Don't Look, Don't Touch, Don't Eat: The Science Behind Revulsion

by Valerie Curtis

  Look at this list of what teenagers at my local school said was morally disgusting:

  Mocking someone in a crowd, bullying, racism, discrimination, standing by when others suffer, cloning babies, drug taking, suicide, bigotry, taking the Lord’s name in vain, soldier deserting, spitting in someone’s face, governments allowing injustice, putting profit before human life, exploitation of the poor, invading privacy, hypocrisy, back-stabbing a friend, poor sportsmanship, swearing, smoking when pregnant, vandalism, rape, adultery, treachery, stealing from the poor or handicapped, terrorism, chemical weapons, child slavery, cruelty to animals, pedophilia, incest, pornography, torture, wife beating, sadism, mistreatment of the elderly and vulnerable, cannibalism, eating dogs, murder, pollution.

  The parasite avoidance theory (PAT) of disgust here seems to fail us. Why should this collection of immoral, hypocritical, cruel, exploitative, and nasty behavior be found to be disgusting? Were the kids just using disgust as a metaphor, or were they using their ancient parasite-avoidance systems in another context entirely?

  There is a link between morality and disgust. To find that link, we have to start with the problem of human cooperation.

  Adam Smith began his famous book, the Wealth of Nations, with the story of the pin maker: A single workman “could scarce, perhaps, with his utmost industry, make one pin in a day, and certainly could not make twenty. But in the way in which this business is now carried on . . . [o]ne man draws out the wire, another straights it, a third cuts it, a fourth points it, a fifth grinds it at the top for receiving the head; to make the head requires two or three distinct operations; to put it on, is a peculiar business, to whiten the pins is another; it is even a trade by itself to put them into the paper.”1

  Smith goes on to calculate that by sharing pin making among ten people, the men could, between them, make upward of forty-eight thousand pins in a day. Cooperation can thus render one individual 250 times more productive than if they worked alone.

  Imagine, for a minute, a world of Smith’s single workmen (and women), who do not know how to cooperate productively. In this world there would be no houses, just caves. There would be no beds, just piles of brushwood. Clothes would be missing; there would only be animal-skin wraps. No one would make breakfast for you; you’d have to dig something up for yourself with a stick and eat it raw. You’d have no transport, couldn’t go to work; you wouldn’t even have the language abilities that would let you speak to your family. You couldn’t call the police, the exterminator, or the gas woman to fix the heating. You couldn’t go shopping, take a vacation, or visit a hairdresser. You wouldn’t be able to use roads, trains, or planes, and there’d be no phones, no computers, no art or music, no recipes or restaurants, and no pubs. In short, you’d be living as our primate ancestors did.

  One way of measuring the difference between the level of advancement of our current way of life and that of our forebears is to compare energy throughput. A single hunter-gatherer uses energy at about the rate of 100 W, the bare minimum of subsistence food energy to fuel an individual living in a small band. However, the energy throughput of the average extant American is more than a hundred times this (12.7 kW). Our most advanced form of life is an astronaut; an inhabitant of the international space station uses energy at a rate five hundred times that of a hunter-gatherer.2 To achieve such feats, we’ve harnessed the energy of plants, animals, water and wind, fossil hydrocarbons, and terrestrial as well as solar nuclear forces. None of this astonishing productivity could have been achieved by Adam Smith’s pin maker working alone. Wherever you are reading this book, the lifestyle you lead and just about everything you see around you (unless you’re a Luddite hermit on a pristine island), including this book, is a product of division of labor, of mass cooperation.

  We are so used to our modern world that we hardly notice how miraculous it is. Whole populations of individuals make individual sacrifices to cooperate and divide their labor, so as to reap the later benefits. According to elementary evolutionary biology, this shouldn’t be possible. In most species, an individual that helps another is just a fool, a nutter to be taken advantage of. Odd genes that throw up helpful tendencies that advance the interest of others over those of the self will simply go extinct. How has our species figured out how to suspend the selfish dictates of the genes and become supercooperative?

  Simple selfless, helping behavior isn’t too hard to evolve. There are lots of good evolutionary reasons why one individual member of a species should help another, even if this comes at a cost to that individual. If we help members of our family, we are helping those who have copies of the same genes, so we are helping our own genes.3 This is the trick used by the eusocial insects, whose ability to cooperate on a large scale stems from the fact that they are closely related to one another. If we help others in the reasonable expectation that we will, in turn, be helped, then this helps our genes too. This is termed reciprocal altruism,4 though the word altruism is something of a misnomer, as the individual interests are still selfish.5 But why should we help unrelated strangers and people that may never reciprocate? A number of solutions to this puzzle have been proposed. It could be that we do such things to enhance our social reputations, hence giving us a chance to reap benefits from our kindly actions later on, so-called indirect reciprocity.6 There is probably also a virtuous ratchet whereby prosocial people do well in a culture that favors helping, and the more prosocial people become, the more culture becomes pro social. This is known as gene-culture coevolution theory.7

  Critics worry that none of these explanations suffice to explain how we maintain such extraordinary levels of cooperation between unrelated individuals. Cooperation breaks down too easily; there is a strong incentive to cheat, to take the benefits of membership in the cooperative group without making your own contribution. Social order turns readily to anarchy. Game theorists have shown that one ingredient is essential to keep societies cooperating—you must make cheaters pay. Economists use public goods games, lab-based simulations of social dilemmas with real people and real rewards, to investigate cooperation. They find that with multiple players and repeated interactions, where people pay in but get more eventual payoffs if they cooperate, cooperation nevertheless gradually peters out, however beneficial it is. If, however, players are allowed to punish others who do not chip in their fair share, then cooperation ramps up rapidly and can be sustained over the long term. It helps if one individual punishes another for harming or failing to help a third person—truly altruistic behavior.8 The evolution of punishment provides a mechanism for the maintenance of hypersociality.

  Beyond kin selection, reciprocal altruism, and punishment, there is one other evolutionary process that may have played a part in helping humans scale the hills of selfishness and drop down into the Happy Valley of supercooperation. That is group selection. It’s easy to see that when different human social groups compete with each other, those groups that are the most cooperative are likely to do best. The societies that can share labor to extract and process resources, that can invent and produce new productive or destructive technologies, and that can train and deploy effective armies will be able to outcompete the neighboring societies that are less cooperative. They will be more productive, better able to defend themselves from attack, and able to appropriate the lands, the goods, and the women of neighboring tribes. Their populations and the genes that they carry will multiply faster than the competition’s. Cooperators will also be likely to find and like one another, shun the cheaters, be more successful, and so breed more cooperators. And if the most cooperative groups are relatively more productive, they will have more leisure time to devote to activities that help groups cohere, such as devising laws, operating institutions, and running religions. It requires quite special conditions of adaptive advantage and population mixing,9 and is deeply controversial, but some sociobiologists are convinced that selection at the level of the group does account for some of our unique ability to be “super-cooperators.”10<
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  Whatever the exact evolutionary path we took to hypersociality, humans must have a mechanism in the mind that drives this cooperative behavior, which gives us our “hive brains.” I’m going to call this the human moral system. Moral systems in brains evolved to make us want to be cooperators, to contribute to the group at our own cost, and to want to punish those who defect—so that we can ultimately reap all of those multiple benefits of being a member of a productive society. And because getting punished for our moral failings isn’t to our own advantage, our moral motives also include shame and guilt—which provide preemptive internal punishment—helping us avoid being punished by others, not just for failings of hygiene manners, but for moral lapses too.

  Gene-culture coevolution then further helped to domesticate us; our ancestors chose to mate with those individuals who were more cooperative and who chose to follow the rules of society, and the cooperators thrived better and had more offspring than the spiky anarchists. We have bred into ourselves qualities of docility, kindness, and sheep-like following of the local norms. It is as if we have antennae that look for and seek to learn the system of rules, of norms of behavior that are followed by others. We take some time to learn the moral rules, but, as with manners, we have a long childhood and adolescence in which to do it. We glean local norms from the evidence of behavior around us—learning from our parents, our friends, our teachers, and the written and unwritten rules of society. Moral systems reside not just in innate prosocial motives and learned behavior patterns in our heads, but also outside of them—in the heads of our parents and our role models, on stone tablets, in religious books and codes of law, and in novels, films, soap operas, newspapers, and tweets. Morality is what allows us to inhabit the superproductive Happy Valley of supercooperation. When morality fails, cooperation fails, and, like Adam, we are ejected from this Eden to fend for ourselves, to try to survive alone and unaided.

  Moral Disgust: A Cheap Form of Punishment

  If morality is the system in brains and in societies that evolved to underpin supercooperative behavior, then what is moral disgust for? How did it evolve and what adaptive purposes does it serve? To answer these questions, we have to return to the question of punishment.

  Punishment, as we saw, is one of the keys to cooperation. Its job is to make it more expensive for someone to not cooperate than to do so. If I throw my old junk into the street, it has saved me the cost of disposing of it properly. If I am then fined or shunned by my neighbors, I pay a price that could well be greater than what I saved by not taking the rubbish to the dump in the first place. There are lots of types of punishment: I could be insulted; I could be beaten up or locked up; I could be fined or even executed.

  Active intervention, however, can be costly. My neighbor might hesitate to tackle me in the street about my rubbish habit because I could retaliate. If I was big and strong and had plenty of allies, I might give him a black eye, despite his efforts to uphold the values of virtue. In this case, what is the best strategy my neighbor can use to punish me and to return me to prosocial behavior? Two routes are left open. One is gossip—he can tell everyone that he meets about Val’s rubbish habits and suggest that her behavior is shameful, a reflection of her bad character, implying that she wouldn’t be great as an associate or trustworthy as a business partner. Contaminating my reputation would probably work well, at least in a stable community, as people tend to pay attention to such reports and use them to make judgments about whom to include in their social dealings. The second route is ostracism—Val can be avoided in the supermarket, and the residents’ committee can debate her crimes and refuse to buy her jam at the street market. This is severe punishment indeed. In the film We Need to Talk about Kevin, the mother of the child sociopath is attacked in the street and has paint thrown over her house and car. But for Eva, played brilliantly by Tilda Swinton, the hardest thing to bear is her social exile; no one will talk to her at work, and the neighbors refuse to even acknowledge her existence.11 By ostracizing someone, we can deny them all of the benefits of living in a social group and run little risk of bearing a cost for doing so, especially if we can “gang up” on that person through gossip.

  When we encounter someone throwing their waste into the street or someone who has had a hand in a crime, we don’t sit down and carefully calculate what cost to impose on the perpetrator. Hot anger and cold disgust well up inside us and tell us what to do. We make a rapid appraisal of the context and produce a suitable behavior: perhaps a loud insult if that person is small and defenseless, or crossing to the other side of the street to avoid meeting her, if not. We take every opportunity to discuss the crime and the criminal with others. Moral disgust wells up and tells us how to act; it tells us to punish the authors of antisocial acts by shunning them. It tells us to use the language of dirt and contamination to encourage others to feel disgust too. Disgust is perfectly suited to this job; it makes us want to stop, to drop the object of disgust, to steer well clear of it, and to recruit others to our point of view by insinuating that there’s something nasty and contaminating around. Anger is the alternative, but anger can get you into trouble; your aggression can be met with retaliation. Disgust is cheap, and there’s not much that the object of disgust can do to get back at you.

  Disgust, then, is an emotion that first evolved to help us avoid parasites and then became an internal signal telling us to avoid and punish others who are sources of parasites—those with bad manners. From there it was but a short hop for the disgust mechanism to become a motive that makes us want to punish those who are behaving antisocially. Disgust expanded its object from microbes to manners and went on to become an essential component of morality.

  Is Moral Disgust Really Disgust?

  Though it makes a good story, this is still just a story. Is there any evidence that can help us determine whether the moral-disgust motive actually uses the same basic pathogen-disgust mechanisms and is not just a metaphor that people employ when they want to express disapproval?

  It does seem as though there is at least some cross talk between moral and pathogen disgust. Psychologists Thalia Wheatley and Jonathan Haidt turned conjurer to hypnotize their experimental subjects into feeling a flash of disgust whenever they saw a particular word. When this word was incorporated into short vignettes about moral infractions, it made their judgments more severe.12 Amazingly enough, the same effect was found in a later set of studies in the UK when subjects were asked to do the judgment task in a disgusting room or were seated next to a smelly waste basket, or when they had been exposed to a disgusting video.13 Simone Schnall then went on to show the complementary effect, that people primed with a word task about cleanliness and purity made less severe judgments about moral infractions.14 And in an intriguing study, Chen Zhong and Katie Liljenquist found that guilt could be reduced by hand washing, what they called the “Lady Macbeth effect.”15

  Some of these studies may be too good to be true, as it turns out that not all of their results can be replicated. A paper in the Journal of Articles in Support of the Null Hypothesis failed to reproduce Zhong and Liljenquist’s results despite a huge sample size.16 The jury is still out on this one.

  Another possible source of evidence for a relationship between physical and moral disgust comes from the face. Exactly the same facial muscles—the levator labii—are employed in the lip curl of disgust at bad taste and images of feces, and at unfair offers in an economic experiment with a one-shot interaction called the ultimatum game. Is this really evidence that the two disgusts are one and the same thing, as the authors claim?17 Again, we have to be a bit skeptical. We humans use facial expression as a communicative act—and the adaptive reason for communication is to manipulate others.18 If you look up some of the wonderful videos of babies tasting lemons on the Internet (e.g., http://www.youtube.com/watch?v=Ixj88urcnIg&feature=related), you’ll see expressively crinkled faces, charming wrinkled noses, and lots of tongues sticking out. The reason that the clips are so entertaining to w
atch is that the images fire our mirror neurons, making us cringe, smile, and laugh with the baby. In the babies’ parents, they produce the effect that the expressions were designed for: to manipulate them into stopping the lemon feeding (though sometimes only after several more video takes!). Of course, the disgust face also works the other way around; a parent pulling a disgust face can manipulate a child, for example, into discouraging it from taking a sweet from a potty.19 And a disgust face directed straight at you tends to produce shame20—encouraging you to behave more morally.

  But the most important, and sometimes insidious, result of the disgust face is the effect on third parties. We are much like the mice that notice and remember which mouse has been in contact with a diseased mouse, so as to avoid it later.21 If I show disgust at what Adam is doing, Helen will pay attention, learn that what Adam is doing disgusts me, and remember that Adam did something disgusting. Helen will store the information to use when she next encounters Adam. The disgust face both manipulates the individual to make him stop the immoral action and manipulates others, encouraging them to treat the offender as morally contaminated and so shun him.

  Providing evidence that the same facial expression is displayed in response to microbial and moral infractions is not evidence that moral and basic disgust is the same emotion. All that it tells us is that the same facial expression can usefully be employed in two different contexts. In an ultimatum game, a player may pull a disgust face, not because of some inner state, but because the face can influence other players—perhaps by making the perpetrator feel shame and so desist, or by implying moral contamination in the cheater—making the other players want to shun her. That same disgust face, as well as disgust sounds and words, worked to spread moral opprobrium when hunter-gatherers discussed Ig’s dirty habits around the campfire, in the same way that it does now when office workers discuss the latest political scandal around the water cooler. “He did what? Ugh! How disgusting!” There is a connection between moral and microbe disgust—expressing it manipulates the behavior of others—but this still doesn’t prove that they are one and the same thing.