Americans who live in states with higher rates of infectious diseases are more likely to hold racist views.

Protesters wave placards against President Donald Trump during a Rise and Resist Against White Supremacy demonstration inside the Grand Central Station in New York on September 18th, 2017. (Photo: Jewel Samad/AFP/Getty Images)

Donald Trump is a self-admitted germophobe. As recent events have made undeniably clear, he is also a racist.

New research into the psychological roots of racial prejudice suggest these seemingly separate traits are, in fact, strongly linked.

A research team led by Harvard University psychologist Brian O'Shea reports people who live in states with higher rates of infectious disease are more likely to hold both overt and implicit prejudices. This finding held true after taking into account a myriad of other factors, including religiosity, ideology, and education.

It further found that, among white people who are highly averse to germs, images that evoke thoughts of infectious disease also increase bias against African Americans.

Previous research has linked fear of disease to anti-immigrant bias, based on the false notion that migrants are carrying germs or viruses into the United States. This new study, in the journal Social Psychological and Personality Science, suggests that same dynamic drives dislike of fellow Americans with different skin color.

O'Shea and his colleagues describe two studies, the first of which featured 355,000 white and 77,000 black Americans who took tests measuring conscious and implicit bias on Harvard University's Project Implicit website.

Participants were asked whether they prefer people of one race to another, and, if so, how strongly. They also indicated how warmly they felt toward both black and white people using a "feeling thermometer," and took a separate test designed to uncover unconscious prejudice.

The researchers then looked at the rate of infectious diseases, and the level of racial segregation, in each of the 50 states, and compared them with the average level of racism among residents who took those tests.

They found that, on average, white people were more biased against black people and black people were more biased against white people, if they lived in states where diseases were more prevalent. Residents of those states were also more likely to find discriminating against individuals based on racial stereotypes to be acceptable behavior.

This remained true even after taking into account other factors associated with racial bias, including age, gender, education level, political ideology, and religiosity. "Infectious disease rates were the best and most consistent environmental predictor of implicit and explicit prejudice," the researchers write.

The second study featured 588 adult Americans recruited online. They were exposed to one of three sets of images: One referencing disease (including photos of mold, feces, and people who were coughing or sneezing); one alluding to terror (including images of the 9/11 attacks); or one showing neutral images such as buildings and pieces of furniture.

The subjects then completed those same racial prejudice tests, and filled out a survey measuring their level of aversion to germs. It featured responses to statements such as "If an illness is going around, I will get it" and "I prefer to wash my hands pretty soon after shaking someone's hand."

The key result: Among white people with high levels of germ aversion, exposure to the infection-related images increased explicit (but not implicit) levels of anti-black prejudice. "This finding further emphasizes the impact that diseases can have on explicit prejudice," the researchers write.

Needless to say, this sort of fear and hatred can be exploited by unscrupulous politicians. Indeed, there is some evidence that the 2014 Ebola scare, which occurred just over a month before the mid-term elections, helped drive support for Republican candidates.

At a time when racist rhetoric is re-entering the mainstream of political dialogue—stoked by the president of the United States—discovering what drives this distorted thinking has become an urgent task. This research adds to the evidence that it is rooted in fear for one's own survival.

That helps explain why, though it sometimes retreats into the shadows, racism never really goes away.