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DON'T LOOK, DON'T TOUCH, DON'T EAT
The Science Behind Revulsion
By Valerie Curtis
THE UNIVERSITY OF CHICAGO PRESSCopyright © 2013 The University of Chicago
All rights reserved.
EVASION OF THE ONE BODY SNATCHERS
Pus, vomit, urination, menstruation, sexual fluids and so on—all substances and acts that, for some reason, many cultures tend to see as repellent and, despite their constant presence in human life, as abnormal.
A. K. Reinhart, "Impurity/No Danger"
Eureka moments are what every scientist lives for. I'd been mired in sheaves of data, mostly long transcripts of interviews with women in India, Africa, and Europe about their personal hygiene. I'd been asking them what they did, why they cleaned up after their kids, why they used soap and detergents, why they groomed themselves and tidied their houses, and why they washed their hands, or—more often—why they didn't. The women found it surprisingly hard to explain their cleaning and preening behavior. Often as not, they said that they felt that they needed to remove things that were "yuck"—the smelly, the clammy, the ugly, or the nasty. But when I asked them to explain why, we got stuck. "They are just yuck!" they would say.
Here are some of the things that Dutch women found revolting: feces, cats, aphids in lettuce, hairs, dogs, pollution, vermin, dog hairs, drug users, vomit, dog saliva, drunkenness, dust, rotten waste, fat people, sweat, bad-smelling food, insulting, stickiness, food leftovers, politicians, off al, worms, dirty old men, fishmongers' hands, flies.
And here is what women in Burkina Faso told me that they found disgusting: feces, dirty latrine, dirty food, unswept yard, diarrhea, impure substances associated with birth, flies on food, sores, rubbish in the yard, worms, sexual relations before a child is weaned, smelly drains, dirty clothes, rubbish heaps, sick people, pigs, vomit.
Closer to home, I asked a group of women in Cheshire, in the north of England, to write about disgust, and these are what they chose: feces, stained kitchen, dirty hotel, dog shit, flies, dirty cafeteria, cat shit, dead sparrow, dirty play area, dog diarrhea, rotten food, drunks, child shit on the sofa, moldy food, drunken louts, vomit, rank smell of old grease, rude people, wounds, dirty nails, foul language, maggots, eating with mouth open, man beating a woman, sweaty person, eating a burger that a stranger had bitten into, body parts in jars, cruelty to a horse, stained toilet, cleaning another's false teeth, wounding of an old lady.
If you compare these Dutch, African, and English examples with the Indian list in the introduction, you'll see that what, at first sight, looks like a ragbag of unrelated rejecta does, in fact, have themes. All of the lists include bodily wastes and body products. All include bad foods, contaminated objects, and immoral behavior. But, like the women who had participated in these studies, I was also stuck for an explanation. What could possibly account for the fact that such disparate items all occasioned a similar response—a feeling of "eugh!," accompanied by a wish to push them all away, to avoid them, to remove them—and what could explain my desire, even as I was poring and puzzling over them, not to think about them at all?
Then came the eureka moment. I'd been asked for advice about a rare disease caused by a lung fluke in Asia. I wasn't certain how to answer the question, but I knew where to go—to the standard reference work on infectious disease: the Control of Communicable Diseases Manual. I flipped through the index, and suddenly a pattern jumped out at me. Interspersed between diseases such as acanthamebiasis and zygomycosis came familiar words: "contaminated food," "dog faeces," "flies," "hairs," "human faeces," "pigs," "rubbish," "sexual fluids," "vomit," "worms," and "wounds." All topics from my disgust lists. There was clearly a pattern here. I went back to the archive of internationally disgusting items, and, sure enough, it seemed that almost everything there could be found in my infectious-disease compendium.
Some of the most disgusting bodily products turned out to be the most deadly. Feces are not just revolting; they are the source of over twenty gastroenteric infections, including cholera, typhoid, cryptosporidiosis, rotavirus, and the other stomach bugs that are responsible for three-quarters of a million child deaths a year. Nasal mucus is not just nasty: it carries the agents of tuberculosis, influenza, measles, leprosy, and the pneumonias that kill even more children annually. Saliva transmits herpes, syphilis, and mumps. Blood carries AIDS; hepatitis B, C, and D; Lassa fever; syphilis; and trypanosomiasis. Sexual fluids transmit chlamydia, herpes, AIDS, gonorrhea, syphilis, and trichomoniasis. Other bodily fluids can pass on impetigo, chicken pox, smallpox, diphtheria, thrush, ringworm, influenza, leprosy, meningitis, and German measles.
And the index contained not just the disgusting body products of humans—disgusting animals were also there. The rat carries plague and a variety of interesting parasites such as those that cause arenaviral hemorrhagic fever, Lassa fever, and Weil's disease. Snails and slugs carry helminthic parasites. Insects such as flies and cockroaches walk about in wastes, spreading the agents of gastroenteric infection. Other insects, like the louse and the scabies mite, are themselves parasites. Some insects are both parasites and parasite vectors at the same time: fleas carry plague and typhus, lice carry relapsing and trench fever, ticks carry encephalitis and a variety of viral fevers. Earthworms are not dangerous to human health, but they look similar to the parasitic worms that can be found in meat and fish and that infect over a third of humanity.
It seemed that I had stumbled on an explanation for the diversity of the disgusting. All of the things that people find revolting seemed to have some sort of role to play in the transmission of infectious disease. Eureka? Almost—only one problem remained. Why did cruelty, rudeness, drunkenness, and politicians appear on my lists of the disgusting? Moral disgust seemed to need a different explanation. I decided to put this problem aside for the moment and explore the role of disgust in protecting us from infectious disease.
The Birth of PAT
Like any scientist with a hypothesis, I needed to test it with more data. Where better to find data than the international departure lounge of Athens airport? A student of mine asked over 250 people from Europe, the Middle East, the United States, and Africa about what they found disgusting. Here is his list:
Dog feces in the street; feces; dirty baby diapers; animal saliva; sputum; spitting; bogies (nasal mucus); sweat; people who do not brush their teeth; yellow teeth; vomit; seeing someone vomit; smokers; rotten food; rotten things in the refrigerator; rubbish everywhere; dirty hands before a meal and before bed; no oral hygiene; bad smells (body odor, drains, mushrooms cooking); bad body smell; smelly feet; stinky feet; bad breath; passing wind; body odor; picking nose in public; bad breath; if someone doesn't blow his nose but sniff s, especially in public transport; mucus; eaten alive by insects; cockroaches in my bed; millions of bugs clustered together; insects; cockroaches; grasshoppers; snakes; childbirth; all kinds of pigeons and birds; seagulls; dirty people; dying person; eating with mouth open; wet people; wet shoes; dirty dishes; dirty clothes; old cars; seaweed; when someone has a cut finger; London Underground; pollution; rude Europeans; all Americans; Serbian government; politicians; people who eat dogs, snakes, etc.; violent verbal abuse; physical abuse; smoking when children are around; smoking in non-smoking areas; lies; spite; killing animals; animal maltreatment; injustice; [being] ripped off for being a tourist; too much beer; poverty; child abuse; masturbation in public; people who don't want to learn things from others; British society in general; materialism; inequality; teenager getting pregnant to get money from the state; housing allocation for single mums in the inner cities; no tenderness with child; pornography; arrogant people breaking the traffic law; pickpocket; fighting; lack of respect.
Setting aside again the rather interesting moral violations (politicians and Americans got the highest disgust scores in this sample), the pattern held up. Bodily fluids and wastes appeared, as well as foods that were off ; the insect, bird, and animal vectors of disease; and poor hygiene, all of which are implicated directly, or indirectly, in infectious-disease transmission. Even seaweed, it turned out when I looked it up, can harbor the vibrion that causes cholera.
So was born the parasite avoidance theory of disgust (or PAT for short). The reason we can't resist wanting to recoil when we meet the nasty, the foul, and the stinky is ancient and instinctual. It's not a reasoned response to "knowing" about germs and disease; rather, it comes from an ancient wellspring of wisdom: the process of evolution. Those of our distant ancestors who tended to avoid feces, nasal mucus, and bad-smelling food did better on average in the reproduction lottery; they were healthier, mated more often, brought up more children to sexual maturity, and hence had more grandchildren. And these grandchildren, the descendants of the disgusted, were more disgustable themselves—and so on, till the present day, and us.
Pathogens and parasites caused the evolution of defense systems. These included not just impermeable skin, toxic secretions, and internal immune systems to kill parasites once they had gotten inside the body, but also behavioral defenses that kept potential hosts away from parasites in the first place. We all come equipped with a motive that makes us pay attention to signs that there might be infectious parasites about, and then to avoid contact. Disgust is a voice in our heads, the voice of our ancestors telling us to stay away from what might be bad for us.
It seemed that PAT was right! But, for a scientist, a eureka moment, with its shaft of light that reveals a previously hidden pattern, isn't enough. A theory has to be able to be tested to destruction; it has to lead to predictions that can be falsified. Just as we'd published a paper setting out PAT, the BBC came knocking, wanting to make a documentary about human instincts. They offered us the chance to carry out a web-based experiment to test the disgust reactions of the good viewers of Britain and others around the world.
So while I was being filmed offering the charismatic TV presenter Robert Winston sterilized parasitic worms for lunch, behind the scenes we were hurriedly pulling together an experiment. The idea was to offer people pairs of photos that were broadly similar, but one of the images would be morphed to enhance the infection content. So we took towels and stained them with food coloring. In one version the stain was blue, in the other the stain was brownish yellow—looking like bodily fluids. In another pair of images, we took photos of Duncan, the series producer, either looking normal or slightly pinked up with damp hair and a few pimples. Another pair of pictures showed an empty Underground train and the same carriage full of people. A few control stimuli were added: a football, a cat, and a revolting image of a mouth in which flies had laid eggs in the gums. We mixed all the photos up and then asked people to rate them for disgust on a sliding scale from 1 to 5. When the survey went live on the night of the broadcast, the response crashed the BBC's server. In the end over 160,000 people from 165 countries completed it. Our analysis gave unequivocal results; every disease-relevant image scored significantly higher than the less disease-relevant image, and this effect held for every region of the world.
One final test on the website involved a large picture of a toothbrush and a question: who would you least like to share a toothbrush with: your partner, the weatherman, your boss, your best friend, or the postman? We predicted that the people who were least personally known would be most likely to carry diseases that had not already been caught, so the weatherman's brush would be the most aversive, and one's partner's toothbrush, the least. We got the order right, except for one detail. Sharing the postman's brush was most aversive, the weatherman's came third. On further reflection this made sense, as people probably feel that they know their weatherman, since he is welcomed into the home every evening on TV, while they mostly don't know their roving postman.
We made other predictions from the initial hypothesis: one that women would be more disgust sensitive than men, because our female ancestors had a double burden, to protect themselves and their dependent children from infection. This prediction held up: women scored every stimulus as more disgusting on average than did the men. We also predicted that disgust would fall off in middle age, at about the time that reproductive activity starts to tail off . However, what we found was a steady decline from a high disgust sensitivity at age eighteen or so, to a low one in old age, with no sign of a change in the slope of the line in middle age for men or women.
Cutting Disgust at Its Joints
Our study, the first to experimentally demonstrate that cues of infection predicted the disgust response, led to a flurry of press interest and brought me PhD student Mícheál de Barra. Mícheál wanted to see if he could use PAT to improve on previous attempts to find regular patterns in the disgusting, such as those by psychologists Paul Rozin and Jonathan Haidt. He decided that we needed to go back to square one, to use the sharp knives of both epidemiology and evolutionary biology to dissect disgust and look for its joints.
We asked ourselves what kinds of places and things it might have been adaptive to avoid, so as to not get invaded by parasites—the ravenous body snatchers. How should the brain carve up the world of the disgusting? How should it recognize what to avoid, and how should it orchestrate its response? In other words, is the world of the disgusting just a jumble, a ragbag of the nasty and the infectious, as it appears at first sight, or does the brain recognize a structure in it?
An infectious-disease textbook became the source of a long and varied list of situations in which people might risk contracting an infection. Highlights included being licked by a stray dog, seeing cockroaches, indulging in promiscuous sex, encountering poor genital hygiene, seeing an oozing lesion on a friend's foot, being in contact with someone who is unkempt, eating food that has gone off , riding on a filthy bus, and hearing about open defecation. (Of course, it would have been neater to have done this with an exhaustive list of the diseases that infected our ancient ancestors, but no such list exists.) We set up a website survey and advertised it on Facebook, getting more than 2,500 people to record how disgusting they found the sixty scenarios. The responses tended to cluster together, and they did so in ways that were subtly different from what we had anticipated.
We had thought that the shape of the disgust response might reflect the routes of entry of pathogens into the body: that there might be a different type of response to the parasitic organisms that we might breath in, that we ate, that entered through the genital tract, and that got in through the skin, because each of these would require a different kind of behavioral defense, with distinct kinds of mental machinery. But the picture that emerged wasn't quite like that. We identified six categories of stimuli: people of unusual appearance who showed signs of sickness or deformity; infected lesions and bodily fluids; people with signs of poor hygiene; having risky sex; unfamiliar and possibly infected foods, as well as certain animals and insects; and finally, things in the environment that had come into contact with infectious agents (fomites are surfaces that vector infection). The table summarizes these categories.
While it wasn't exactly what we were expecting, this pattern still made sense. If you are an animal that cannot directly see infectious agents, then you need to be able to reliably avoid the places and things that tend to harbor pathogens. Those animals in our evolutionary past that did so would have passed on more genes, on average, than those who did not. Since other people are the prime source of human pathogens, avoiding people whose appearance or behavior hints that they might have a disease is paramount. Any bodily substances that they exude should also be dubious, especially if these look or smell infective. It's particularly important to avoid the sorts of contacts with other people that might give you a sexually transmitted disease (as well as avoiding unsuitable mates). Animals and insects that vector disease, as well as foods that are unfamiliar or that show signs of being rotten, are best avoided because they are likely to carry pathogens. The data suggested that we also have a kind of disgust that is reserved for objects that have come into contact with things that are contaminated. Finally, people who behave unhygienically—who appear unkempt and poorly groomed or who cause contamination of the shared environment—are also the object of a separate kind of disgust.
These six types of disgust were distinct; we think that each reflects a slightly different way in which evolution has organized the brain's response to recurring patterns of disease threats in the environment.
Excerpted from DON'T LOOK, DON'T TOUCH, DON'T EAT by Valerie Curtis. Copyright © 2013 The University of Chicago. Excerpted by permission of THE UNIVERSITY OF CHICAGO PRESS.
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Table of Contents
Preface: Unweaving the Rainbow
1. Evasion of the Body Snatchers
2. Into the Hot Zone
3. Disgust’s Diversity
4. Maner Mayks Man
5. Moral Disgust
6. Disgust Matters
Epilogue:Disgust: The Unfinished Story
Appendix: The London Disgust Scale