The Biology of . . . Disgust

Warning: This article could make you sick to your stomach

By Josie Glausiusz|Sunday, December 01, 2002


A crowd has gathered at the New Museum of Contemporary Art in downtown Manhattan. Before them sits an intricate ramble of tubes, flasks, cogs, wires, and gauges 33 feet long and more than six feet high. The structure mimics all the digestive processes of the human gut. Twice daily, local chefs prepare meals that are fed to the machine, which quietly churns them through six glass vats that contain a carefully calibrated mix of enzymes, acids, bile, and bilirubin. Now the moment has arrived. As two little red-haired girls stare, bug-eyed, a long brown lump of excrement emerges silently from the end of the apparatus. It rolls a few feet along a green conveyor belt, and stops. A Japanese tourist solemnly snaps a photo.

Designed by Belgian artist Wim Delvoye, the exhibit, known as Cloaca, arouses strong emotions. Some people are derisive. Many are disgusted. But all are fascinated. Indeed, the line that divides disgust from fascination would seem to be a slim one. Disgusting objects fascinate. So, too, does the emotion of disgust itself. It fascinated Charles Darwin, who wrote about its origins in his 1872 book The Expression of the Emotions in Man and Animals. It fascinates Valerie Curtis, an epidemiologist at the London School of Hygiene and Tropical Medicine who travels the world with a lump of artificial feces in her suitcase. It fascinates neuroscientists, who are pinpointing the areas of the brain that process it. And it fascinates psychiatrists, who want to know what happens when we can no longer recognize it.

Moral disgust—the repugnance people feel toward child abuse, say, or political corruption—is an especially elusive quarry. Some scientists think that it evolved as a means of policing social parasites—those who have power and use it to prey upon the poor or the vulnerable.
Unlike fear or anger, disgust is an emotion that until recent years has been largely neglected by neuroscience. One researcher, Mary Phillips, has described it as "the forgotten emotion of psychiatry." Yet disgust appears to be universal. Darwin described examples of it from Abyssinia to Greenland, in people responding to bad tastes, bad odors, and—in his case—the sight of his food "being touched by a naked savage." For Darwin, disgust was, quite literally, something offensive to the taste. For Valerie Curtis, disgust is an emotion crucial to human survival.

Curtis studies diarrheal diseases—the number two killer of children in the world today—and is keen to understand how disgust helps motivate people to maintain good hygiene. (She uses her plastic feces as a prop in talks on the topic.) In a recent global survey, Curtis asked people in five places—India, the Netherlands, Britain, the West African country of Burkina Faso, and Athens International Airport—to describe what disgusts them. The results revealed some regional variations: "Lower castes" and "kissing in public" aroused disgust in India, whereas the British were particularly repulsed by dead sparrows and cruelty to horses; politicians and dog saliva revolted the Dutch, while airport travelers named everything from "wet people" to being eaten alive by insects. Yet it was the common threads that intrigued Curtis. Every region considered feces disgusting, while vomit, sweat, spittle, blood, pus, and sexual fluids inspired nearly universal loathing, closely followed by body parts and animals such as pigs, rats, maggots, worms, lice, and flies.

"The same stuff was coming up again and again in every country," Curtis says. "There had to be a reason why we were seeing this very clear pattern." That reason, she believes, is a universal need to protect oneself from disease. Feces carry at least 20 known viral, bacterial, and protozoan pathogens, from polio to salmonella. Saliva can transmit herpes and syphilis. Nasal mucus harbors whooping cough and tuberculosis. Fleas transmit the plague, lice transmit typhus, and flies trachoma, while animals carry a motley crew of pathogens from tapeworms to anthrax. "Back in the Stone Age, the ancestors that successfully avoided disease-causing substances had more success passing on their genes to the next generation," concludes Curtis. "Therefore, we're more likely to have the disgust emotion that they had."

If sensing disgust does enhance survival, then it is likely to be encoded in our genes. Some researchers believe that the most basic level of disgust—the urge to vomit in response to especially sickening experiences—may be controlled not only by the brain but also by a collection of nerve cells commonly known as the gut brain. A complex mesh of hundreds of millions of nerve cells spread like chicken wire throughout the walls of the digestive tract, the gut brain operates independently of the brain to control the movement of food through the gut. Eat a slice of rotten meat and the gut brain puts the process of peristalsis into reverse.

Psychiatrist Mary Phillips at the Institute of Psychiatry in London and neuropsychologist Andy Calder of Cambridge University have found that disgust has its own department in the brain, one separate from that used to process fear. Using magnetic resonance imaging, they found that two regions of the brain light up when subjects view either facial expressions of disgust or disgusting scenes such as cockroaches or decaying meat. One is the striatum, which is involved in movement, cognition, and reward. The other is the insula, which plays a key role in our ability to taste.

Stimulating the insula during brain surgery induces sensations of nausea and a foul taste in a patient's mouth; damaging it can eliminate disgust altogether. Calder cites the case of a young man whose insula was injured by a stroke. Afterward, he was willing to eat soup stirred with a washed flyswatter and chocolate shaped like dog feces, as well as sleep in a bed in which someone had died the previous night. Dysfunction of the insula may also play a role in obsessive-compulsive disorder. Some people with the disorder spend many hours a day washing or bathing to rid themselves of contaminants. Most psychiatrists see these habits as a classic case of fear and anxiety working overtime. But Phillips's research and that of others suggest that it is primarily an abnormal sense of disgust.

Phillips asked a small group of people with washing compulsions to examine a series of scenes while lying in an MRI scanner. Some of the pictures were neutral in content (jewelry or pastoral vistas), some were clearly disgusting (decaying food or open wounds), and some fell on the borderline (bags of garbage or unmade beds). The obsessive-compulsives rated the borderline scenes as far more disgusting than did people without the disorder, and their insula was unusually active during the periods in which these scenes were screened.

Still, the implications aren't as obvious as they seem. Reiner Sprengelmeyer, a cognitive neuroscientist at the University of St. Andrews in Scotland, examined a group of 12 people with obsessive-compulsive disorder and 12 people with Tourette's syndrome, a condition characterized by impulsive motor and vocal tics. (Five of the Tourette's patients also displayed symptoms of obsessive-compulsive disorder.) Sprengelmeyer showed all the subjects a series of photos of people expressing a range of emotions: happiness, surprise, fear, sadness, disgust, and anger. All the subjects with signs of obsessive-compulsive disorder had a difficult time distinguishing disgust from fear or anger. The more likely they were to be disgusted themselves, the more trouble they had recognizing the same reaction in others.

"The findings are indeed contradictory," Sprengelmeyer says. But they also suggest an avenue for future research on abnormal disgust. "The first step is to get a clear view of the brain systems that are involved in emotion," Sprengelmeyer says. "Once you have a clear understanding of that, you can find the neurochemical basis of each emotion. And if you understand that, you can develop pharmaceutical strategies to treat this disorder. This is the first observational step in a long chain."


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