I. “My Jaw Dropped”

Why do we have the political opinions we have? Why do we embrace one outlook toward the world and not another? How and why do our stances change? The answers to questions such as these are of course complex. Most people aren’t reading policy memos to inform every decision. Differences of opinion are shaped by contrasting life experiences: where you live; how you were raised; whether you’re rich or poor, young or old. Emotion comes into the picture, and emotion has a biological basis, at least in part. All of this and more combines into a stew without a fixed recipe, even if many of the ingredients are known.

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On rare occasions, we learn of a new one—a key factor that seems to have been overlooked. To a surprising degree, a recent strand of experimental psychology suggests, our political beliefs may have something to do with a specific aspect of our biological makeup: our propensity to feel physical disgust.

In the mid-2000s, a political scientist approached the neuroscientist Read Montague with a radical proposal. He and his colleagues had evidence, he said, that political orientation might be partly inherited, and might be revealed by our physiological reactivity to threats. To test their theory, they wanted Montague, who heads the Human Neuroimaging Laboratory at Virginia Tech, to scan the brains of subjects as they looked at a variety of images—including ones displaying potential contaminants such as mutilated animals, filthy toilets, and faces covered with sores—to see whether neural responses showed any correlation with political ideology. Was he interested?

Montague initially laughed at the idea—for one thing, MRI research requires considerable time and resources—but the team returned with studies to argue their case, and eventually he signed on. When the data began rolling in, any skepticism about the project quickly dissolved. The subjects, 83 in total, were first shown a randomized mixture of neutral and emotionally evocative pictures—this second category contained both positive and negative images—while undergoing brain scans. Then they filled out a questionnaire seeking their views on hot-button political and social issues, in order to classify their general outlook on a spectrum from extremely liberal to extremely conservative. As Montague mapped the neuroimaging data against ideology, he recalls, “my jaw dropped.” The brains of liberals and conservatives reacted in wildly different ways to repulsive pictures: Both groups reacted, but different brain networks were stimulated. Just by looking at the subjects’ neural responses, in fact, Montague could predict with more than 95 percent accuracy whether they were liberal or conservative.

The subjects in the trial were also shown violent imagery (men pointing revolvers directly at the camera, battle scenes, car wrecks) and pleasant pictures (smiling babies, beautiful sunsets, cute bunnies). But it was only the reaction to repulsive things that correlated with ideology. “I was completely flabbergasted by the predictability of the results,” Montague says.

His collaborators—John Hibbing and Kevin Smith at the University of Nebraska at Lincoln, and John Alford at Rice University, in Houston—were just as surprised, though less by the broad conclusion than by the specificity of the findings and the startling degree of predictability. Their own earlier research had already yielded a suggestive finding, indicating that conservatives tend to have more pronounced bodily responses than liberals when shown stomach-churning imagery. However, the investigators had expected that brain reactions to violent imagery would also be predictive of ideology. Compared with liberals, they’d previously found, conservatives generally pay more attention—and react more strongly—to a broad array of threats. For example, they have a more pronounced startle response to loud noises, and they gaze longer at photos of people displaying angry expressions. And yet even in this research, Hibbing says, “we almost always get clearer results with stimuli that are disgusting than with those that suggest a threat from humans, animals, or violent events. We have an ongoing discussion in our lab about whether this is because disgust is simply a more powerful and more politically relevant emotion or because it is an emotion that is easier to evoke with still images in a lab setting.”

Findings so dramatic, especially in the social sciences, should be viewed with caution until replicated. The axiom that extraordinary claims demand extraordinary proof clearly applies here. That said, Hibbing, Montague, and their colleagues are scarcely alone in linking disgust and ideology.

Using a far cruder tool for measuring sensitivity to disgust—basically a standardized questionnaire that asks subjects how they would feel about, say, touching a toilet seat in a public restroom or seeing maggots crawling on a piece of meat—numerous studies have found that high levels of sensitivity to disgust tend to go hand in hand with a “conservative ethos.” That ethos is defined by characteristics such as traditionalism, religiosity, support for authority and hierarchy, sexual conservatism, and distrust of outsiders. According to a 2013 meta-analysis of 24 studies—pretty much all the scientific literature on the topic at that time—the association between a conservative ethos and sensitivity to disgust is modest: Disgust sensitivity explains 4 to 13 percent of the variation in a population’s ideology. That may sound unimpressive, but it is in fact noteworthy, says David Pizarro, a psychology professor at Cornell who specializes in disgust. “These are robust, reliable findings. No matter where we look, we see this relationship”—a rarity in the fuzzy field of psychology. The trend stands out even more, he adds, when you consider all the other things that potentially impinge on “why you might have a particular political view.”

II. The Behavioral Immune System

Broadly speaking, studies of possible connections between ideology and susceptibility to disgust fall into two categories. The first involves measuring subjects’ sensitivity to disgust as well as their social or political ideologies and then calculating the correlation between the two. The second category explores whether exposure to disgusting subject matter can actually influence people’s views in the moment. But whatever the type of study, the same general finding keeps turning up. “We are at the point where there is very solid evidence for the association,” says Michael Bang Petersen, a political scientist at Aarhus University, in Denmark. His own research finds that “disgust influences our political views as much as or even more than long-recognized factors such as education and income bracket.”

So many scientists have thrown themselves into this line of research in recent years that it has become an accepted discipline, sometimes jokingly given the aptly unappetizing name “disgustology.” Their conclusions raise a lot of questions, chief among them: Why in the world would your reaction to mutilated animals, vomit, and other unwelcome things somehow be associated with your views on transgender rights, immigration, or anything else stirring debate in the news?

Jeff Brown

Researchers have theories rather than answers. At a deep, symbolic level, some speculate, disgust may be bound up with ideas about “them” versus “us,” about whom we instinctively trust and don’t trust. In short, this research may help illuminate one factor—among many—that underlies why those on the left and the right can so vehemently disagree.

There is nothing inherently political about disgust. It evolved not to guide us at the ballot box but rather, it is widely theorized, to protect us from infection. As we move about in the world, a sizable volume of research shows, our minds are constantly searching our surroundings for contaminants—moldy leftovers, garbage spilling out of trash cans, a leaky sewage pipe—and when the brain detects them, it triggers sudden feelings of revulsion. Confronted, we withdraw from the threat. The mechanism is part of what’s known as the “behavioral immune system,” and it is as vital for survival as the fight-or-flight response. Our pathogen-tracking system does its job largely beneath our conscious awareness—and pays close attention to those walking germ bags we call human beings.

This dynamic was highlighted in a pioneering series of experiments launched in the early 2000s by the psychologist Mark Schaller, of the University of British Columbia. Like a smoke detector, Schaller discovered, our germ radar operates on a better-safe-than-sorry principle. It is error-prone in flagging danger—it produces a lot of false positives. Any physical oddity displayed by the people around us—contagious or not—can set off an alarm. Just as a pink eye, a hacking cough, or an open wound may activate our behavioral immune system, so too can a birthmark, obesity, deformity, disability, or even liver spots. Furthermore, having germs on our mind can affect how we feel about people we perceive to be of a different race or ethnicity from ourselves.

In one notable experiment, Schaller showed subjects pictures of people coughing, cartoonish-looking germs sprouting from sponges, and other images designed to raise disease concerns. A control group was shown pictures highlighting threats unrelated to germs—for instance, an automobile accident. Both groups were then given a questionnaire that asked them to assess the level of resources the Canadian government should provide to entice people from various parts of the world to settle in Canada. Compared with the control group, the subjects who had seen pictures related to germs wanted to allocate a greater share of a hypothetical government advertising budget to attract people from Poland and Taiwan—familiar immigrant groups in Vancouver, where the study was conducted—rather than people from less familiar countries, such as Nigeria, Mongolia, and Brazil. Familiarity does make a difference. Schaller, whose landmark studies are credited with sparking the initial interest in the relationship between disgust sensitivity and prejudice, says: “If I grow up in an environment where everybody looks pretty much the same, then someone from China, for example, might trigger my behavioral immune system. But if I grow up in New York City, then a person who comes from China is not going to trigger this response.”

If pathogen cues of this kind can indeed intensify prejudice, the explanation could be biological adaptation. Some scientists—notably the psychologist Corey Fincher, at the University of Warwick, in England, and the biologist Randy Thornhill, at the University of New Mexico—theorize that foreigners, at least in the past, would have been more likely to expose local populations to pathogens against which they had no acquired defenses. Other scientists think germ fears piggyback on negative stereotypes about foreigners common throughout history—the notion that they’re dirty, eat bizarre foods, and have looser sexual mores.

Whatever the explanation, an online study launched by Petersen and Lene Aarøe, also at Aarhus University, and Kevin Arceneaux of Temple University suggests that a dread of contagion is not just a personal matter. It can have an impact on society. The investigators began by evaluating the disgust sensitivity of nationally representative samples of 2,000 Danes and 1,300 Americans. The participants were then asked to fill out a questionnaire that assessed their views about foreigners settling in their respective countries. As the researchers reported in 2017, opposition to immigration in both the Danish and American samples increased in direct proportion to a participant’s sensitivity to disgust—an association that held up even after taking into account education level, socioeconomic status, religious background, and numerous other factors.

The team expanded the part of the study that focused on the U.S. It got state-by-state breakdowns of the prevalence of infections, and also analyzed statistics compiled by Google Trends, which tracks internet searches related to contagious illnesses in an effort to spot early signs of outbreaks. Crunching the numbers (the results are as yet unpublished), the researchers found that resistance to immigration is greatest in states with the highest incidence of infectious disease and where worry about this, as reflected by internet activity, has also been high.

More recent investigations by Petersen and Aarøe suggest that those with high disgust sensitivity tend to be leery of any stranger, not just foreigners. They view casual social acquaintances with a certain amount of suspicion—a robust finding replicated across three studies with a total of 4,400 participants. The implication is clear: Disgust and distrust are somehow linked. And maybe, again, the link is defensive in origin: If you shrink your social circle, you’ll reduce your exposure to potential carriers of disease.

III. The Smell Test

Interest in disgust sensitivity extends beyond its potential role in fostering xenophobia and prejudice. As the social psychologists Simone Schnall, at the University of Cambridge, and Jonathan Haidt, at NYU, have shown, disgust sensitivity may also help shape beliefs about right and wrong, good and evil. In one experiment, Schnall, Haidt, and other collaborators sat subjects at either a clean desk or one with sticky stains on it as they filled out a form that asked them to judge the offensiveness of various acts, such as lying on a résumé, not returning a wallet found on the street, and resorting to cannibalism in the aftermath of a plane crash. One subgroup of participants seated at the filthy desk—those with high “private body consciousness,” meaning they were particularly sensitive to their own visceral reactions—judged the transgressions more severely than those seated at the pristine desk.

Foul odors can be just as effective as a sticky desk. Another experiment involved two groups of subjects with similar political ideologies. One group was exposed to a vomitlike scent as the subjects filled out an inventory of their social values; the other group filled out the inventory in an odorless setting. Those in the first group expressed more opposition to gay rights, pornography, and premarital sex than those in the second group. The putrid scent even inspired “significantly more agreement with biblical truth.” Variations on these studies using fart spray, foul tastes, and other creative disgust elicitors reveal a consistent pattern: When we experience disgust, we tend to make harsher moral judgments.

Jeff Brown

In thinking about why disgust sensitivity may be associated with conservative moral values, researchers have considered the potential connection between the behavioral immune system and religion. Religious strictures and other traditions may have the hidden function of protecting us from disease, some theorize. Our urge to respect certain culinary practices, sexual prohibitions, and injunctions about washing and hygiene may not be just about achieving spiritual or symbolic purity, but may be the result of an evolutionary drive to avoid contamination.

Could a predilection toward revulsion indicate how we vote? A team led by Cornell’s David Pizarro and Yoel Inbar, at the University of Toronto, set out to answer that question by conducting an online study during the 2008 U.S. presidential contest between Barack Obama and John McCain. In the run-up to the election, the researchers assessed the contagion anxiety of 25,000 “demographically and geographically diverse” Americans and then surveyed the attitudes toward the candidates held by a random subset of the larger group. Those with the highest germ fear reported that they were more likely to vote for McCain, the Republican nominee and the more conservative candidate. Further, the actual proportion of votes that went to him in each state directly scaled with that state’s level of contagion anxiety. The researchers eventually extended studies of this kind to 121 countries and found that disgust sensitivity correlated with a conservative ethos basically everywhere there were sufficient data for analysis. As Pizarro, Inbar, and the other authors of the study write in the journal Social Psychological and Personality Science, this result suggests that disgust sensitivity “is related to conservatism across a wide variety of cultures, geographic regions and political systems.”

IV. “It’s a Little Hack”

Disgustology is a young endeavor. Not all the pieces fit together neatly, and some suppositions (as always) may turn out to be wrong. But a few clues have recently surfaced that suggest a useful framework for interpreting this sprawling mass of findings. One of them, in hindsight, is obvious: the etymology of disgust. The English word is derived from the Middle French desgoust, which literally means “distaste.” As it turns out, what tastes foul to us is typically a sour or bitter substance—which can be a marker of contaminants (think of spoiled milk). Several years ago, Pizarro learned that people vary tremendously in the number of bitter receptors they possess on their tongue, and thus in their taste sensitivity. What’s more, the trait is genetically determined. This got him wondering: If conservatives have a greater disgust sensitivity, are they also better at detecting bitter compounds? “It seemed like a really long shot,” Pizarro says. But he, Inbar, and Benjamin Ruisch, a grad student at Cornell, decided to put the idea to the test. They recruited 1,601 subjects from shopping malls and from the Cornell campus and gave them paper strips containing a chemical called Prop and another chemical called PTC, both of which taste bitter to some people. Sure enough, those who had self-identified as being conservative were more sensitive to both compounds; many described them as unpleasant or downright repugnant. Liberals, on the other hand, tended not to be bothered as much by the chemicals or didn’t notice them at all.

The researchers went a step further. Taste receptors, they knew, are concentrated in fungiform papillae—those spongy little bumps on your tongue. The greater the density of papillae, the more acute your taste. So they dyed subjects’ tongues blue (which allows the papillae to be more easily observed), pasted a paper ring on them like those used to prevent pages from tearing out of a metal binder (to create a standard area to be evaluated), and recorded the number of circumscribed papillae. The degree to which subjects’ views tilted to the right was, they found, in direct proportion to the density of papillae on their tongue. This result may have bearing on a puzzling partisan split in food preferences. A 2009 survey of 64,000 Americans revealed that liberals chose bitter-tasting arugula as their favorite salad green more than twice as often as conservatives did. It may also have a bearing on conservative President George H. W. Bush’s famous hatred of broccoli—an unusually bitter vegetable. Of course, sometimes a stalk of broccoli is just a stalk of broccoli.

No doubt your own political allegiances will heavily influence what you extract from the bulk of this research. If you’re liberal, you may be thinking, So this explains some of the other side’s nativism and hostility to immigration. But it’s just as easy to flip the science on its head and conclude, as conservatives might, that the left is composed of clueless naïfs whose rosy-eyed optimism about human nature—and obliviousness to various dangers—will only lead to trouble.

The research itself does not speak to the relative merits of a conservative or liberal ethos—how could it? Conservatism and liberalism are not monolithic, and they rest on deep intellectual traditions. In terms of gut reactions, the relative appeal of each philosophy can depend significantly on context—for instance, on whether times are kind or cruel. When tensions are high and groups split into factions, as they inevitably do, we can depend on our family and friends to defend our interests—but the outsider is an unknown quantity and, from an evolutionary perspective, may be seen as a source of contamination or, more generally, a threat.

One defining characteristic of disgust, though, is that it occupies a blind spot in our psyche. As Pizarro notes, “It’s such a low-level, almost noncognitive emotion that you really aren’t thinking that much about it.” Compared with anger, happiness, and sadness, he says, disgust is also “less open to change based on your judgment, your thoughts, your reasoning.” Chocolate in the shape of dog poop, he points out, is still gross. The emotion is more reflexive than reflective. “That is the rhetorical strength of disgust,” Pizarro says. “It’s a little hack. You hack into brains pretty quickly and easily by making them feel disgust,” bypassing logic and reason to sway judgment.

Aristotle may not have found this idea surprising. As he intuited millennia ago, a human being “is by nature a political animal”—uniquely endowed with the capacity for deliberation and speech, but at the same time governed by instincts we share with other living creatures. Like bees, he noted, we have a desire to congregate—to form societies. Aristotle could not have anticipated the germ theory of disease, or the role infection avoidance might unconsciously play, but his fundamental insight about the animal side of our politics remains prescient. Even the most rational among us might not always be as rational as we’d like to think.

This article appears in the March 2019 print edition with the headline “The Yuck Factor.”