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It’s easy to blame the anti-immigrant impulses driving so much Trump administration policy on basic bigotry. But a recent line of research has asked whether this visceral disdain for outsiders is not just psychological, but biological.

Evolution, after all, has programmed us to be wary of potential sources of disease or infection. For people who are particularly sensitive to such threats, that can translate into a desire to stay far away from suspect strangers — such as immigrants from a far-away land.

Strongly felt, but largely if not totally unconscious, this predisposition makes it virtually impossible to have a rational discussion about the pros and cons of immigration reform. It also hinders the best known antidote to prejudice — getting to personally know people of other ethnicities or religions. If you don’t feel it’s safe to shake someone’s hand, you’re probably not going to develop a meaningful relationship.

That sad insight comes from University of Aarhus researchers Lene Aaroe and Michael Bang Petersen and Temple University’s Kevin Arceneaux. They are the authors of a newly published paper that provides the best evidence yet linking pathogen avoidance with opposition to immigration.

Their research, published in the American Political Science Review, details a series of studies featuring four distinct sets of participants — two in the United States and two in Denmark, where the first two researchers are based. Together, they provide the strongest evidence yet that the desire to avoid potentially unsafe encounters is “a potent and distinct obstacle to inclusive attitudes and tolerance.”

In each of the four studies, participants revealed their opposition to immigration by responding to a series of statements, including “Immigrants improve American (or Danish) culture by bringing in new ideas and cultures.”

Contamination fears were measured in a variety of ways. One group revealed their conscious choices to keep clean, responding to statements such as “I never let any part of my body touch the toilet seat in public restrooms.”

Others took more subtle surveys designed to measure their level of disgust and perceived vulnerability to disease. In addition, 42 Danish university students had their skin conductance measured “while viewing six images related to infection risk and disease.”

No matter how you measured it, or which country you lived in, “contamination disgust correlated with opposition to immigration,” the researchers report. This remained true even after taking the participants’ political ideology into account.

In one study, participants were primed to feel disgust by reading a scenario in which a hospital employee described a day in which he came into contact with bodily fluids. The researchers found that, when they added an additional scene in which he vigorously washed up, it “decreased the influence of behavioral immune sensitivity on anti-immigration attitudes by 47 percent.” This strongly suggests that pathogen-related fears are real, and not just proxies for prejudice.

The results shed light on why racists and xenophobes so often refer to outsiders as dirty, or compare them to disease-carrying creatures such as vermin. While horrifying, these descriptions are also revealing of the underlying fears that are driving their disdain, at least to a degree.

“Our results suggest that people high in disgust sensitivity are not opposed to immigration because they are conservative,” the researchers conclude. “More likely, these people tend to be conservative because their behavioral immune system propels them to oppose immigrants and related (immigrant-friendly) policies.”

The researchers offer no solution to this mismatch between ancient drives and contemporary conditions. It isn’t at all clear why some people are so hypersensitive to infection that they view almost anything, or anyone, that is unfamiliar as a potential threat. In 2014, University of New Mexico biologist Randy Thornhill told Pacific Standard that climate probably plays a role. Hot, humid conditions (as found in the American south) tend to produce more infectious diseases, which would presumably produce more threat-sensitive cultures.

One thing is clear: The first step toward getting this or any other unconscious drive under control is to bring it to consciousness, and determine whether it is (a) rational and (b) in our best interest. When it comes to pathogens and prejudice, the answer to both questions is clearly no.