Immigrants have helped start more than half of America's "unicorn" start-ups, including Uber, Palantir Technologies, and SpaceX, according to a new study by the National Foundation for American Policy.

The study finds 55 percent, or 50 out of 91 of the billion-dollar privately held tech companies, had at least one immigrant founder. A quarter of those founders came to the U.S. as international students.

Valuations of immigrant-founded private tech companies are notable, led by Uber, currently valued at $76 billion. The ride-hailing giant was co-founded by Garrett Camp, who immigrated to the U.S. from Canada before co-founding the company with Travis Kalanick.

SpaceX, founded by South African-born Elon Musk and valued at $27.5 billion, comes in second. Those companies are followed by WeWork, co-founded by Israeli-born Adam Neumann and valued at more than $20 billion, and Palantir Technologies, valued at more than $20 billion and co-founded by Peter Thiel, who was born in Germany.

The study comes amid the Trump administration's indications it will revoke work visas for spouses of H-1B visa holders. Immigration attorneys say H-1B applications are receiving more scrutiny and experiencing longer delays under the current administration, imperiling the work status of high-demand technical workers, upon which many Silicon Valley companies rely.

Entrepreneur Al Goldstein and his family were political refugees who moved to the U.S. from Uzbekistan in 1980, Goldstein was 8 years old at the time. He went on to study at the University of Illinois, and founded four companies, including Avant, an online loan company. Together, he says the companies he has founded have created 2500 jobs.

He says finding talent is a challenge, and hopes reports like the NFAP study shed light on the value of immigrants to the U.S. economy.

"We always struggle with the same thing, with talent, with finding talented engineers and talented data scientists," says Goldstein. "The reality is, the U.S., we don't produce enough of those people, But there are lots of those people who want to move to the U.S., so I'm a huge supporter of programs like H-1B, and L-1, and O-1 that help really talented people move to the U.S. I think investment on that is incredible. The return on that investment is massive."