Like many features of the human condition, the first psychological account of disgust comes from Charles Darwin, who in The Expression of the Emotions in Man and Animals defined it this way: “Something revolting, primarily in relation to the sense of taste, as actually perceived or vividly imagined; and secondarily to anything which causes a similar feeling, through the sense of smell, touch and even eyesight.” Theories of disgust bounced around following Darwin. Throughout the 20th century it was a niche area of research, but by the 1990s disgust was popular in psychology. Spearheading this movement was Paul Rozin, a clever psychologist who devised several experiments that revealed what elicits disgust. Think about eating soup from a sterilized bedpan or eating chocolate molded to resemble dog feces. Not pleasant, right? Rozin’s insight was that disgust is the “fear of incorporating an offending substance into one’s body.”
Disgust’s evolutionary origins are not a mystery. Humans are omnivores (we eat just about anything we can digest), so disgust acted as a food rejection system – a helpful emotional reminder that it’s not safe to feast indiscriminately. This is why carrion, vomit, feces, mucus, rotten meat, effluvia and other things loaded with dangerous microbes and parasites are so repulsive. Hundreds of thousands of years before Louis Pasteur discovered germ theory, natural selection had already endowed us with an implicit knowledge of it, which is why we not only refuse to eat said contaminates but also touch and think about them.
Disgust is universal but humans don’t express it until they are between three and four years old. In a slightly evil experiment Rozin and his colleagues found that children happily gobbled up dog feces (it was really peanut butter and smelly cheese) and grasshoppers. For parents, this study confirms the obvious: children younger than two put virtually everything in their mouths – a behavior Freud thought linked to sexuality (it doesn’t). Because disgust emerges a few years after birth it differs from culture to culture beyond a few universals. The mystery is: Why do different cultures develop disgust for different foods?
One line of reasoning is that disgust is a reaction to health issues. Many Jews believe that Judaism forbids pork because pigs are dirty. Some Muslims likewise think that the Islamic code that designates what foods are permissible for Muslims, Halal, bans the consumption of pork for health reasons. This explanation is plagued with inconsistencies. It’s true that pigs wallow in their own urine and eat feces. But this is also true of cows, dogs, and chickens under certain conditions.
Another possibility is that disgust was used to strengthen community bonds. As Steven Pinker puts it, food taboos “make the merest prelude to cooperation with outsiders – breaking bread together – an unmistakable act of defiance.” Judaism might have forbidden pork because the Philistines, who were the one of the Israelites’ main opponents, ate a lot of it. (H/T Geoff Mitelman)
The more plausible explanation comes from the anthropologist Marvin Harris. He argues that ecology played the dominant role, namely, that what food a culture deems disgusting is determined by the value of the animal the food comes from. In his 1974 book Cows, Pigs, War and Witches Harris observes in a chapter titled “Pig Lovers and Pig Haters” that Semites refuse to eat pork while people of highland New Guinea crave it. What explains this porcine paradox? Harris points out that North Africa and the Middle East, where Semites are from, lack vegetation including essential foods like nuts, fruits and vegetables. Pigs eat these foods as well, so domesticating them would be a burden on human nutritional needs. In contrast, vegetation in New Guinea is plentiful but protein is scarce. Pigs in New Guinea were therefore more valuable dead, cooked and eaten. All of this is consistent with the fact that kosher animals, including cattle, goats and sheep, survive off desert plants that are not valuable to humans. A similar example comes from Hinduism where slaughtering cattle is prohibited because (if Harris is correct) cattle pull plows and provide milk and manure. They are, in sum, worth more alive than dead.
Another question is how disgust and morality are related. A key piece of literature that addresses this question comes from a 2008 paper by Rozin, Jonathan Haidt and Clark McCauley. Building on previous research, they argue that communities co-opted a physical disgust for food and bodily functions into moral codes to establish rules about purity. If this is true it explains why cleanliness is a virtue in several cultures and religions including Hinduism where people are prohibited from wearing shoes when they walk on the courtyard of a temple. It also helps explain why the Abrahamic texts have so many rules concerning menstruation and sex. Western secular liberals might have trouble relating, but they are also disgusted when, for example, a person’s rights or dignity is violated.
Under this paradigm our disease avoidance system “spilled over” into our moral codes. This seems like a reasonable theory. For example, there are plenty of things I find disgusting that I don’t make a moral judgment about. In a recent Bloggingheads conversation between Paul Bloom and David Pizarro (leading researchers in the field), Pizarro points out that he finds nose picking disgusting but he does not make moral judgments about nose picking or nose pickers. Similarly, Bloom says cheekily, a poopy diaper might be gross but no one would blame the kid for pooping. Another idea is that the disgust for dangerous foods and bodily functions and the disgust for other things including people, practices and ideas are one in the same. However, a lack of evidence makes it difficult to determine which one of these theories is more plausible at this point in time.
Disgust, it should be said, is not necessarily a good guide for morality. Liberals in the United States criticize homophobic conservatives for deeming homosexual sex immoral just because they find it disgusting, implying that disgust is not a sufficient justification. But when the same liberal thinkers are pressed to explain why things like child molestation, incest or having sex with chickens are immoral they encounter the same problem: moral dumbfounding – what’s intuitively obvious is not always morally correct. Disgust, in other words, is not a reliable source for moral guidance. Leon Kass makes this point in an essay he penned many years ago:
Revulsion is not an argument; and some of yesterday’s repugnancies are today calmly accepted — though, one must add, not always for the better. In crucial cases, however, repugnance is the emotional expression of deep wisdom, beyond reason’s power fully to articulate it. Can anyone really give an argument fully adequate to the horror which is father-daughter incest (even with consent), or having sex with animals, or mutilating a corpse, or eating human flesh, or even just (just!) raping or murdering another human being? Would anybody’s failure to give full rational justification for his or her revulsion at these practices make that revulsion ethically suspect? Not at all. On the contrary, we are suspicious of those who think that they can rationalize away our horror, say, by trying to explain the enormity of incest with arguments only about the genetic risks of inbreeding.
A scary consequence of morality based on disgust is what happens when it is extended to out-groups. Sometimes a community will lump members of an out-group into a category and equate it with what’s physically disgusting. This is one hallmark of ethnic cleansings and it occurred during the Rwandan genocide when the Hutus equated Tutsis with “cockroaches.” To paraphrase Haidt, moral rules based on disgust bind and blind.
So what is disgust? It is a disease avoidance system put in place by natural selection to prevent us from consuming harmful food and bodily fluids. Effluvia, vomit, feces, rotten flesh, and urine are disgusting to people around the world. It can’t be a coincidence that these substances contain dangerous diseases. The question is how disgust emerges in different cultures. Harris postulates that it relates to ecology and economics. I mentioned that it’s possible that disgust evolved not just as a disease-prevention system but also as a tool to distinguish “us” from “them”. However, it seems more likely that disgust for anything that is not food or a bodily fluid is a byproduct of a disease-prevention system.
Sometimes disgust results in quirky behavior. People are disgusted by the thought of wearing the socks of a rapist or Hitler’s sweater. Other times disgust is more significant, especially when large groups of people label other groups disgusting. From the trivial to the consequential, it’s important that disgust doesn’t guide morality. I hope people are rational enough to realize this.
In the last two decades psychological science has conducted brilliant research to uncover what Darwin described nearly 150 years ago in The Expression of the Emotions in Man and Animals. A lot of credit goes to Paul Rozin, but other researchers including Bloom, Pizarro (and their colleagues Yoel Inbar and Ravi Iyer), and Haidt are providing insightful findings with clever experiments. If the next twenty years are as fruitful as the last we’ll have a much more complete picture of this nauseating corner of human psychology.
 From Pinker 1997.
 Leviticus 11:7-8 “And the swine, though he divide the hoof, and be clovenfooted, yet he cheweth not the cud; he [is] unclean to you. Of their flesh shall ye not eat, and their carcase shall ye not touch; they [are] unclean to you.”
 Leviticus 15:19-30 “And if a woman have an issue, and her issue in her flesh be blood, she shall be put apart seven days: and whosoever toucheth her shall be unclean until the even.”
 Kass does not conclude that it is correct.