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.3 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 sniffs, 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.4
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.5
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.6 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.7 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.8
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,9 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.10 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 lo
ng 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.11
Table 1. Categories of disgust elicitors emerging from factor analysis of sixty stimuli
Source: M. de Barra, “Attraction and Aversion: Pathogen Avoidance Strategies in the UK and Bangladesh” (PhD diss., London School of Hygiene and Tropical Medicine, 2011).
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.
When we started this research, there were few people who agreed that it made sense to see disgust as an evolved adaptive system, and I had to battle my way through barrages of critiques in lectures and teaching. People would tell me lovely stories about how their babies would explore their own poo, arguing that this disproved the idea that disgust was innate in humans. Yet to take a comparable example, babies have little in the way of sex drive; it appears when needed, in adolescence, with the pubertal hormone spurt. Yet few would deny that a sex drive is innate, since it appears despite attempts in many cultures to quash it (at least in females). Babies only need disgust when they are old enough to explore the world independently, and when their mothers use disgust expressions to help potty train them, in about their second year of life. Something that is innate does not necessarily have to manifest itself at birth.12
And what about disgust and food? How can human disgust be a part of our evolved natures when surely it is culture that determines what people eat? Why will the inhabitants of Iceland eat reindeer nose worms, the Chinese rotten eggs, the Swiss rotten milk, and the Ugandans grasshoppers? In fact, all foods, and especially those of animal origin, are potentially disgusting, but we make exceptions for what is familiar. In every society infants accept what their mothers feed them, and then they grow up with that particular food repertoire.13 The culturally tried and tested can safely be regarded as fit to eat. But any novel foods are treated with suspicion, to be rejected, sniffed at, or tasted only in small bites. Disgust of food is the default, for good reason; the system works on the precautionary principle that it is better to be safe than sorry.14 After all, if you don’t eat what is offered, you may miss a meal, but if you do eat something suspect or infected, you may lose your life or make yourself seriously ill. Better stick to Mama’s cooking!
And what about toxins? It is often claimed that the disgust system protects us from toxins as well as pathogens. Yet the only toxins that are generally found to be disgusting are those that are associated with signs of potential infection, such as hydrogen sulfide, which betrays bacterial or fungal decomposition processes. Bitter or sour flavors are distasteful, but are they actually disgusting? They certainly didn’t make any appearance on our disgust lists.15
Explanations of Disgust
If PAT explains most human disgust and also cuts it at its joints so neatly, how come this evolutionary-epidemiological approach isn’t standard wisdom? There are lots of reasons. Historically, it was thought that our aversions were simply products of culture. More recently, evolutionary explanations of behavior have been given short shrift by social scientists, and psychologists in particular, at least until now.16 Studies of disgust have been dominated by a few famous psychologists, and the standard model has gone unchallenged.
One of the earliest writers on disgust was Charles Darwin. On his specimen-collecting visit to Tierra del Fuego on the Beagle in 1832, he collected observations of interesting human behavior alongside his other specimens: “a native touched with his fingers some cold preserved meat which I was eating at our bivouac, and plainly showed utter disgust at its softness; whilst I felt utter disgust at my food being touched by a naked savage, though his hands did not appear dirty.”17
From the safety and comfort of Down House, Darwin sent out letters to his global collection of correspondents. They confirmed that the disgust face—turning up the nose, wrinkling the mouth, and making as if to spit, as well as the emission of an “eugh!” sound—was common to Greenlanders, Native Americans, Malays, and Australians. He even recorded the same face in his own infant son. Based on the etymology of the word dégoût, however, Darwin proposed that disgust’s original purpose was the avoidance of things offensive to the taste.18 This idea that disgust has oral origins has become orthodox in the field of disgust studies.19 Yet if PAT is right, then its origin must predate humans (Darwin himself thought baboons and turkeys could show disgust), and it must serve to defend all of the portals of the body—the skin, the airways, the genitals—from infection, not just the mouth.
While Darwin emphasized the universality of the emotion, the philosopher Herbert Spencer was one of the earliest to focus on the differences between what different cultures found disgusting: “Here human flesh is abhorred, and there regarded as the greatest delicacy; in this country roots are allowed to putrefy before they are eaten, and in that the taint of decay produces disgust; the whale’s blubber which one race devours with avidity will in another by its very odour produce nausea.”20
Anthropologists from Spencer’s time onward made the differing taboos and prohibitions of different cultures the stock of their trade. A famous book by the social anthropologist Mary Douglas entrenched this view of disgust as a product of culture. In Purity and Danger: An Ana
lysis of Concepts of Pollution and Taboo, which appeared to great success in the 1960s, Douglas suggested that dirt was the mirror of culture. According to her, anomalous objects and events that do not fit the local cosmology have to be rejected, and classed as dirty or impure, as otherwise they would pose a threat to the social order. “Dirt then, is never a unique, isolated event. Where there is dirt there is a system. Dirt is the by-product of a systematic ordering and classification of matter, in so far as ordering involves rejecting inappropriate elements” (36).
The caste system in India was Douglas’s paradigm case. Human wastes are anomalous materials, neither alive nor dead, both belonging to, but rejected by, the person. Hence wastes such as feces are dirty and polluting. Humans that deal in wastes become symbolically impure; these castes then have to be kept apart, for fear of social disorder. Douglas’s approach is still influential in anthropological and cultural studies circles.21
Sigmund Freud also thought a lot about disgust. For him disgust was a mode of repressing desire. In an essay on sexuality dating from 1902, he proposed that disgust served to defend the brain against its own baser tendencies.22 For example, disgust of the bodies of others, especially of relatives, reduces the temptation to give in to lust at every opportunity. Learning disgust of feces during toilet training suppresses any temptation to want to contact or eat them. Children keep their animal id in check by learning to be disgusted at their own baser urges.
Don't Look, Don't Touch, Don't Eat: The Science Behind Revulsion Page 2