Last week the Atlantic published a story about liberals and conservatives and the scientific evidence that you can often tell one from the other by looking at how people respond to disgust. If you aren’t familiar with this area of research, the idea is that some political opinions may stem from more fundamental biological or psychological differences in the population. Disgust or revulsion is just one of those. No one is saying that people are absolutely determined to be right or left based on this alone, but the relationship between the two things seems pretty reliable:

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.”

Disgust is believed to have evolved for a specific purpose: to keep us away from potential pathogens. So when you see something that looks like decay—maggots crawling on meat for instance—your brain tells you to stay away from it. All of this precedes any theory of the cell or understanding of virology, perhaps by millions of years. It’s a kind of blunt instrument to keep you healthy, what the article calls a behavioral immunity system. And there’s some evidence that disgust makes us wary, not just of maggot-ridden meat, but of strangers. Why would this make sense? Because in the past encountering a new population was a potential threat, not just physically but in terms of disease transmission.

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…

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.

Other experiments have used smells rather than images and they get a similar result:

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.

The most interesting part of the research into disgust is the evidence that what we call disgust, literally that which is distasteful, may stem partly from our physical ability to taste bitter and sour things:

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’s relatively common parlance these days for people on the left and the right to talk about the other side as people living in a different world. What’s interesting about this research is that, to some degree at least, it suggests there may be some truth to that idea. Yes, we all inhabit one world but there may be cohorts of people who experience it, literally and physically, in different ways. It’s interesting to think about the degree to which our partisan differences could be determined by more fundamental aspects of our biology.