White people who support Donald Trump are feeling threatened by growing cultural diversity in the United States, says a psychologist who studies social identity.

"Many whites are feeling very threatened by the increasing ethnic and racial diversification of America," according to psychologist Brenda Major, a professor at the University of California Santa Barbara. "Donald Trump understands that."

Major has just published a study on the phenomenon. Trump, she said, "plays to" the angst of of white supporters. "The threat of diversity among white Americans who highly identify with their ethnicity," she said, "is playing a key role in shaping this year's presidential contest."

The study highlights that many white Americans see race as a "zero-sum" game where status gains for minorities only come at a cost for white people.

Major ran an experiment back in March, 2016, with 450 white people — 262 who were Democrats and 114 Republicans — and confronted them with information about demographic shifts in the country that show whites will lose their majority status in the country in the next few decades.

According to demographic data (which shows the make up of societies, by age, religion, ethnicity and other factors) published by the PewResearchCenter early this year, by 2055 the US will not have a single racial or ethnic majority.

This is being driven by immigration. Trump has consistently called for curbs to the number of people allowed in to United States, saying he will deport up to 11 million undocumented immigrants, and has spoken about "extreme vetting" of Muslims who want to enter the country. The religious group have been under intense scrutiny over the past 15 years following the 9/11 terrorist attacks and several other smaller-scale attacks and attempts by Islamic extremists.

The Pew study shows although the Muslim population in the US will continue to be small, it is projected to grow rapidly from 1% of the total American population in 2015 to 2.1% in 2050.

"If you're white and you're highly identified with your ethnicity, reminding you of increasing diversity moves you toward Trump," Major said, adding that it "makes you endorse anti-immigrant policies more, whether you are Democrat or Republican."

Far from being a blip on the political landscape, Major predicts that identity politics will only grow in importance, potentially playing a roll in the next election in 2020.

"I predict that white identity politics will be on the stage more and more," Major said, "and become a larger part of the political discourse in the United States in the coming years."