate from places of conflict or economic malaise might be basic human decency—which is one reason there is so much uproar over President Trump’s executive order limiting immigration from seven majority-Muslim countries. But immigrants are also essential to America’s innovative spirit: Tech companies including Apple, Google, and Facebook filed an amicus brief protesting that the ban “threatens companies’ ability to attract talent, business, and investment to the United States.”

Immigrants have historically helped boost the ingenuity of some American regions and industries, according to a new paper by the University of Chicago economists Ufuk Akcigit and John Grigsby, and Harvard Business School’s Tom Nicholas. The authors looked at sectors with higher shares of foreign-born inventors between 1940 and 2000, and found that those areas produced more patents and inventions than those with a higher share of native-born inventors. (This is similar to findings in a 2014 paper that found that fields welcoming a large number of Jewish emigres from Nazi Germany saw patents rise 31 percent.) Why count patents? Patents are associated with economic growth: In a separate paper, the authors find that the states with higher shares of patents filed experienced more economic growth than those that were less inventive.

Akcigit and colleagues also looked at geographic areas with high numbers of immigrants and found those produced more inventions and patents. In the most inventive states—New York, New Jersey, Illinois, and Massachusetts—20.6 percent of the population were international migrants. In the least inventive states, just 1.7 percent were international migrants. (The authors controlled for population density by examining the number of patents filed and number of immigrants per capita.)

These findings are similar to a widely-cited paper from 2009 by Jennifer Hunt of McGill University and Marjolaine Gauthier-Loiselle of Princeton, which found that immigrants file patents at double the native rate, primarily because more immigrants hold degrees in science and engineering. A one-percentage-point rise in the share of immigrant college graduates, they found, increases patents per capita by 6 percent.

There are a few reasons why immigrants have produced these innovation bumps, Nicholas, of Harvard, told me. Immigrants might have come to the U.S. specifically to work in growing fields, and so had ideas and expertise they had cultivated before they arrived. When they arrived, they were able to quickly turn those ideas into patents in a way that would have been more difficult in other countries with weaker intellectual-property protections. For example, the amount it cost to hold a patent to a term of 17 years in 1900 cost just 11 percent of what it cost in the United Kingdom in the same year. What’s more, as Hunt points out, many immigrants have expertise and interest in fields like science and engineering where there are fewer native-born specialists,, and which are more conducive to patents and invention.

In addition, because they had some discretion in choosing their destination city, some immigrant inventors might have sought out geographic areas where there were other people working in similar fields. This would create what economists refer to as spillover, which is when the presence of a bunch of experts in one geographic area leads to the  generation of more ideas because people share notes and collaborate. For example, a number of immigrants, including a Canadian, a Belgian and a Russian, worked together at the Radio Corporation of America to create the first electron microscope.

All this innovation is about more than just advancing science—technological change drives economic growth, and so immigrants have been key to helping produce growth in the U.S. economy, which can raise living standards and produce widespread prosperity.

A reduction in the number of immigrants coming into the U.S., then, could affect U.S. growth. “Any lack of innovative capacity will lead to economic growth drying up,” Nicholas said. “If a major source of innovation is immigrants, and that source stops, you would be worried about economic growth more generally.”

Many leaders, most notably President Trump, have worried about the American economy’s slowing growth numbers. Economists are trying to come up with an explanation for why the U.S.’s gross domestic product isn’t growing at a rate of 3 percent, as it did in the past. Allowing in more immigrants is probably not the answer; the U.S. is now accepting more high-skilled immigrants than it did in the past. But that could change, too, and not just because of the recent ban: Trump has spoken about revamping the H-1B visa program, which allocates visas to skilled foreigners, in a way that could allow in fewer immigrants. The administration doesn’t seem to be aware that any further reductions in immigration could have very real impacts on American innovation. And that could mean less, rather than more, growth, in the U.S. economy.