Campaign insiders and TV pundits may have overlooked an important factor coloring our politics: germs.

Worries about all manner of pathogens — disease-causing bacteria, viruses, and larger parasites — are an underappreciated contributor to prejudice, distrust of foreigners, and resentment toward those who spurn traditional values, according to a growing body of psychology research.

To understand why, it helps to be acquainted with the behavioral immune system, our defense against infection that’s shaped by natural selection and further embellished by learning. Largely below the level of our conscious awareness, we constantly scan our surroundings for any potential source of contagion. If we encounter one — a bug, garbage, dog feces — our minds warn us to retreat by triggering feelings of disgust. But this germ radar is not guided by sophisticated reasoning, and it pays particularly close attention to other people, a leading source of infection. It can lock onto any abnormality — contagious or not. Someone with an oozing eye, runny nose, acne or even excessive body fat can raise our disgust level.

So too can a person whom we perceive to be of a different race or ethnicity — particularly if we’re squeamish by temperament or prompted to think of germs. This finding, brought to light by psychologist Mark Schaller at the University of British Columbia and confirmed by several labs, may stem from a protective adaptation or prejudice rooted in history. Especially in centuries past, the researchers point out, foreigners were more likely to carry exotic germs that could be highly virulent to local populations.


When we’re experiencing high levels of visceral disgust, we’re also more likely to be repulsed by those who violate societal norms. Schaller and other scientists speculate one reason could be that many customs — washing before prayer, marrying only within one’s religious group, bowing instead of handshaking — arose at least in part to protect us from infection.

So how does this connect to politics? It turns out that conservatives typically require less provocation to feel nauseated. An experiment conducted by John Hibbing and collaborators at the University of Nebraska provides a simple demonstration of this physiological response at work. Compared to liberals, they found, conservatives who looked at stomach-turning images such as a person eating worms sweated more profusely (as measured by galvanic skin conductance).

Large population surveys have shown a relationship between germophobia and xenophobia.

Further, large population surveys have shown a relationship between germophobia and xenophobia. In a study led by Danish political scientists Michael Bang Petersen and Lene Aarøe, a representative sample group of roughly 2,000 Danes and 1,300 Americans took an online test that ranked their sensitivity to disgust, and then their attitudes toward allowing foreigners into their respective countries was assessed. After controlling for every variable the researchers could think of — race, age, gender, education, socioeconomic factors — scientists found that opposition to immigration increased in direct proportion to individuals’ disgust sensitivity in both samples.


A habit of fretting about germs may even predict voting behavior. A Cornell University team led by psychologists David Pizarro and Yoel Inbar conducted an online disgust-sensitivity survey of 25,000 Americans during the 2008 U.S. presidential election. Respondents with the highest contagion-anxiety scores were the most likely to indicate that they would vote for Republican nominee Sen. John McCain, the more conservative candidate, over Democrat Barack Obama. Moreover, a state’s average level of disgust-sensitivity actually predicted the proportion of votes cast for McCain. The Cornell team went on to show that disgust sensitivity similarly tracked with political beliefs in 122 other countries — basically wherever a nation’s response rate was high enough to obtain a statistically meaningful result.

Microbes and parasites may even affect geopolitical trends more broadly. A radical new theory posits that the prevalence of infectious disease is a major sculptor of geopolitics. Proponents of this view — called the parasite-stress model of values and sociality — believe that where pathogens are rampant citizens are more pressured to obey hygiene rules, sexual practices and other cultural traditions that have the hidden purpose of shielding them from germs. As a result, anyone who bucks convention is treated with hostility — and those conditions favor repressive political regimes.

Two champions of this controversial perspective — evolutionary biologists Randy Thornhill at the University of New Mexico and Corey Fincher at the University of Warwick in England — sought support for their thesis by correlating nations’ levels of infectious disease and their systems of governance. Countries where parasites (including tiny pathogens) were a huge threat, they discovered, were indeed more likely to be ruled by dictators. In contrast, nations with much lower parasite loads were overwhelmingly democracies. Thornhill and Fincher have since gone on to connect parasitic hot zones around the globe to higher levels of social unrest, ethnic violence, religious intolerance and racial and class stratification. Whether their theories — currently a topic of intense scientific debate — will hold up to scrutiny remains to be seen.

That being said, it certainly makes sense that the trauma parasites have inflicted on us down through the ages has left a deep mark on the human psyche. Malaria alone has claimed the lives of half of all humans born since the Stone Age. Next to parasites, saber-toothed tigers and the world’s worst tyrants look like pussycats.


Whether or not pathogens shape the contours of entire societies, we can say with confidence that a dread of contagion can warp our personal values. If people are made aware of this unconscious bias, will it tilt attitudes leftward? Democrats might want to find out because Donald Trump — a self-professed germophobe — is doing an excellent job exploiting the disgust of the Republican base.

Kathleen McAuliffe is a science writer and author of the recently- published book “This Is Your Brain On Parasites: How Tiny Creatures Manipulate Our Behavior and Shape Society.”

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