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How Germs Help Explain the Rise of Donald Trump

The fear of "outsiders" pushing Trump toward victory comes in part from our fear of disease.
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Donald Trump addresses the media following victory in the Florida state primary on March 15, 2016, in West Palm Beach, Florida. (Photo: Rhona Wise/AFP/Getty Images)

Donald Trump addresses the media following victory in the Florida state primary on March 15, 2016, in West Palm Beach, Florida. (Photo: Rhona Wise/AFP/Getty Images)

With big wins in four of last night's primaries, Donald Trump further tightened his lead on a Republican presidential nomination that looked impossible to political leaders—and many Americans—six months ago. Think pieces on his rise just keep coming. One big question they all seem to tackle: Why do so many Americans support Trump's xenophobia? One strange but possible answer: Blame our bodies' systems for fighting diseases.

Research has stacked up over the past decade supporting the parasite stress theory of values—a theory that seeks to explain human culture through biology. The underlying idea of the parasite stress theory is that, to survive and reproduce, people behave in ways to reduce their risk of disease, showing what researchers call a behavioral immune response. The more a population is threatened by disease, the more its culture will revolve around actions to minimize that threat—say, keeping out perceived "outsiders" by building a wall. Communities facing higher disease stress will be the most conservative and show the strongest behavioral immune response, the theory predicts. That means demonstrating more xenophobia, traditionalism, and conformity—values linked to authoritarianism, the political profile some researchers believe fits Trump's voter base.

We reported in January on the evidence that Trump is riding the waves of authoritarian beliefs—a deep desire for order and a return to some kind of status quo. Authoritarians are also more likely to support the punitive use of violence, a noteworthy feature of recent Trump rallies. These extreme reactions stem from the behavioral immune response, according to Randy Thornhill, a University of New Mexico researcher and pioneer of parasite stress theory who was profiled by Ethan Watters in the May/June 2014 issue of Pacific Standard.

The biological drive to stave off disease, Thornhill says, has fueled our psychological capacity for hostility toward anyone seen as an outsider—so, in the Trump camp, that's everyone from Syrian refugees to Mexican immigrants (the latter of whom he has specifically blamed for infectious disease). That psychological capacity flares up when we feel most threatened. Right now, plenty of Americans perceive a threat in nationwide social change and the specter of terrorism, as explored in a recent Vox essay connecting Trump's rise to American authoritarianism.

If Trump's popularity stems from authoritarian leanings driven mostly by an anti-disease response, then his support should be strongest in the states with the highest rates of infectious disease. Thornhill has ranked states (and Washington, D.C.) using 1993–2006 data from the Centers for Disease Control and Prevention on rates of disease caused by parasites. (Parasite load tends to stay constant within a region, Thornhill says, meaning the rates should look pretty similar today.) Based on those numbers, D.C. and Mississippi have the highest parasite disease loads, exacerbated by wealth gaps, Thornhill says.

Mississippi's Republican primary called for Trump, as parasite stress theory predicts, but D.C.'s small population of Republican voters went with Marco Rubio. Maine and New Hampshire have the lowest disease rates and generally lean left, in keeping with the theory; their Republicans voted for Ted Cruz and Trump, respectively. These discrepancies speak to the limitation of any theory in fully explaining culture.

Some researchers strongly question parasite stress as a major causal force in human behavior. But as Watters wrote in 2014, the coming years will provide plenty of real-world tests:

Higher temperatures, elevated sea levels, and increased precipitation in some areas—all predicted to accompany climate change—are expected to bring tropical diseases to higher latitudes and elevations in the coming decades. Pathogens that once perished in cold climates and dry soils may find new congenial zones of heat and moisture, and new host populations. Incidents of dengue fever in the U.S., for example, are expected to spread beyond Hawaii and the Mexican borderlands as climate change creates expanding habitats for the mosquito that carries the virus. Unless effective health interventions ward off these new threats, humans in ever higher latitudes may again have to resort to their embedded psychological and cultural defenses. Collectivist group behaviors may yet stage a comeback. Behold the power of perceived disease threat.

That sort of comeback played into the 2014 mid-term elections, when paranoia around the Ebola virus prompted more conservative voting, as we reported on Monday:

Three additional [Ebola] cases were announced in October [2014], leading to massive news coverage that produced levels of fear disproportionate to the actual risk. "One mid-October poll showed that 65 percent of Americans feared a widespread Ebola epidemic," researchers note. While this threat was never real, Google Trends data found that "on one day during mid-October, Americans searched for 'Ebola' even more than they searched for 'porn'."

So to clinch the GOP nomination, Trump might not even need another disease outbreak—just the rumor of one would do.


Since We Last Spoke examines the latest policy and research updates to past Pacific Standard news coverage.