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Have you ever taken a sip of sour milk? Or watched a Saw movie? Or spotted a spider in your soup? Why do these different experiences all produce the same repulsed reaction? Rachel Herz's new book, That's Disgusting, explores the psychology behind disgust, touching on everything from cannibalism to competitive eating along the way. Here, she shares some of her insights into the complex emotion.

Why did you choose to write a book about disgust?

Well, it actually began sort of as a joke. My first book was called The Scent of Desire and it's all about the psychology of smell, and from writing that book I got popular within smell circles. One of the results was that I was asked to be the judge of the Rotten Sneakers Contest, where I had to sniff smelly sneakers.

People were asking me: 'How can you do this thing? It's so disgusting!' and I made a joke that it was for my next book, which was going to be called The Scent of Disgust, but one thing that happened from that little quip is that I started thinking about what I was doing more deeply, and started thinking about what this experience was going to be like … and in fact it was not as bad as I thought it was going to be. I realized how much thinking had influenced my perceptions.

You mention in the book that disgust is culturally subjective. Why is that?

Disgust has to be learned. Newborns don't have disgust; children before the age of 3 really don't show disgust. So, depending on what your culture is, you will be taught that certain things are disgusting. For example, it might be fine to eat earthworms in particular cultures, and, in fact, insects are a major staple in diets of people around the world.

What's the evolutionary advantage of disgust?

Well, I believe, and a number of other people believe this too, that disgust evolved from the emotion of fear to deal with threats to our survival that are slow and that require a lot of thinking to understand. Our very long lifespan means that, coupled with the fact that we don't have any natural predators, our actual natural predator is germs.

So if we want to stay alive for as long as possible, we need to stay away from germs for as long as possible, and the idea that disgust evolved to protect us from that comes from where you think that disgust would be a motivator for, for example, why you don't want to sit beside the guy with the pussy, oozy red sores.

People often talk about being disgusted by others' bad behaviour. Is moral disgust actually disgust?

It is not the same, except when bad behaviour also involves visceral things. So, if someone set fire to your home and killed your cat, then that action involves a visceral thing, your cat burning up. That is a disgusting thought and so that quality could make the arsonist's behaviour seem disgusting.

If someone was just an arsonist and lit fire to an empty building, you would maybe think that's a moral violation and feel anger and outrage, but I do not believe the emotion of disgust would be elicited.

What's also interesting, though, is we say 'that's disgusting' for things like the arsonist who burned the building down, and when we say 'that's disgusting' it seems as though we can condition a physiological pang of revulsion just by using the word. This is because the baseline response we have when we are disgusted is actually physical – we feel nauseated, we feel sickened, and so the brain is conditioned to elicit this physiological reaction of feeling sickened when the word disgust is invoked.

Why are we more grossed out by other people's bodily fluids than by our own?

With respect to disease again, what's inside me is obviously not harmful to me, but the saliva or the blood from somebody else could be contaminated, could make me sick, could bring in foreign germs and so is threatening from that disease perspective.

It's also the case that disgust is basically preventing the outside from getting into your inside. If you think about the human face, when we make the facial expression of disgust, the mouth sort of purses together, so it's basically like you're trying to close yourself off from the outside so that it doesn't get in.

What I think is really interesting is that if you think of your saliva right now, inside your mouth, that's probably not grossing you out at all, but if I told you to spit in a glass and then drink from that glass, how would you feel?

That's really gross.

But it's no different; that's your saliva. Two seconds ago it was sitting on your tongue being totally neutral and benign and now that it's suddenly gone to the outside, it becomes this threatening, nasty thing. There's this aspect of how the outside going back into you is a lot worse than the inside coming out.

Your book gives fairly graphic descriptions of some pretty gross things. Did you become less easily disgusted in the course of researching That's Disgusting?

I realized while I was writing it that if I stopped to think about it my face was in a disgusted position, but apart from that, I became less sensitive to disgusting things.

What's interesting is that I've become more germ-phobic. I'm much more careful now to open the public restroom door with my elbow. Realizing from researching and writing this book how disease can be such a threat, and it is really our No. 1 predator, has made me much more conscious of that component, although I'm not disgusted, I'm just more germ-phobic.

This interview has been condensed and edited.


From Rachel Herz's That's Disgusting:

My favourite fermented challenge, because I'm a cheese lover but am mortally repulsed by worms, is casu marzu. Casu marzu is a sheep cheese popular on the Italian island of Sardinia. The name means "rotten cheese" or, as it is known colloquially, "maggot cheese," since it is literally riddled with live insect larva. To make maggot cheese you start with a slab of local sheep cheese pecorino sardo, but then you let the cheese go beyond normal fermentation to a stage most would consider infested decomposition – because it is. The larvae of the cheese fly (Piophila casei) are added to the fermenting cheese and the acid from their digestive systems breaks down the cheese's fats, making the final decomposed product very soft and liquidy. By the time it is ready for consumption, a typical casu marzu contains thousands of larvae. Because locals actually consider it unsafe to eat casu marzu when the larvae have died, casu marzu is served with the live larvae actively squiggling.

The larvae are translucent white worms – maggots – about one-third of an inch long. Some people clear the maggots from the cheese before consuming it, others do not. Those who eat it with the maggots still milling about, cover the cheese with their hands to prevent the maggots from leaping onto them or anything else, since when they are disturbed the maggots can jump distances of up to six inches.