Yuck! Why humans are hardwired to feel disgust

Evolution has primed us to avoid anything icky to ward off disease. But science also shows that disgust can be a double-edged sword.

Published 31 Mar 2021, 13:59 BST
digust
Children who physically interact with a dog, under the age of one years old or so, will have a 13 percent reduction in the likelihood of developing asthma.
Photograph by Louise Johns

In the late 1860s, Charles Darwin proposed that being grossed out could have an evolutionary purpose. Disgust, he wrote, was inborn and involuntary, and it evolved to prevent our ancestors from eating spoiled food that might kill them. Darwin hypothesised that the early humans most prone to revulsion survived to pass on their genes, while the more nutritionally daring died off.

For many years afterward, though, scientists didn’t pay much attention to disgust. It wasn’t until the early 1990s, a decade when gameshows eagerly slimed contestants, that disgust garnered more attention in psychological and behavioural research. Since then, scientists have identified different types of disgust and have explored how they affect the way we behave.

The research shows that Darwin was basically right: Disgust is a major facet of the behavioural immune system, a collection of actions influenced by some of the most primal instincts that keep our bodies in prime condition.

“In terms of keeping us healthy, disgust is associated with fewer infections, so it is a helpful emotion in disease-relevant contexts,” says Joshua Ackerman, an associate professor of psychology at the University of Michigan. In January, for instance, researchers reported that people more innately prone to disgust have indeed fared better during the COVID-19 pandemic, probably because they are inclined to engage in more hygienic actions like hand-washing.

However, disgust is far more complex than even Darwin imagined. Studies also show that what, exactly, we find disgusting stems from a patchwork of innate responses and a variety of life experiences that depend on our culture and environment. And for some people, disgust can go too far, preventing us from doing the gross things that actually keep us healthy, such as eating probiotic-rich fermented foods. 

“It can be a double-edged sword because it also is associated with aversion to unfamiliar things, like food, some of which could actually improve our health and immune functioning,” Ackerman says.

Here's what the latest science says about the protective effects of disgust, why some people—especially kids—are drawn to gross stuff, and the ways humans have hijacked this psychological response to fit various cultural norms and reap some intriguing health benefits.

The roots of disgust

On the Amazon rainforest’s Ecuadorian edge, a team of anthropologists ventured out in 2005 to meet the Shuar, an indigenous tribe once known for creating shrunken heads from their slain enemies. They disavow the practice now, and many communities welcome commerce, tourism, and scientists from around the world hoping to learn from their way of life. One such visitor was Tara Cepon-Robins, an expert in parasites from the University of Colorado, Colorado Springs.

Almost a century and a half after Darwin penned his proposal about disgust, Cepon-Robins had set out to study how culture, environment, and emotion influence the ways humans shield their bodies from disease. Until then, most similar studies had focused on industrialised cultures. But to better understand disgust's evolutionary purpose, researchers had to study it in a high-pathogen environment that more closely resembles how our ancestors lived.

“Childhood is effectively a boot camp for the immune system—at least, until a certain age. ”

Tucked away in the misty shadows of the Andes, some of the Shuar volunteers lived in traditional huts with dirt floors, while others lived in houses with concrete floors and metal roofs. Many participated in subsistence activities such as hunting, fishing, horticulture, and foraging that brought them in contact with possible pathogens, including roundworms and whipworms, which thrive in soil contaminated with excrement. Cepon-Robins surveyed 75 participants about what disgusted them.

“They were most disgusted by things like directly stepping in faeces and drinking chicha, a drink that’s made by chewing up yuca and spitting it out,” Cepon-Robins says. Chicha is a traditional fermented drink, and it’s one of the main sources of water in more rustic Shuar communities. Chicha itself isn’t what disgusted the respondents, but rather who made it. “Drinking chicha from someone who’s sick or who has rotten teeth is what they found disgusting,” she says.

The researchers then collected blood and faecal samples from the participants and compared their health to their level of disgust. As the scientists reported this February in Proceedings of the National Academy of Sciences, the individuals who scored highest in disgust sensitivity had the fewest viral and bacterial infections.

In the communities studied, respondents couldn’t avoid things that some people in industrialised cultures might find gross, like dirt, and their disgust didn’t protect them from larger parasites. Still, disgust helped them minimise contact with bodily excretions that might carry microbial diseases, leading Cepon-Robins to believe that disgust evolved to defend our ancestors from illness, just as Darwin hypothesised.

If that’s true, what makes many kids so enthusiastic about slime and grime?

Why do we love being gross?

In a somewhat counterintuitive twist on Darwin’s theory, kids might love being gross because it gives them an evolutionary advantage.

We already know that not all germs are bad for us. From our gut flora to the germs on our skin, microbes work with our immune systems to maintain our body’s equilibrium, protect us from pathogens, and more. Science also tells us that wallowing in a little filth, especially with activities that bring kids close to the soil or in contact with animals, helps them build stronger immune systems that can more readily fight disease.

“It is less about getting dirty than it is enabling them to interact with the world around them,” says Jack Gilbert, a paediatrics professor at the University of California, San Diego. Gilbert doesn’t run after his kids with disinfecting wipes. He lets them experience nature’s bouquet of microbes, because he knows their future immune systems depend on it.

“Children that physically interact with a dog, under the age of one years old or so, will have a 13 percent reduction in the likelihood of developing asthma,” he says. “Kids who grow up on a farm interacting with lots of farm animals have a 50 percent reduction. That exposure is actually very important for stopping chronic allergic diseases.”

Childhood is effectively a boot camp for the immune system—at least, until a certain age. One study from 2014 shows that for most kids, disgust sensitivity starts to kick in around five. That’s right around the time kids are more likely to be exposed to more dangerous forms of microbial life, such as the respiratory syncytial virus and Giardia, a microscopic parasite that causes diarrhoea.

“This is an age at which they've been weaned, and so they're starting to find food for themselves and put a lot of things in their mouths, but their immune systems aren't fully developed,” says study author Joshua Rottman, an assistant professor of psychology at Franklin & Marshall College in Lancaster, Pennsylvania. “A lot of really young children die every year due to pathogens and parasites. That might be in part because they're not disgusted.”

Some adults find gross things compelling, too. We judiciously inspect the contents of our tissues, watch gory movies, enjoy slimy food, and take strange enjoyment in squeezing our spots. What’s wrong with us?

The jury’s still out on this one. But researchers have a few ideas. Some experts, including Rottman, attribute our zeal for the ick factor to “benign masochism,” in which our brains find pleasure in negative things. Others hypothesise it’s our subconscious bent for problem solving that makes the gross so compelling.

“It’s related to the value of learning about a threat, so that you may better protect yourself in the future, or neutralising the threat now,” says Laith Al-Shawaf, an assistant professor of psychology at the University of Colorado, Colorado Springs. “So, if your kid gets an open wound and there's pus oozing out, you have to go learn more about it and attend to your kid and help them.”

Both of these could be true. There’s also a third hypothesis: Dirt could still be good for adult immune systems, says Gilbert. “I think of the immune system like a gardener,” he says. “It’s there to maintain the garden of microbes we come in contact with every day, to keep the good ones around and the bad ones out. The good ones have a massive impact on our health.”

Highly cultured

For most adults, though, what humans find disgusting differs by culture and environment—aside from a short list of common items.

“A number of things that are pathogenic are disgusting universally, so faeces, vomit, open wounds, pus—sorry, this is kind of a gross conversation,” says Al-Shawaf, laughing. “Also, rotten foods—rotten meat is almost universally disgusting, and the commonality between all these things is they pose a pathogen risk.”

However, even things humans are primed to find gross might confer health benefits.

“Many nomadic arctic cultures in places like Greenland and northern Scandinavia routinely eat rotted meat,” says Rottman. “It actually helps them get vitamin C and prevent scurvy. So it’s a common part of their diet; they don't think it's disgusting.”

As unbelievable as it might sound, rotted meat was an integral part of the palaeolithic diet. Allowing meat to putrefy makes it easier to digest and helps it retain vitamin C, aka ascorbic acid, bv lowering its pH. By contrast, the more common practice of cooking meat degrades that crucial vitamin. Ancient arctic people who found putrefied meat revolting might not have lasted through winter.

An overly heightened sense of disgust, a phobia of unfamiliar foods, or a lack of cultural education can also prevent some people from eating and living more adventurously—and perhaps enjoying similar benefits. Many people in Western societies enjoy eating shrimp, but they might turn up their noses at eating other arthropods such as crickets, a dietary staple in other parts of the world. There’s nothing wrong with eating crickets. They’re just different from what we’re used to, and increasingly people are promoting crickets as a more environmentally friendly source of protein.

Researchers across fields continue to explore disgust so we can better understand our world and ourselves. Disgust is part of society’s equilibrium. Too little, and we might get sick. Too much of it can isolate us and even harm our health. Continuing to unravel this complex tapestry could help researchers decipher a variety of human behaviours.

“There are things across the board that we find disgusting, but habituation can occur,” Cepon-Robins says. “Nurses get used to dealing with bodily fluids. Our fear of things that might be a little gross wears off when we’re exposed to them over and over again—and they don’t kill us.”

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