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

by Valerie Curtis

  Finally, if I am right that different pathologies of the disgust system relate to its different components (sick people, hygiene, sex, food/animal, and fomite), then we have promising new avenues of research to help understand, and ultimately alleviate, these conditions. In appendix 1, our new disgust scale, which makes these distinctions, could prove a useful tool for this purpose.65

  The Moral of Moral Disgust

  We’ve looked for a moral to our disgust story in the microbe and manners domains, but what about in the moral domain? What can understanding the origins and purposes of moral disgust tell us that is useful?

  First, our story reminds us that disgust, as one of the wellsprings of our sense of right and wrong, has an evolutionary history. It is an ancient system that arose in animals to help them avoid infection. This emotional system predates the evolution of “rationality” and still operates, to a large extent, independent of it. Though many prefer to believe that rationality is the source of our morality, evolutionary thinking has put paid to this idea.66 Moral sentiments are just that—sentiments.

  Emotional moral disgust underpins our very ability to live the ultrasocial human way of life. When we feel a flash of disgust at the individual who sneezes in our face in the elevator, or at the bully who pushes for an unfair share of the research fund, we are receiving messages from our ancestors telling us to act. They encourage us not to let wrongdoing, whether on a minor or major scale, pass. When we act on this disgust, we are behaving as moral beings, making social defectors pay the price of their defection. Each such disgust-driven act is a small contribution to the moral framework of social life.

  The cognitive revolution of the 1960s undermined popular faith that it is right to act on our emotions. Rationality has become king. Professionals tell us that children should be given reasons to use a potty (nasty germs make you sick), when all the caregiver needs to do is to pull a disgusted face. Children or criminals who do wrong are counseled—ineffectually—about the consequences of their actions, when a simple dose of emotional learning via disgust and shame would be a surer way to set them on the right path.

  Comparing the way I lived in traditional societies in Africa during my early professional career to the life I live now, among the UK’s “chattering classes,” I see a big difference in the place given to the moral emotions. There, I saw that those who did wrong were likely to meet with disgust and immediate social ostracism. Perhaps as a result, people seemed to pay more attention to social obligations, for example, always turning up for a wedding or funeral, than they do here. Here, I see folks dither about whether and how to respond to misdemeanors, apparently not trusting their emotions.

  While trusting more to the emotions may have some upsides in terms of increased social cohesion in traditional societies, there are also downsides. Discrimination is a major problem for traditional values.67 We know that disgust responses can be elicited by the sick, the homosexual, the sex worker, those with dirty jobs, and the social marginal or outsider.68 Because moral disgust grew out of microbe disgust, does discrimination on the basis of potential infection leak into our moral systems?

  In her book Hiding for Humanity: Disgust, Shame and the Law?, legal philosopher Martha Nussbaum argues that it does. She is suspicious of disgust as a moral emotion, saying that it engenders prejudice and stigmatization of the weak, the lower caste, the outsider, and the homosexual and that it can be used to contaminate the whole person.

  Indeed, the unscrupulous and the power-hungry throughout history have long exploited this fact, whipping up minority, race, and out-group hatred so as to gain standing among the in-group as they close ranks.

  Because we are hypervigilant to cues that might signal infectious substances or individuals, we pay attention when we are offered social information about potential disease threats. A common tactic for the playground bully is to label another child as infected or as having “cooties”; the victim then suffers shunning by their peer group. As the bullies grow up, they continue in the same vein, sometimes targeting not just individuals but whole groups. Ethnic strife abounds with the language of disgust; Tut-sis are cockroaches, Jews lice, the “Japs” vermin, and the neighbors need ethnic “cleansing.”69 The bullies salve their consciences by thinking of their adversaries as “the great unwashed”: dirty, degraded, and less than human. Robbed of claims to human dignity, the oppressed see their living conditions worsen: in slums, ghettos, camps, and marginal lands, hygiene becomes harder, fueling a vicious cycle of neglect and oppression.70

  Throughout history, politicians have gained advantage by labeling members of out-groups as polluting, dirty, unhygienic, disease carriers. The rhetoric of disgust is called on to justify caste and class divisions, cruelty, exploitation, pogroms, ethnic cleansing, genocide, and war.71 Such problems persist globally because the old tricks still work. The anti-gay politician or anti-abortion religious leader whips up disgust at an out-group; the implied disease threat then causes the in-group to close ranks. The effect is particularly visible at election times, when intercommunal violence peaks, as does the discussion of immigration.72 By labeling the outsider as dirty and diseased, racists and nationalists find that they can also, to some extent, recruit morality to their side.73 Those who campaign against abortion, homosexuality, and GM foods exploit the imagery and language of disgust and its ability to contaminate; they employ pictures of aborted fetuses, talk of “dirty” sexual practices, and raise the specter of “Frankenfoods.”

  This is the dangerous downside of disgust. But to entirely dismiss moral disgust as a basis for social justice, as Nussbaum suggests we do, is to throw the baby out with the bathwater. Leon Kass argues that there is wisdom in repugnance.74 I would go further and say that without moral disgust of cheating, violence, and exploitation, we would be unable to function as a social species. None of the astonishing products of our modern supercooperative societies would be available to us, we would be plunged back into the Stone Age, and life would again become nasty, brutish, and short.

  So the moral of moral disgust is that we need to listen to some kinds of disgust and not to others, and that we need science to help us to distinguish between them. The tricks of the politicians need to be exposed as such, and we all need access to the science that shows how our moral responses can be influenced by microbe disgust.

  I have no doubt that such moral progress is possible.75 Looking back at the list of what the kids in my local school found morally disgusting, the most frequently nominated category was discrimination against others, whether on the basis of race, gender, looks, age, religion, or sexual orientation. Over our history, the circle of what is regarded as the in-group has gradually grown from the clan, the tribe, and the nation to humanity as a whole, and even beyond.76 This progress has come from expanded economic links, trade, and communication, from storytelling, to TV, literature, and art—all products of the group imagination. Amazingly, for most of our history, humans probably thought of neighboring tribes as different species.77 Science and art has now proved this to be wrong. We now know that we are all one. Strangers are no longer seen as less than human, to be disregarded, exploited, or treated as contaminated parasites. They are, instead, people like us.

  EPILOGUE

  DISGUST: THE UNFINISHED STORY

  What does the future hold for disgust? The blooming field of disgustology is going to be an exciting place in the near future. We are not just going to learn a lot more about behavior relating to microbes, manners, and morality, but we will also blaze a trail for a new approach to brains and behavior. The implications are tremendous.

  Psychology still has a long way to go before it can be regarded as a proper scientific discipline. The tools of psychological investigation are in their infancy; we’ve had the delicate probes and the huge rumbling scanners that can peer into living brains for only a couple of decades. Human brains are hard to study, as they are perhaps the most complex objects in existence. But these are only partial reasons for our slow progress.r />
  In physics, the search for the Higgs boson began only after theoreticians had predicted that this fundamental particle should, of necessity, exist. The Large Hadron Collider was built to test a hypothesis that came out of theory. And theory is the right place to start in all of science. Patterns are noticed, a theory that explains the pattern is propounded, hypotheses are generated, and these are tested on new evidence. If the results do not fit the prediction, the hypothesis is wrong and so, probably, is the theory (or maybe the observations are mistaken—replication is important—either way).

  Psychologists, however, distrust theory. Perhaps there have been too many wrong theories, like the work of Freud or Jung, or too many partial theories, like Pavlovian conditioning, cognitive dissonance, or the theory of reasoned action. Perhaps it is hard to theorize about brains because we are too close to them, and thinking about thinking about thinking soon becomes mind-bogglingly recursive. Or perhaps we’ve been too subjective, seeing human brains as special and therefore beyond theorizing.

  The good news is that psychology is in the process of acquiring objective theory, and so is becoming a proper science. The theory that scientific psychology needs has been around for 150 years, but it has been misunderstood or has been rejected as simplistic or politically incorrect.2 Evolution by natural selection provides our grand unifying theory for explaining brains and behavior.

  In this book I’ve used evolutionary theory to propose a function for a class of behavior. Since parasites are a problem for all animals, infectious-disease-avoidance behavior must feature in the repertoire of all animals.3 I found that the PAT of disgust neatly predicted the classes of behaviors that animals, including humans, engage in to avoid becoming prey to parasites. PAT gives us a simple, coherent, and parsimonious story about disgust that avoids some of the mental gymnastics employed by others who have tried to explain it.

  The Disgust Agenda

  But is PAT right? There is still a huge amount of research that we need to do to test the theory to destruction. What hypotheses can we test to check it out?

  First, the story of animal infectious-disease avoidance needs completing. I predict that every species of fish, bird, or mammal will be found to avoid sick conspecifics, animals with infected lesions or signs of deformity, parasite eggs and cysts, and waste-contaminated foodstuffs (unless the benefits of contact outweigh the possible costs of infection). The epidemiology of infectious-disease transmission for different species can offer detailed predictions about their behavioral aversions.

  If we look, I expect that we will also find that social animals will all display behaviors that help prevent parasite transmission. We should find that social animals have “manners” (meaning hygiene behaviors that reduce the likelihood of transmission of disease to others). Social animals, from insects to fish to birds to mammals, should display grooming and waste hygiene so as to make themselves more welcome as group members. They should sequester themselves when sick, and punish and/or avoid those who pose disease risk to others—the deformed, the sick, the incontinent, or the poorly groomed. Ethologists still have a lot of work to do to map disease-avoidance behavior in species other than our own.

  Humans have a recently evolved prefrontal cortex that lets us metarepresent the world, model it in our heads, and learn from this virtual experience. Some argue that our special abilities have allowed us to dispense with our ancient animal “instincts.” From the perspective of evolutionary theory, I predict that this “human exceptionalism” will be proved wrong. It will be established that humans come preadapted to (learn to) avoid poo, lesions, unhealthy mates, ectoparasites, and people who are unhygienic, and that many of the mechanisms by which we do these things are shared with the animals with which we have recent common ancestors. Eventually, when we’ve been able to pick apart the genes involved in disgust, it’s my bet that DNA analysis will show that parts of our disgust system are deeply ancient, sharing much with reptile, or even invertebrate, aversion systems, because the tasks it has to do are similarly ancient.4

  When it comes to human disgust mechanisms, there is so much still to learn. I expect that we will find that the neural basis for each of the subdomains of disgust is slightly different. Responses to stimuli concerning poor hygiene, lesions, deformity, sex, and contagion will share some neural mechanisms and have some that are different.

  Another area that it will be possible to disentangle in the near future is the development of human disgust beyond disease avoidance. Disgust responses help avoid suboptimal mating—couplings with those of the “wrong” age or sex, those of poor mate quality, or with a high degree of relatedness. Incest avoidance disgust has received the most attention,5 but suboptimal mating provides a simple explanation for why homosexual behavior is often reported as disgusting,6 as is mating with someone of poor genetic quality (i.e., ugly) or displaying signs of sickness, or a young person having sex with an old person.7 These “disgusts” should share features and brain mechanisms.

  Other outgrowths of the disgust system will also be accounted for. In the next few years, we’ll use fMRI to map disgust for outsiders and social defectors, and learn how much wiring belongs to the ancestral functions of disgust and how much is shared with its newer outgrowths. We’ll figure out if moral disgust really is an outgrowth of microbe disgust, or something else entirely.

  In short, we will soon be able to build a complete disgust map, telling the story of disgust in terms of function, mechanism, and behavior across species including our own. To do this correctly, we will have to let theory guide us, using the epidemiology of infectious disease as our guide.

  Another area where we will see huge progress is in psychiatry, where we will understand and be able to treat the pathologies of the subdomains of the disgust system. I expect that overactive disgust for sex will be shown to cause problems with intimacy, overactive disgust for other people to cause social phobias, and overactive animal disgust to produce specific animal phobias. Disgust will also come to be more widely recognized as causing a distinct form of post-traumatic stress, which will be treated as such. Evolutionary thinking needs to blaze a trail into psychiatry,8 giving us more effective chemical or cognitive alleviation for those with debilitating mental conditions.

  Disgust as a Model Emotion

  Disgust also has a lot to teach us about the emotions in general. Disgust’s time has come, and where it leads, the other emotions will follow. The theoretical approach we’ve used to unweave disgust’s rainbow works just as well for the other emotions. We can nail down thorny questions such as what emotions are for, how many emotions there are, how far we share them with related species, what subdomains they have, and ultimately how they actually work.9

  If we want to use this evolutionary approach on other emotions, however, we’ll have to leave some of our preconceptions at home. For example, if we want to understand love, we’d do better to start with the adaptive purposes of pair-bonding behavior rather than with how we feel around a significant other. We’ll have to jettison common assumptions, especially the confusion of feelings with emotions. In the same way that disgust has an adaptive function, so too does love. Clear your head of that seductive, yearning, mushy feeling for the (wo)man in your life, and think instead of the mammals with offspring that are born so immature that they require huge parental investment to rear to adulthood. Accomplishing this task works best with two parents cooperating to rear the kids. But each parent might do better genetically if it left the other to do the job and went off to get more matings and more offspring. How to solve this problem? The only way is sacrifice. Each parent has to try to make it worth the other’s while to stay around to help. Hence each offers what he or she can, paid in the currency of grooming services, food and shelter provisioning, sexual availability, entertainment, ego massage, and the cognitive economy of a regular, reliable domestic routine. The motive that makes each want to invest in the other in this way can be labeled “love.”

  Following the disgust example,
we can make further predictions. We might expect that the love motive might be stronger in women than in men on average, because women are more heavily invested in their current children. We might expect that love would falter with the prospect of better reproductive options, and that social devices would have been invented to bolster the fragile pair-bond. In terms of mechanisms, we might expect to find a chemical arms race between males and females, with each trying to ensnare the other while remaining bond-free themselves.10 And we might expect that individuals would be motivated to demonstrate their suitability as a potential mate with expensive displays of self-sacrifice, whether through risking disease by kissing and close contact, through buying expensive gifts, or through displaying their cooking abilities.

  These types of behavior involve sacrifice of time and resource, but are necessary because the stakes in reproduction are so high. But why should we have feelings about love? Being able to reason consciously about the benefits and costs of any long-term plans is highly adaptive. If you can include what your ancient animal motives are telling you in this high-level cogitation, then you can make better plans. Hence a strong love feeling about someone is a signal between different parts of your brain; your lower-level motivated brain is drawing the attention of the reasoning brain to the fact that something important is going on down there, which you should pay attention to as you daydream about your long-term plans.