Most of us mature adults can agree picking your nose is disgusting. So is defecating in public—no matter how much you’ve had to drink, or what philosophical point you’re trying to make. Rancid food, weeping sores, cockroaches—most of us can agree that these are pretty gross, too.
But what makes us feel that way? It’s a deep-seated, visceral response; we don’t consciously reason through it, but most of us seem to share the same sense that certain things are just repulsive.
Researchers have long believed that disgust has an evolutionary origin: It may have helped steer our ancestors away from disease. A new study builds on that idea, suggesting that disgust protects us from people, behaviors, and things that may pose a risk of disease. It also establishes six categories of disgust, each linked to the risk of disease.
We find poor hygiene abhorrent, for example, because picking your nose and not bathing increases your chances of getting sick—and thereby our chances of catching something from you. It’s the same with avoiding food that looks (or smells) spoiled, animals that carry illness (cockroaches, fleas), and wounds or lesions that signal infection. And when it comes to people, we tend to be put off by atypical appearances and risky sex practices, both of which can imply heightened risk of disease.
“It shows us that disgust as an emotion evolved to make us do things that would have been good for us in the evolutionary past,” says Val Curtis, professor at the London School of Hygiene & Tropical Medicine and co-author of the study. “Rather than being organized by category of infectious disease, the brain’s architecture is organized by categories of things that it’s a good idea to avoid coming into contact with.”
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To reach that conclusion, researchers surveyed more than 2,500 people online, providing them with a list of 75 potentially disgusting situations, from hearing someone sneeze to seeing pus-filled skin lesions. The participants then rated their responses on a scale from “no disgust” to “extreme disgust.” (Infected wounds producing pus were overall the most disgusting, and hygiene norms being violated were also particularly gross.)
One interesting finding is common to disgust studies, Curtis says: Men tend to be slightly less grossed out than women, with about a 12 percent difference in their disgust scores. That difference may also have an evolutionary origin. “On the whole men tend to take more risks,” Curtis says, “because in the past men who took more risks had more offspring, while women who were more careful were able to bring up more children successfully.” Men, the research suggests, might be less likely to be grossed out by signs of sexually transmitted diseases, and more likely to go forward with risky sexual behavior.
Curtis notes, however, that this is an online survey—a review of what people say disgusts them. While it would be great to do more empirical testing to see how people really respond to disgusting situations, “That is a big, expensive and perhaps ethically challenging study,” Curtis says.
But that doesn’t mean the results are totally abstract. The research could be used to better tailor public-health messaging, for example, to encourage handwashing or destigmatize certain illnesses. And ultimately, Curtis says, it’s about understanding how emotions in general may have evolved to promote certain advantageous behaviors. If disgust helps keep us from risking our health, what might other emotions do for our survivability? That’s what Curtis and her colleagues want to explore in the future.
This article originally appeared on Tonic.