The Origins of Disgust

Being impressed by the cognitive abilities of a chimpanzee isn't just good for them; it is good for us, because it helps us learn about our own evolutionary history. Comparing the psychology of humans to the psychology of other primates is a great way to illuminate what it is we maintained, lost, or changed as our brains diverged from other primates.

Now, I'm really excited right now because, in the spirit of sharing, The Primate Research Institute has agreed to show me some of the other ways they're learning about us by studying those who are almost us. I'm heading to the parasite lab to meet up with associate professor Andrew McIntosh and doctoral student Cecile Serabian. Their work with Japanese macaques has led them to theorize that primates' feelings of repulsion or disgust could have served a protective purpose in our evolution.

To study disgust, we will be working with one of the most famous disgust inducers: poop. Andrew, Cecile, thank you for meeting up with me; I can now tell people that my time in Japan was crappy. [both chuckle] [Andrew] Just like every day for us.

So tell me, why is there a bunch of poop on the table? Because we were studying the social behavior and parasite infection of the macaques. [Cecile continues] Right. If we want to say that humans are somehow especially hygienic or disgusted by those kinds of objects that might make us sick, it's quite difficult to separate whether that's an evolutionary response to something or just a cultural phenomenon that we've kind of created on our own.

[Michael] Now these are clearly toys. They're made of plastic. These look kind of different. Okay, so you have different types here on the table. This is the type for carnivores; it's not the type of primates. Primates have a long-shaped type of poop. It's called type 9. -So those are all-- -Wait, poops have numbers? -[both chuckle] -I only know number 2.

Why make play poop when you have plenty of chimps out there giving you real poop to use? Ah. -So there's no odor or texture here. -Yeah, exactly. And what differences did you see? [Cecile continues] Ah, so it's like a surface to put the food on top of. -They would avoid the replica? -Yes. [Cecile continues]

[Michael] Oh, really? Right, right. If you expose a bunch of individuals to a range of different kinds of avoidance experiments, we found that those individuals that are much more hygienic actually were exhibiting lower infection, so... It sounds like a really strong argument in support of the idea that we evolved an emotion like disgust - or a behavior like avoidance of infectious-looking stuff. - Mm-hmm. -Because it had an advantage. -You get infected less. -[Andrew] That's right.

So you have some kind of experiment for me? -Yep. -Is it safe? -Let's see. -Don't play that way! -[all laugh]

Cecile and Andrew wanted to show me just how hard-wired our sense of disgust is. [Cecile] There you go. Okay. Those are edible? -Yes, those are edible. -Mm-hmm. -I'm just going to place one in each of the holes here. -Okay. If you are ready to eat them, just try to reach out for them. Okay. Good. Good. What do I do once I pull the treat out? You will tell us whether you are ready to eat it or not, basically. Oh, okay.

-So, you'll put one in there. -One goes here. Sure. -One goes here... -Okay. And one goes there. All right, I'm going to start right in the middle. Hmm. All right, so I might just blow on this before I eat it, but I feel pretty ready to eat it. -Yeah? -Because what I felt was dry, and it was solid; it was like a foam. Watch. -Okay.

-Oh, it's really good. It's got kind of a coffee taste. So far so good. I'm going to go to the left. No, I don't want this one. -Okay. -Whoa, you still went for it. I still pulled it out. [sniffing] What was it? So I'm just going to set it down. Okay. All right, it smells... it's kind of fishy. All right, now... [chuckles knowingly] I... okay.

I'll tell you what, that was disgusting in terms of how it felt, but when I see the color, I see this kind of bright green color that makes me feel less disgusted actually because it's sort of a fun color and it reminds me that it probably isn't truly contaminated. But this is the one that I'm least likely to want to eat. Even stronger than the visual cues or the olfactory cues, it seemed that touching some substrate like this gave us the strongest aversive response. Yeah.

So the first one you touched was this piece of foam here. -Yeah. -So yeah, it's basically dry. -Solid. Um, then I think you went here, and that is called konnyaku. [Andrew] Yeah, konnyaku is a kind of starch-based gelatin which is part of the cuisine here. You said it smelled fishy, so in here you probably have parts of seaweed.

Yeah, right, that's where that's coming from. It's soft, a bit wet, but not very... -It's not sticking to me. -Not sticky, yeah. And the last one here was what maybe kids in the '90s, including myself, were playing with—a slime toy. Which is like slimy and sticky. And this, it's just so gooey. Well, and the fact that it's pink makes it so much more fun than if it was like, brown.

Mm-hmm. Now, the research you're doing with macaques, is that something I could observe? We'd be willing to take you out to the macaque habitat and we could run some of these similar experiments with them for you to observe. -Ooh. I would love to. Yeah.

Cecile's work with macaque monkeys on Koujima Island led her to develop an experiment that she and Andrew have been running on the macaques here at the Primate Research Institute. They want to know if macaques avoid feces because a feeling of disgust evolved in order to reduce the risk of their exposure to parasites.

-[Andrew] And here they are. -[Michael] And here they are. Oh look at that, the babies are so cute. -Here it's feeding time, almost. -Mm-hm. Which is a good time for an experiment. Yeah, we're a bit lucky; they didn't eat that much in the morning, so hopefully they'll have that extra motivation to participate.

And we're going to try to play a bit with the value of the food, so we'll start with their regular monkey chow. -Right. -Which they see a couple times a day on a daily basis. And then we'll see what happens when we introduce that nice, tasty peanut. [Cecile speaking] [Michael] Right. And so, I'm assuming they like peanuts. -Right, so-- -They don't like peanuts in or on poop.

Well, hopefully, we'll find that out today. [Michael] Okay, so it looks like Andrew is dumping the, um... [Cecile] Those are real feces that he is going to pair with the replica feces that we have seen in the lab, and a control brown wood chip. -[Michael] Near each other? -[Cecile] Yes, near each other. Aha, all right. -Let the games begin. -Yeah.

For the first trial, Andrew places monkey chow on the control block of wood, the replica feces and the real feces. Whether or not the macaques eat it may indicate their level of disgust. [Michael] They're looking... now here comes one. -[Michael] No... oh, oh! -[Cecile] Yeah. [Cecile] No, he's smelling the wood chip. [Michael] He smelled the wood chip; the monkey chow fell off of it.

[Cecile] Yes. [Michael] Is he... gonna eat it? No. I think regular old normal food on poop is just-- it's a hard sell. [Andrew over radio] [Michael] Shelled peanuts, okay. -[Cecile] So that's on the fake poop. -[Michael] Yep. -[Michael] That's on the real poop. -[Cecile] Yeah. -[Michael] And that's on the wood block. -[Cecile] Yes.

This time around, Andrew introduces peanuts, which the macaques really love. Will this special treat override their inner instincts? Oh, oh, they're already running in. [both] Ooh. [Michael] But he threw it. -And he's washing it now. -He's washing it. All right, here comes another one. Oh, he went for the poop one! -[Cecile] Yeah. -[Michael laughs]

[Michael] The first one; he just wants it to roll. -He doesn't even want to have to carry it. -[Cecile] In the water. This is rolling, so, they do some of those behaviors. And now he's washing it in the water, -[Cecile] Yeah. -[Michael] Is it clean enough? Oh, still gotta roll. That was incredible because the peanuts that were just on the wood chip, those went straight into monkey mouths.

Was this an aberration? Andrew reloaded the peanut baits to find out. -[Michael] Decoy. -[Cecile] A female is approaching. [Michael] Yep, here she is. She smells it first, and then-- and then-- -[Cecile] Smells it again. -[Michael] Believes it, eats it. -[Cecile] On the fake.

-[Michael] That was on the fake, yeah. Very cool. Now she already ate off the decoy and it was safe. She hasn't, oh she put the control right in her mouth. -[Cecile] Yes. -[Michael] And she's going to wash the real feces. [Cecile] Yeah. And she went for the real feces last. Yes, she did. [Michael] She's got a baby with her, so I really hope she cleans that.

[Cecile] Or maybe that's the disadvantage; she's carrying a baby, she cannot afford, like, cleaning everything. Yeah. I'm really amused by how they visually recognize feces. -I mean we're far away; it's hard for me to tell, -Yes. but they can see that shape or texture-- -"Be wary." -Yeah.

Over the course of several rounds of the experiment, the monkeys would eat off of the control, wash food from the replica, and be extremely hesitant towards anything that touched poop. The macaques demonstrated a clear and strong sense of disgust that was driving their decision making and potentially keeping them safe from pathogens.

With the experiment complete, Dr. McIntosh returned topside so we could discuss our findings. So did you enjoy the view from up here? [Michael] They did exactly what we talked about earlier. Ready to avoid parasite-infested potential things.

Yeah, I mean it's pretty clear; there's not a complete hesitation to pick up things on actual feces, but the reactions that the monkeys give when they do so is just so clearly different than when they are picking things off the ground or off the control items that they have here.

Yeah, yeah, in fact, I, you know, I only saw a few trials here, but as soon as the peanut or the monkey chow on the feces is picked up and they realize that the feces is coming with it, it's a sticky substance, and that was when their revulsion got larger. It almost seemed like they might be absent-mindedly going "I don't care," -and something deeper in them was like, "you should care, you should throw this," - Yeah. roll it, wash it, so it makes sense from an evolutionary standpoint.

Well, Cecile, Andrew, thank you again so much for letting me come out here to see a primate experiment in action. This was really phenomenal. Thank you. -[both] You're welcome. -Thanks for coming.