Ask a man of the Shuar tribe in Ecuador’s Amazon what disgusts him the most, and he might say eating raw meat; stepping in human feces; or drinking chicha, a traditional alcohol, made “with the spit of a toothless woman,” says Lawrence Sugiyama, an anthropologist at the University of Oregon.
Feeling grossed out by potentially contaminated food or spit from an unhealthy person turns out to be a wise reaction for the Shuar, Indigenous people who hunt, gather, and grow crops in the rainforests of south-central Ecuador. In the first study of disgust and the health of Indigenous people, Sugiyama and his colleagues have found that the Shuar who felt the most revolted by raw or spoiled food, or other potential sources of pathogens, are less likely to be fighting a viral or bacterial infection. “Those individuals who scored higher on the Shuar disgust scale had lower levels of immune activation,” says biological anthropologist Joshua Snodgrass of the University of Oregon, a co-author of the study.
In 1872, Charles Darwin proposed that disgust was an innate emotion that evolved because it helped our ancestors avoid eating tainted food. People with disgust thus had more chances to reproduce and so passed on the genes that make us feel revulsion. Researchers have since shown disgust does appear to protect the health of humans in relatively wealthy and sanitary cultures. But no one had studied whether disgust was protective for people living in traditional subsistence societies under conditions similar to those in which our hunter-gatherer ancestors evolved.
Tara Cepon-Robins, an anthropologist at the University of Colorado, Colorado Springs, surveyed 75 Shuar men and women in three Indigenous Ecuadorian Shuar communities as part of her dissertation at the University of Oregon. The Shuar she interviewed all lived in environments with many pathogens, such as roundworm, whipworm, and tuberculosis. Their communities had differing levels of economic development, ranging from open huts with dirt floors to government-built houses with concrete floors and closer proximity to food markets. Cepon-Robins and her colleagues adapted a survey that ranks the objects of disgust, and then gave it to the Shuar. Individuals rated such events as seeing people vomit, touching raw meat, finding maggots in their food, and seeing a rodent where they stored their food.
The researchers then analyzed blood and fecal samples from their subjects that had been collected since 2005 by the Shuar Health and Life History Project. When they examined the samples for molecular markers of acute immune responses to bacterial or viral infections, they found that the Shuar who scored highest on the disgust scale had the lowest signals of infection, they report today in the Proceedings of the National Academy of Sciences. (The Shuar don’t usually suffer from chronic inflammation like Americans and Europeans, so markers of an acute immune response are a reliable indicator that their immune systems are fighting an infection.)
In an interesting twist, the researchers also found that sensitivity to disgust can change with the environment. Those Shuar who lived in the most basic conditions—thatched huts with dirt floors, which often had direct exposure to soil pathogens, animal feces, and contaminated water—showed less disgust than Shuar who lived in homes with concrete floors, clean water, and easier access to food markets. “If you have an open house with chickens coming in, and you can’t really clean up the animal feces in the dirt floor, you can’t afford to be too disgusted,” Cepon-Robins says. “But if you can afford to avoid those things, your disgust will elevate to allow you to avoid the exposure to pathogens.”
Those findings are significant, say researchers not involved in the work. “This paper is so exciting because it’s filled one of those missing pieces of evidence that is really difficult to gather in the Western world where parasite stress and exposure to infectious disease is relatively low,” says evolutionary psychologist Josh Tybur of Vrije University, Amsterdam.
Evolutionary psychologist Debra Lieberman of the University of Miami agrees: “What I really like about this study is that it provides provocative evidence that disgust sensitivity evolved to adapt to the local environment.” Even Shuar who grew up in huts with dirt floors have become more sensitive to disgust over their lifetimes as they moved into houses with concrete floors, and closer to markets where they could buy food, Sugiyama says.
sciencemag.org, 15 February 2021