Speaking of Psychology: Yuck! What disgusts us and why, with Paul Rozin, PhD

“Disgusting” is a flexible word—it could describe everything from a putrid smell to your least-favorite food to a behavior you find immoral. But what does it really mean to be disgusted? Paul Rozin, PhD, talks about where disgust comes from, why some people are more easily disgusted than others, universal triggers of disgust, why the foods we consider disgusting vary by culture, why is gross-out humor can be funny, and more.

Paul Rozin, PhD, is a professor emeritus of psychology at the University of Pennsylvania and one of the world's leading experts on the psychology of disgust. His research has touched on human food choice, the development of food aversions, attitudes toward meat, how disgust carries over to morality, and attitudes toward recycled water. Rozin has published hundreds of scientific papers and has won many awards for his work, including the APA's Distinguished Scientific Contribution Award for 2007.

Kim Mills: What’s the last really gross thing you encountered? Was it a cockroach? A dirty diaper? Yesterday’s smelly garbage? Most of us are lucky if we go a single day without encountering something we find at least a little revolting.

But what does it mean to call something disgusting? The word can describe everything from a putrid smell to our least favorite food to the word “moist.” Over the past several decades, psychologists have begun to dig into the concept of disgust, exploring where it comes from and how many things it can stretch to encompass.

So why do we feel disgust? Why are some people more easily disgusted than others? Are babies born with the ability to feel disgusted or does it develop later in life? Is there anything that all humans find disgusting? Why do the foods that we consider disgusting vary so much by culture or even by individual? Why is gross humor funny, at least to some people? And how might understanding disgust help us address some important public health issues?

Welcome to Speaking of Psychology, the flagship podcast of the American Psychological Association that examines the links between psychological science and everyday life. I’m Kim Mills.

My guest today is Dr. Paul Rozin, a professor emeritus of psychology at the University of Pennsylvania and one of the world’s leading experts on the psychology of disgust. He’s been studying disgust in its many forms since the 1960s. His research has touched on human food choice, the development of food aversions, attitudes toward meat, how disgust carries over to morality, attitudes toward recycled water, and much more. Dr. Rozin has published hundreds of scientific papers and has won many awards for his work, including the APA’s Distinguished Scientific Contribution Award for 2007. 

Dr. Rozin, thank you for joining me today.

Paul Rozin, PhD: Pleasure to be here.

Mills: Now, before we dig too deeply into some really revolting topics, I want to warn our listeners that we might talk about some things that will gross them out. So if you have a weak stomach, maybe this isn’t the episode for you and you can come back next week. 

That being said, Dr. Rozin, where does disgust come from? Why did we evolve to feel this unpleasant emotion?

Rozin: We’re not really a hundred percent certain how it, disgust, originated in human evolution. It seems to be uniquely human, and it’s quite clear that it serves a function which is to keep us away from microbes, dangerous microbes. Whether that’s the reason it was originally adopted, we don’t know.

Mills: Now are babies born with the ability to feel disgust or does it develop later in childhood?

Rozin: Well, disgust is not innate, not present at birth. Little infants have an innate aversion to bitter tastes and other tastes, and the face that they make to that is similar to the face that we make to disgust, but disgust doesn’t appear until maybe 3 to 5 years of age.

Mills: What are the things that kids at that age are disgusted by? Is there any pattern?

Rozin: It’s—surprisingly we don’t know that well, and the reason is that feces is the universal disgust, and it’s probably true that toilet training is the experience which makes children have their first disgust. But I think it’s because toilet training is itself disgusting that most people don’t study it. So there’s very little research on toilet training and what actually happens between the mother and the child and the diapering and all of this that communicates to the child what a terrible thing this is.

Mills: There are some things that are gross to other people, but not to ourselves. For instance, I would venture to say that most people don’t find their own poop as disgusting as other people do. People pop their own zits, but if they see somebody else do it, they’re going to vomit it. Yuck. What is the reason for that?

Rozin: We don’t really know. Now, it’s true that your own poop or your own—any of your own body waste is less dangerous to you than to other people because you’ve already got that stuff in you. So that’s a functional reason why you might not as be as upset about your own body waste as others. And of course, you’re living with it. You’ve got a colon filled with feces and you’ve got a mouth filled with spit and you sort of live with it. But even then, most people will not drink a glass of water if they spit into it first. So that was something of their own, but as soon as it’s out of their body, it’s disgusting. 

Mills: Is disgust an emotion? Is it a physical sensation? Is it both? 

Rozin: Well, it is generally considered one of the basic emotions, and that’s because has the features of emotions. It has a facial expression, which you’re all familiar with. It has a physiological presentation, which is nausea, and it has a behavior, which is withdrawal from the source of disgust that’s like fear or anger or other emotions, and it has some sort of appraisal. That’s the other feature of an emotion that is say you have to—your mind has to appreciate what it is that’s causing it.

Mills: You said earlier that there’s one thing that’s universally disgusting and that’s feces. Is that the only thing that’s universally disgusting to humans?

Rozin: Well, nobody has again looked at this carefully across cultures. So it’s conceivable that, for example, vomit is also a universal disgust. By the way, one of the important things about disgust is that little infants don’t find anything disgusting. They don’t find feces disgusting. In fact, one function of diapers is to keep them from eating it. And so until toilet training, they don’t seem to have any issue with body waste. So I think that there are probably other body waste that might be disgusting. They could probably—vomit is one of them. I don’t know that anyone’s actually looked at that, believe it or not.

Mills: You mentioned bodily fluids. I mean, some people, and I’m one of them, will faint at the site of a lot of blood. Can you overcome that sort of disgust?

Rozin: Well, in general, you can overcome any disgust, including feces. Well, the main reason you do that is because you’re exposed to it enough. For example, we’ve shown that medical students who dissect a cadaver in their medical training originally are very disgusted by the cadaver, but they get used to it. People get used to things that are disgusting that are in their lives all the time. Perhaps one reason you get used to your own feces and saliva is that you live with it. You get used to it. So people get accustomed to anything, not just disgust, anything, if you expose them often enough, except maybe extreme pain.

Mills: You mentioned before that humans seem to be the only animals that feel disgust. And I know I walked my dog this morning and I have to watch her because she will eat her own feces and she’ll sniff other dogs’ butts, they’ll sniff hers. They are really into all of that. What about our closest relatives, chimps? Do they experience disgust or will they do the same thing as say, my dog?

Rozin: We don’t know absolutely for sure, but many primates eat their feces. So that’s a start, which would be like human infants, of course. The other thing is there’s one feature of disgust that was far as we know no other animal shows, and that is what we call contamination. That if something disgusting touches an edible food, it renders it inedible. So you think about a cockroach touches your favorite juice, drops in your favorite juice, that’s the end of the juice, right? You won’t eat it.

Now, there’s no evidence that any animal shows anything like that. So it’s a very potent thing about the disgust. It can take a pile of delicious food and spoil it with one touch of a feces or a cockroach or something. By the way, there’s no opposite to disgust. If you take a pile of cockroaches, there’s nothing you can touch to it to make it good to eat. So it’s just the opposite. The disgust is very potent and very negative and there’s no positive equivalent.

Mills: But I thought I’ve heard you say that for example, say Hitler had a sweater. I mean, people wouldn’t touch Hitler’s sweater because it was contaminated by belonging to him. And no matter what you did, no matter how many times you cleaned it, it’s still contaminated. Unless say Mother Teresa wears it, right? Have you found that?

Rozin: Yes. We found that for most people, and that’s what’s called positive contamination. When Mother Teresa or your favorite rock star or something, touches something, it becomes more valuable, but it’s weaker than the negative. So no one wants to wear Hitler’s sweater, but many people don’t care about having the sweater of somebody who’s a hero of theirs. And the hero is less potent than the evil person, but you can overwhelm it with enough exposure up to the positive person. We call this negativity dominance—that negative things in general are stronger than positive things.

Mills: Now, you’ve done a lot of work around the idea of meat being disgusting. Can we talk about that? I mean, why is meat disgusting, as much as we eat a lot of it?

Rozin: Yeah. Actually, the question in a way, to me really is why do we eat any meat? Because we are in fact, say take Americans, we eat cow, pig, maybe lamb. Okay, out of 4,000 mammals, we eat three, and we don’t eat most of them. We only eat their muscle. Most people won’t eat their liver. Nobody eats their eyeballs. In fact, we reject almost all the meat in the world and consider it disgusting. And there’s a small subset which we come to love. So the question really is not why do we find meat disgusting, but rather why do we find a small set of meats very attractive?

Mills: And do you have the answer to that?

Rozin: No, but it is clear that little children when they start eating meat don’t know where it comes from. I mean, what’s disgusting about a piece of meat is that it’s a piece of an animal, and it’s an animal that we generally find disgusting. So it takes a while for kids to realize that this red stuff that they get from the supermarket and cook up is actually a piece of a pig or a cow. In fact, my son, when he was about 6 years old, was eating chicken at the table, which by the way has the name chicken to help you think that this is actually from a chicken. And he suddenly said, My God, this is a piece of chicken. I mean, he made that discovery. Now, it’s interesting that we don’t name most meats by the animal they’re from. In Spanish, you say beef is the meat of cow. That’s the word for it. But in the English, we say beef, which is not cow. We don’t say we’re eating cow. We don’t say we’re eating pig. So that’s one way of making distancing from ourselves, from the fact that we’re eating a piece of what was once a living animal, is to change the name.

Mills: What about things that are more culturally specific when it comes to food? Why don’t we all find the same foods disgusting across cultures and nationalities?

Rozin: Almost all disgusting things are animal products. First of all, very few people are disgusted by asparagus. They may not like it, but they don’t call it disgusting. Many Indians are put off by meat, and there are hundreds, millions of them are vegetarians. So there is variation within culture for sure. One issue is that certain disgusting things become very desirable. So for example, in the United States, many people, not all, like smelly cheese, which is rotted milk, and that has the smell of disgust. The decay smell is the smell of disgust. That’s the quintessential disgusting smell. And other people from other cultures can’t believe we eat that. But on the other hand, in Southeast Asia, most of their food is seasoned with fermented fish sauce, which smells rotten. And they love that, and many other people can’t imagine why they’d eat that or rotten eggs. So cultures take a very small subset of all the foods they can eat, and they actually make them rotten and disgusting to most people. Little kids will eat them, of course, babies, because they don’t know about disgust yet. Kids have to learn to eat those things after they’ve learned not to eat feces. And anything that smells like that, we say, whoa, whoa, whoa.

Mills: This cheese is a little bit like that. But it’s great. So if you become an adult, then it becomes that much harder to eat something like Marmite, for example, which I think is disgusting.

Rozin: Marmite or Vegemite, they’re both the same thing. They’re not animal products. The question is whether you’ll, let’s take a food that you really like and put a little drop of Marmite in it. You won’t taste it, tiny amount, when you eat it. People will say, sure. It’s not like a cockroach. So it doesn’t have that disgust quality to it. And almost everything that has that quality is an animal product. And animals are, of course, are highly nutritious, highly favored food. And we’ve gone through great difficulty, a great to disguise that, particularly in the United States. In Europe, you’ll see in the market animal carcasses hanging up or the fur will be on rabbit meat when they sell it. But in the United States, we wrap it in plastic. You don’t have any idea where it’s from. We don’t do that with fish. You can buy a whole fish very easily, and some people are put off by that. And if you serve them a whole fish in a restaurant, they’ll be put off. But other people have no trouble with that. As I say, we get used to disgust.

Mills: How wide is the range of disgusted sensitivity? Why are some people so much more easily disgusted than others?

Rozin: First of all, there is an enormous range. Let me just give you an example. Some people will have no trouble eating rotten things and any kind of meat, raw meat, things like that. And they’re not upset about body waste. They don’t like them, but they don’t get upset about it. Other people, for example, would not blow their nose in a brand new piece of toilet paper, which is pretty sensitive. So you have this enormous range of sensitivity disgust, but we don’t know why this is true. We can see there’s some genetic components, some early environment, but a lot of the variation we say can’t be explained in anything and liking dogs and all kinds of things that people do.

Mills: Might it be that somebody has a more acute sense of smell and taste and that that person would be more easily disgusted? Has that been looked at?

Rozin: There’s no evidence that that’s true, just as you wouldn’t say that an artist like Picasso, did Picasso have very good eyesight? Probably not. And this is the same thing with disgust. You might be able to detect a decay odor in the refrigerator, say more rapidly than anyone else, but whether it bothers you or not is another matter. It’s not the detection. It’s how much it upsets you.

Mills: In many ways, it feels like we’re more removed from disgusting things today than we used to be. I mean, we have indoor plumbing and we don’t have to—most of us don’t have to kill our own food. Do you think that we have a lower disgust tolerance as a species as a result of this?

Rozin: It’s probably true that we’ve been protected from disgusting things. On the other hand, you could say if you’re exposed to a lot of disgust early, you might get more tolerant of it. So there’s every reason to believe that we’ve had less experience with disgusting things than people a hundred years ago, no question because of all things you mentioned, the plumbing and so on. But the question is whether that makes you more or less sensitive to disgust is another question. You could become adapted to it or you could become sensitized to it. We don’t know. Those are two opposite processes. When you’re exposed to something like an allergen, you get sensitized to it, right? Your exposure makes you more sensitive. There are lots of things that we learn to pay no attention to because we get adapted to them. So you can’t always tell when someone’s exposed to something, whether they’re going to be traumatized or they’re going to get used to it.

Mills: Now, what got you interested in this topic to begin with and why have you stuck with it for so many years?

Rozin: I was working on human food choice starting in the late 1970s, but I wasn’t working on disgust. And one of the things I got interested in was meat, because it’s such a favorite food of man, of humans. And then I discovered that, isn’t it interesting that almost all the taboos of food are about animal products? And then I started looking at that and I realized that many of the taboos were associated with disgust, but people didn’t want to, they got nauseous and thinking about them. They didn’t want to have anything to do with them. They’d get upset if other people were eating them in their presence. Nobody who hates asparagus has a problem with the person across the table eating asparagus. So this was a very powerful thing, and I realized it was one of the most powerful reactions that people have to anything, not just food. It’s so powerful, as I said, that if it touches something you love, it spoils it. So that’s what got me into disgust. It was starting out with meat.

Mills: The fact that we experience disgust as much as universally as we do, I think could be deleterious to us going forward in terms of water. I mean, we are in a situation where at some point in the not too distant future, there are going to be parts of the country and if not the world where people don’t have enough water, and yet it’s very easy to recycle human waste. I mean, we could recycle human waste within seconds and make it drinkable. Why is that such a difficult thing to overcome? I mean, in fact, aren’t we all essentially drinking dinosaur pee anyway?

Rozin: Well, one of my colleagues calculated for me, a geologist, that everyone in Europe who takes a glass of water are drinking a few molecules that went through Adolf Hitler. Now, Adolf Hitler is a great contaminant. People don’t want anything to do with it. So what happens is all of our water that we drink is contaminated because it’s been in toilets and all sorts of things. And yeah, it goes down streams and it gets processed, but there’s still molecules of water in there that were in very bad place. So we get used to that. We just don’t think about it. Now, if you happen to go to visit a sewer plant, it might put you off for a while because you have in your mind the origin of the water. But of course, it’s very important to drink water. And if you were put off by disgust and water, we would be in very bad shape. So what we do is we don’t think about it.

But the movement to purify water, to solve the water problem by taking fouled water and reprocessing it into pure water, safer water than the water that comes into your forest. Now, people are very put off by it because it’s a bad image, and they think of it that way. It reminds them that water is sewage, processed. Now, Singapore has been a leader in this area because they are short of water and they’ve had to recycle water. And what they’ve done is a very good job of convincing people that it’s okay. They have a facility, a water purification facility that you can visit. You can get bottled water made from it, they’ll provide it to you for your parties and so on. It tastes fine, of course, and they’ve done a good job of selling it, and they mix it with their groundwater. They don’t actually go right from the purification. They don’t have enough groundwater, but they have some. So they’ve done a good job of selling it.

And in southern California where there’s a water shortage, there have been some cases where it’s gotten acceptable, but it takes some work because the people who were opposed to it, who were opposed to this process, they have a saying.

Mills: Oh, “toilet to tap.”

Rozin: Toilet to tap. Yes, toilet to tap. That’s the one. And so they push it in your face and they can sue. And in our country, you can sue and stop anything for a while. So this has happened. But from the public health point of view, producing purified water from sewage is a very positive thing, and disgust gets in the way of it a little.

Mills: So it sounds like a marketing problem. 

Rozin: Yes. 

Mills: What is the answer? I mean, how can we get people to get over that disgust factor so that, I mean, this is about public health, right?

Rozin: Well, it’s interesting that reason doesn’t work very well with disgust. Explaining to people that it’s not really disgusting. When we drop a cockroach into juice and people won’t drink it, they say, well cockroaches are disease vectors. We say, okay, we’ll do it with a sterilized cockroach. This cockroach is safer than your fork. So we put it in, we take it out—I still don’t want it. Okay. So we have a situation where reasoning with people doesn’t help. The best thing is exposure. If you get exposed to something that’s mildly disgusting, not terribly disgusting, to start with, you get used to it and then you don’t pay any attention to it. And that’s certainly true with water, and I think it’s also true with insects. Something else I’ve discovered, insects are a very good source of food, and they’re inexpensive and they’re better for sustainability, and they don’t come with a killing problem. Most people don’t mind killing mealworms or something like that. So the main thing is to get people to try them and then try them a little more and then get used to them because that’s a powerful way of doing things.

Mills: In some sense, the idea that they came out of the dirt—in addition to that, I mean, but all animals at some point are in something dirty.

Rozin: You can make stories about this. I mean, people think pigs are sort of dirty animals because they eat garbage. Cows don’t eat garbage. But people are very happy to eat pork. Not orthodox Jews, but most people. It’s very hard to figure out why some animals are more disgusting than others. It is true that if they have slimy habits, or if there are carnivores, we tend not to eat carnivores like cats or dogs. And that’s maybe because they eat meat and they also have much more putrid feces. Elephant dung is not that smelly, less disgusting than leopard dung. So there are a lot of things going on there. There’s a lot of people interested it, but no one’s gotten too far on why certain animals are eaten and others are not.

Mills: Why is it that a lot of people like really gross humor? I mean, every little kid and a lot of adults will laugh at poop jokes, but what’s the connection between disgust and humor?

Rozin: There is a big connection. Everyone who’s had a little boy knows that little boys love to gross out their parents and gross out little girls. Anyway, so yeah, it is a source of humor. I think it’s part of a much bigger picture that humans get a lot of pleasure out of doing things and enjoying them that are originally negative. So they like to go down rollercoasters because their body thinks that they’re dropping to their death, but they know they’re not. There’s something pleasant about experiencing something negative, but knowing it’s not really threatening. So poop jokes are not threatening to you. You’re not experiencing it directly. So if you are walking down the street and someone in a tux and his partner in a gown are going to an event and they step in dog doo, that’s sort of funny. But if you step in dog doo, it’s not funny. So the defensive, I call it benign masochism, being safe, feeling that you experiencing something negative and your body is telling you, get out of here. But you know that it’s really okay and there’s a certain pleasure in that. And I think that’s partly what’s going on with disgust humor.

Mills: We’ve talked about how disgust may have started as a way to keep ourselves from eating unsafe foods, but now we use the term a lot more broadly. We will talk about being disgusted by a person’s selfish behavior, for example. Is that the same kind of disgust that we feel viscerally when we see something like poop?

Rozin: Well, it’s a big discussion in the literature now as to whether what we call moral disgust is really just a name “disgust” attached to it, or it really involves a similar reaction. So some people will make disgust faces when they hear about certain kinds of crimes, particularly bodily crimes like murder. The big issue is—are all moral events, immoral events disgusting? The answer seems to me no. It’s only a certain subset of them. Those subsets, that’s subset that has to do with body and purity.

So people might say that, for example, using a crucifix as a doorstop is disgusting because that’s a violation of what Richard Shweder calls the ethics of divinity. There’s certain things that are sacred and you don’t defile them, and that’s the kind of thing that disgust—other people think it’s more about bodily things like being slashed or murdered or cannibalism.

But it is clear that in some cases of moral disgust, people do seem to feel nauseous. Incest is a great example. People tend to feel nauseous about insect, but that’s not true for things like theft. There’s bloodless crimes. So it looks like anger is the emotion that’s mostly involved in most what we call immoral behavior. But there’s a subset of it having to do with bodies or divinity we might call, which is disgust related. That subset is much less in the United States than in Hindu India, where divinity is a major issue, sacredness of food, very big issue. But in the United States, we limit that and most of our laws and crimes are about harming other people. Most of the moral violations that disgust is involved in don’t necessarily harm other people. For example, burning the American flag doesn’t really harm any other people, but it’s a divinity violation that’s disgusting.

Disgust is used in public loosely to mean bad. You say, That’s disgusting. You say, I heard that so-and-so does something minor, I don’t know, cheats on their income tax, and Oh, that’s disgusting. But that’s the very loose use of the word where you’re just saying bad. And so one of the problems is you have to distinguish the use of the word disgust to mean bad, to disgust in a more narrow, interesting sense.

Mills: So what are the big questions that remain to be answered about disgust? Are you still doing research in this area? And what would you like to know that you don’t know yet?

Rozin: Well, first of all, my own research, I’ve gotten out of most disgust research, and the reason is it’s become very popular. I like to work on things that not many people are working on and disgust is a hot subject now. So I’d have to read too many things to be able to do my research. I am interested in one feature of disgust, which not many people are interested in, which is contagion, the idea that disgusting things are contaminating. What does it mean that the juice that I cockroach was dipped into is now bad? What is it in your mind that has happened to that juice? Essentially, its cockroachness has entered that juice just like Hitler has entered his sweater even after you launder it. So I’m interested in what the thoughts are. What is it that stuff that comes in? Some of it’s like germs, which is of course the origin of disgust, but others are not.

So for example, you can do anything you want to the Hitler sweater and people don’t want to wear it. You can kill all the germs and you can wash it and wash it. So there’s something more spiritual in people’s head that enters from Hitler that is not washable, and I’m interested in that sort of thing. That’s an issue in disgust.

But the biggest issue in disgust now is what—first the issue is what are the brain mechanisms of it? And what’s interesting about that is it’s very easy to get people disgusted. It’s much easier to get people disgusted than to get them angry. So if they’re in a scanner, for example, all I have to think of is a piece of dog poop on a plate, and they’re disgusted. You can’t get them really angry so easily. So it’s been a very good tool to study how people react to emotions. The other big issue is what moral disgust is. Is it really the same kind of thing as disgust, it’s the same system that’s involved, or is it really just the word disgust that’s tagged onto these things?

Mills: Dr. Rozin, I want to thank you for joining me today. I think we had a perfectly disgusting conversation.

Rozin: Okay, good. Well, thanks. So nice talking to you.

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