What Monkeys Can Teach Us About Humans What makes the mind of a human different from that of other animals? Psychologist Laurie Santos says we can't know the answer to that question if we only study humans. This week, we turn to Laurie's work with monkeys to understand which parts of human behavior are distinct, and which we share with other species.
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What Monkeys Can Teach Us About Being Human

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What Monkeys Can Teach Us About Being Human

What Monkeys Can Teach Us About Being Human

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SHANKAR VEDANTAM, HOST:

From NPR, this is HIDDEN BRAIN. I'm Shankar Vedantam. Many of the most iconic characters in children's stories are talking animals.

(SOUNDBITE OF MONTAGE)

UNIDENTIFIED ACTOR #1: (As Bugs Bunny) What's up, Doc?

UNIDENTIFIED ACTOR #2: (As Tweety) I tawt (ph) I taw (ph) a puddy (ph) tat (ph).

ELLEN DEGENERES: (As Dory, singing) Just keep swimming. Just keep swimming. Just keep swimming.

UNIDENTIFIED ACTOR #3: (As Tigger, singing) The wonderful thing about Tiggers is Tiggers are wonderful things. Their tops are made out of rubber. Their bottoms are made out of springs.

VEDANTAM: As children, we love to imagine that animals think as we think and do as we do. We continue to love animal videos as we get older. If you need evidence, look at YouTube. Maybe you're responsible for one of the 22 million views of this video.

(SOUNDBITE OF VIDEO)

UNIDENTIFIED ACTOR #4: (As character) Would you like to open wide, sir?

UNIDENTIFIED ACTOR #5: (As character) (Unintelligible).

VEDANTAM: One monkey lies flat with its mouth wide open. A second monkey hunches over the first, peering into its mouth. It's pretty typical stuff for monkeys. Animals groom each other all the time. But this video went viral for one very clear reason - the human voiceovers.

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UNIDENTIFIED ACTOR #4: (As character) Are you going anywhere nice on your holidays?

UNIDENTIFIED ACTOR #5: (As character) (Unintelligible).

UNIDENTIFIED ACTOR #4: (As character) Yeah, yeah, don't try and talk when my hands are in your mouth, sir.

VEDANTAM: The voices turn an ordinary occurrence in the animal world into a conversation between a dentist and a patient, except this dentist's office is in a tree and both doctor and patient are monkeys.

(SOUNDBITE OF VIDEO)

UNIDENTIFIED ACTOR #4: (As character) You could do with a filling here, actually, at the back. Is it mainly bananas you eat?

UNIDENTIFIED ACTOR #5: (As character) (Unintelligible).

UNIDENTIFIED ACTOR #4: (As character) I won't tell you again, sir. Please don't try and speak when I'm working inside your mouth.

VEDANTAM: This is just one part of the viral video. There are pelicans that look like they're laughing...

(LAUGHTER)

VEDANTAM: ...A squirrel that's beatboxing...

(SOUNDBITE OF BEATBOXING)

VEDANTAM: ...Two owls that complain they've had too much coffee.

(SOUNDBITE OF VIDEO)

UNIDENTIFIED ACTOR #6: (As character) I can't even blink anymore. And I can hear my heartbeat in my ears. I don't get it. Mary had the same as me, and she went to bed hours ago.

VEDANTAM: The joke here is that only humans are supposed to do stuff like this. Although we love anthropomorphized animals, we don't actually believe that other creatures think like us, care about the things we care about or act the way we do. When animals behave like people, we find it hilarious. In recent years, scientists have begun to actively ask, is what happens in our minds really that different from what happens in the minds of other animals? Researchers working with nonhuman primates are finding areas of connection as well as important areas of difference. This week on HIDDEN BRAIN - what other animals can teach us about us.

(SOUNDBITE OF VIDEO)

UNIDENTIFIED ACTOR #4: (As character) Almost done. Just need to smell your breath. Oh, that is rank. I think I need a lie down. Oh, God.

(SOUNDBITE OF MUSIC)

VEDANTAM: Yale University psychologist Laurie Santos has a long commute between her home in Connecticut and her lab. It's so far that it takes three forms of transportation and the better part of a day to get there. But there's no need to feel sorry for her. Laurie's lab is a Caribbean island, Cayo Santiago.

LAURIE SANTOS: It's this beautiful island right off the southeast coast of Puerto Rico.

VEDANTAM: To get there, she flies to San Juan, drives about an hour to the small town of Punta Santiago and then hops in a motorboat. Not long after she arrives on the dock, she's surrounded by monkeys. The sandy-colored creatures are everywhere, some lounging around, others jumping off a tall branch into the water below. About a thousand free-ranging rhesus monkeys live on this island.

SANTOS: And they're monkeys that have been used in research for the last, like, 80 years. And so they're incredibly habituated to humans, which means we can go down there and test monkeys in this natural setting much like we would test human subjects.

VEDANTAM: So you and your team simply walk out onto the island with your clipboards looking for rhesus monkeys to study.

SANTOS: That's exactly right. And we kind of, you know, find ones that look kind of bored and like they might be willing to do something, and we plop down and run studies with them.

VEDANTAM: How do you get the animals to sit still and take part in the experiment? Why would they want to sign up?

SANTOS: Well, they're naturally curious animals, you know, and usually many of the studies involve showing the monkeys something really interesting or something really unexpected. It's kind of like a little roving theater show that shows up, you know, when they're sitting there eating and they're like, OK, you know, what are you going to show me? Not all the monkeys sit there, you know, so we definitely get some who kind of walk away and don't want to deal with us. But many are interested enough to stick around.

(SOUNDBITE OF MUSIC)

VEDANTAM: Laurie Santos has spent most of her career trying to capture the attention of monkeys, not because she's especially excited about monkeys. She didn't come to this work after a childhood of being obsessed with "Curious George." No. Laurie is fascinated by humans.

SANTOS: There's no way to study what makes humans special if you only study humans. You actually have to turn to all the other critters in the animal kingdom. Monkeys are a really important population because they can tell us the kinds of things that are unique to the human species.

VEDANTAM: You can get very valuable information by comparing monkey minds with human minds. Doing this can help us distinguish between things in our heads that are the product of nurture from things in our heads that are the product of nature.

SANTOS: You know, we have lots of human culture. We have lots of human teaching. You know, if we're really looking for the pure things that the human mind has and the absence of those different processes, it's not great to study adult humans.

VEDANTAM: Now, presumably, it would be easier if you could actually study human ancestors, if you could go back and look at, you know, human beings who lived several hundred-thousand years ago. But they're not around anymore to run research experiments on.

SANTOS: Yeah. That's the joke. You know, we comb undergrad study pools to see if there are any Neanderthals or Homo erectus, and (laughter) they're not there. It would be fantastic to study them. But honestly, for many of the things that we've been studying - you know, these basic aspects of social cognition or these basic aspects of decision-making - these are capacities that are old enough that you might expect them to be shared with our common ancestor, with rhesus monkeys, say. And so in some ways, for the particular cognitive capacities we're studying, you know, it's good to kind of rewind the evolutionary tape, as it were, as far back as we can.

VEDANTAM: I want to get to your experiments in a moment. But just being on this island has apparently revealed all kinds of similarities between humans and other animals. One problem you've had to guard against on the island is unethical behavior. Tell me about the monkey thieves that you've encountered on Cayo Santiago.

SANTOS: Well, you know, it's kind of sad to admit that you're getting ripped off by monkeys, but (laughter) it happens more than you'd think on the island. You know, they're wily creatures who are often pretty hungry. And humans have backpacks filled with things like lunches and delicious fruit objects for studies and so on. And one of the big inspirations for some of the work we were trying to do on deception and kind of how monkeys think about other minds came from this act of theft on behalf of one of the monkeys. We were running a study about number where we were showing monkeys different numbers of objects. And there was one research day that we actually had to go home from the island early because the monkeys had ripped off all of the fruit we were using to display the numbers in the study.

VEDANTAM: (Laughter).

SANTOS: And I think it was really on that boat ride home that I started thinking, you know, they're doing this in a successful way. You know, it's not just that we're dumb researchers and they can outsmart us. They're specifically trying to steal from us when we're not aware of what they're doing, or maybe even when we have a false belief about what they're doing. And so it really launched this line of research to be, like, OK, how are they thinking about that problem, you know, that they're duping us in? You know, what are the representations they're using to solve this task?

VEDANTAM: And is it true that they are not just simply stealing but in some ways going after the easiest targets, in some ways what criminologists might call a rational model of crime?

SANTOS: Yeah. In lots of ways. In fact, we set this up as a study. This was work, early work that I did with John Flombaum. We basically set up an experiment where we gave monkeys the opportunity to steal rationally. What we did was we had them experience - they're kind of walking around, and they see two people who are standing in front of a grape, which is a tasty piece of food for monkeys. One of those people is kind of looking at the grapes so if you tried to steal it, he'd probably stop you, whereas the other person is not paying attention, either because he's turned around or he has a barrier in front of his face and so on.

And we just gave monkeys one trial. And what we found is that, even on that first trial, monkeys selectively stole from the person who couldn't see them. In other words, they're rationally calculating, you know, whether or not someone could detect that they're about to do something dastardly.

VEDANTAM: That's fascinating. And you've done some experiments revealing some fairly sophisticated things that the monkeys are doing, including things involving basic math. Can you tell me about some of the experiments where you've tested whether the monkeys have an ability to count? Very simple counting, for example.

SANTOS: Yeah. So this is some early work that was done at the field site back when I was an undergraduate, actually. It was studies that were really trying to see whether or not monkeys shared the basic capacities that human infants seem to have. This is work by Karen Wynn and her colleagues at Yale. She found that even by about 6 months of age, babies can track the outcomes of arithmetic events.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED SCIENTIST: Look.

(SOUNDBITE OF SQUEAKY TOY SQUEAKING)

UNIDENTIFIED SCIENTIST: Here's Mickey.

SANTOS: So if babies see one object added to a box and that box is covered up...

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED SCIENTIST: And up goes the screen.

SANTOS: ...And a second object is put in...

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED SCIENTIST: And look.

(SOUNDBITE OF SQUEAKY TOY SQUEAKING)

UNIDENTIFIED SCIENTIST: Who's this?

SANTOS: ...They seem to expect exactly two objects will be in the box.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED SCIENTIST: Watch the screen.

SANTOS: And they seem to be surprised when the box is opened to reveal only one object or three objects.

(SOUNDBITE OF BABY WHIMPERING)

SANTOS: And so we can do the same kinds of studies out in the field with the monkeys. We literally set up a little magical box for the monkeys. We put on our little monkey magician hats and head out and then show them magic tricks. The magic isn't all that exciting. You know, we're putting, you know, one or two eggplants into a box and revealing different outcomes. I can give you my magician secrets, if you need them.

VEDANTAM: (Laughter).

SANTOS: But basically, what we found is that monkeys show exactly the same performance on these studies as human infants. In other words, they expect the outcome of one plus one to be exactly two, and they're surprised if they see three or one.

VEDANTAM: And in some ways, it's interesting, the connection you're making between studying monkeys and studying infants. Because in some ways, they both tell us - when you study a child, a small child, an infant, and you see this kind of rudimentary math skills, it actually tells you, you don't actually have to go to school to learn that basic math. Some of that is actually potentially programmed into how the brain operates.

SANTOS: That's right. It's yet another way of kind of removing all the training and teaching and cultural learning that we humans have. If you turn to human infants, what you find is capacities that are there in the absence of experience.

(SOUNDBITE OF MUSIC)

VEDANTAM: These studies suggest that there are more similarities than we imagine between humans and other animals. They challenge our assumption that the human mind is unique. For example, Laurie says, one longstanding belief was that nonhuman primates were bad at thinking about other animals' perspectives.

SANTOS: Even our closest living relative, chimpanzees, had failed on really simple tasks, where a human's trying to get the chimpanzee to look for food in one particular location. There's a bunch of cups that are turned over. And the human would point at one of these overturned cups. And what the research showed is that primates were really bad at this task. They didn't seem to realize that pointing meant some information, that a human was trying to communicate with you.

And this really led to the view that primates were bad at understanding others' perspectives. They didn't seem to know what other individuals knew. And that line of research kind of, it didn't really settle well with a lot of the people who were watching primates in the field. You know, like, you know, it's bad to be reading this paper that, you know, primates don't understand other people's perspective when you're getting ripped off every day...

VEDANTAM: (Laughter).

SANTOS: ...(Laughter) On this island and have to go home early. And so - you know, so that was what launched a lot of our studies and other studies by researchers like Brian Hare and colleagues trying to look at, you know, what are the basic capacities primate - nonhuman primates have for understanding, like, what other people's perspectives are? And as part of these studies, we've developed all kinds of different things to test whether monkeys share our understanding of what others perceive, what others hear and what others know. And all of those studies by and large seem to be showing that nonhuman primates are pretty good at that. They seem to know something about someone else's perspective.

VEDANTAM: Can you give me one or two examples of what these studies are and what they find?

SANTOS: Yeah. One of the studies was, again, built off a study that had been done in human infants. This was a study looking at whether or not human infants predict that a person will reach where she knows an object is. So imagine the following scenario. You're watching somebody who seems to really like a particular object. There's a toy that they're really interested in. And that toy rolls over and falls into one of two boxes. Question is, where do you expect that person to reach? You probably expect her to reach in the box where she knows it to be. You know, people reach for things where they think they are.

And that's what the research found with babies. Babies are surprised if that person who saw a toy roll into one box reaches into a different box to look for the toy. We just imported that same study for monkeys. We had monkeys watch as a human looked longingly at a delicious apple, and then they saw the apple move into a particular box. And we showed that the monkeys are surprised if, when a person sees an apple move into a box, they look to the opposite location. They kind of expect a person to reach for something where they know it's going to be.

(SOUNDBITE OF MUSIC)

VEDANTAM: When we come back, more from Laurie's experiments and what they may tell us about the human mind.

(SOUNDBITE OF MUSIC)

VEDANTAM: During the summer, psychologist Laurie Santos takes her research outdoors. Under palm trees and sunshine, she studies rhesus monkeys on the island of Cayo Santiago. But that's only one setting for her research. For a decade, she also studied monkeys at a lab she built at Yale University. Unlike the rhesus monkeys in Puerto Rico...

(SOUNDBITE OF MONKEY CHIRPING)

VEDANTAM: ...The monkeys at Yale were capuchins.

SANTOS: We mostly chose a different species because in some ways, capuchins are very easy to kind of house socially in captivity. So you can have them in kind of a big zoo enclosure, and they're very comfortable. And that was one of the reasons we picked that species. A second reason was that with the capuchins, we wanted to look at a set of different questions. We did work on what they knew about others' perspectives, but we also wanted to look at their decision-making strategies.

And there was lots of work showing that capuchins were really good at kind of making decisions and using tools, but in particular that they were good at a method that we really wanted to use, which was a method involving token trading where monkeys could pay us for different goods and services. There was lots of work suggesting that capuchins might be good at that. And we thought that might be an interesting avenue to start looking at some of monkeys' economic decision-making.

VEDANTAM: Laurie created what she called a monkey marketplace. She wanted to teach monkeys to use money.

SANTOS: I mean, when we first started talking about these studies, you know, to friends and stuff, they're like, you're going to teach monkeys to use money? (Laughter) And I was like, you know, it sounds crazy, but in fact it's pretty simple to get the monkeys to do this kind of trading. They're naturally really curious critters. And so at the start, we just put inside their enclosure little metal tokens that kind of looked like, you know, little fake pieces of money. And the capuchins would, naturally, pick these up, but they kind of, you know, didn't have any use for them. And they quickly realized that if they handed them back to a human experimenter, we'd give the monkeys back a piece of food for the token.

So it was kind of like a token for a piece of food sort of exchange. And what was surprising is that the monkeys picked this up incredibly quickly. You know, within just a few exposures, they realized, like, wait. I give tokens for food? Got it. And then once we had them doing that, we could ask all kinds of economic questions about what they knew about real markets.

VEDANTAM: Once the monkeys learned the tokens were currency, they began hunting for bargains.

SANTOS: The first thing we did was give them opportunities to find good deals. And that meant giving them a choice between different goods offered at different prices. So the monkey marketplace was when the monkeys, you know, they came in for testing from their big social enclosure and entered a little smaller enclosure where they had access to a few of these tokens. And then monkeys had a choice between two experimenters who sold different goods at different prices.

You know, so someone might be selling a grape for one token and someone might be selling, say, a cucumber for one token. And the question was whether the monkeys maximized their food dollar, as it were. You know, did they shop rationally, given the amount of food that they were getting and the kind of food? And the amazing thing that we found is that the monkeys seemed to be relatively rational in this market. That is, they seem to substitute rationally. They bought more food with their token dollar. And they seemed to prefer shopping for foods that they liked better.

VEDANTAM: Did they actually, you think, enjoy the process of shopping? I mean, humans, as you know, sort of lots of humans actually enjoy the process of going to a store and buying things. Was there a pleasure in sort of doing this? Because of course, what the monkeys were doing here is not just simply part of an experiment where I press - you know, where I press a lever and I get some food.

In this case, you actually can choose. I can go to Person A, or I can go to Person B. I have choices. I have autonomy as a consumer.

SANTOS: Yeah. It's hard. I mean, we didn't have a direct empirical measure of pleasure, but they really did seem to like it. You know, they would, you know, rush in to the enclosure to try to do this. And what I loved most is that they tended to pay a lot of attention to, you know, what the different options were. You know, they'd kind of go on both sides of the enclosure and look at what each experiment was offering very carefully. So they seemed to be very, like, careful shoppers.

VEDANTAM: I'm wondering whether the monkeys ever traded tokens with each other. Did they say, you know, I'm going to pay you three tokens for you to groom me?

SANTOS: Yeah. Well, you know, the remarkable thing is that the monkeys never really did that. I mean, to be fair, we didn't give them opportunities to do that. You know, 'cause they were kind of with the tokens mostly by themselves. But what's striking is it doesn't seem like the monkeys maybe need a token economy in their own social system. They kind of have an economy that works. They have this really strict dominance hierarchy. And that determines who gets goods and services.

So in other words, you know, if you're a low-ranking male and you had a big wallet of tokens, if the alpha male saw that, you might kind of naturally let them go and let him take it because, you know, that's the social system you're part of. You know? It's equivalent to, you know, human systems of, you know, back in the days of feudalism. I mean, these kinds of things where only certain individuals are supposed to get access to goods and services. So in some ways, it kind of taught us that, you know, to be a species that uses money, you really have to be a relatively egalitarian species.

(SOUNDBITE OF MUSIC)

VEDANTAM: One of the next questions Laurie wanted to answer gets at the heart of how humans make economic decisions. Economics has long rested on the idea that humans are rational when money is involved. But we now have plenty of evidence that this often isn't the case.

SANTOS: Humans are amazing species. But, you know, pick up any article of The Economist, and you figure out that we're not great at money. (Laughter) You know, we kind of mess up a lot of times. You know, we overpay attention to losses and risk. We don't necessarily invest rationally. And researchers have found a set of standard cognitive biases that humans show in the markets.

(SOUNDBITE OF MUSIC)

SANTOS: And so we wanted to use the monkey market to see if monkeys are biased in the same way as humans are. And to do that, we gave them options that objectively gave them similar outcomes so they would get the same amount of food. But those options were framed differently. Basically, we're stealing from classic work in human psychology with adult participants from people like Danny Kahneman and Amos Tversky, who gave people problems where things were framed as gains or losses.

(SOUNDBITE OF MUSIC)

VEDANTAM: Danny Kahneman and Amos Tversky famously found that people don't think of gains and losses the same way. Let's say I was going to give you $10. In one scenario, I tell you I'm going to give you $15 and instead give you $10. In another scenario, I tell you I'm going to give you $5 and instead give you $10. In both scenarios, you receive the same amount, $10. But they feel different. In one case, you feel something's being taken away from you. In the other, you feel like you got a little extra. Laurie wanted to see if monkeys responded the same way.

SANTOS: And so to do that, we would show monkeys a case where, ultimately, they're going to get, say, two pieces of food, but that two pieces of food is framed differently. One experimenter just shows two pieces of food, and the monkeys get two. So that's kind of what they expect. Whereas another experimenter, say, shows three pieces of food, and then the monkey only gets two pieces of food. And that second case, you know, you get the same amount of food in the end, but it kind of feels like you lost something.

And what we would find in studies like that is that the monkeys seem to systematically avoid the loss. Even though they were getting the same amount of food, they would avoid the guy that gave them what felt like a loss. And this is exactly what humans seem to do in a lot of standard decision-making studies. We tend to avoid losses even when they're not objectively a loss, even when they're just framed that way.

VEDANTAM: And what about the situation where maybe someone showed only, you know, one item of food but when the monkey paid up a token, the monkey got two?

SANTOS: Yeah. And so the monkeys also seemed to pay a little bit of attention to gains, what looked like gains. In fact, what we gave the monkeys a choice between was a gain that they would get for sure - so a sure gain - and a risky gain. Let me give you the following problem that was given to humans by Kahneman and Tversky. I'm first going to give you $1,000, which seems great. But now you're going to have to figure out a way to get more money. On one hand, you could just get $500 extra with certainty. So you get $1,500. Or you could take a risk. I'm going to flip a coin, and if it comes up heads, you get another $1,000. But tails, you get nothing.

When you pitch this problem to most undergraduates, most people pick the safe option there. They want to go with the gain that they get for sure. And that's the problem we gave to monkeys. We gave them a choice between a sure gain - like, they saw one piece of food but knew they were going to get two - versus a risky option, where they saw one piece of food and they were always going to get that one, but they might get two extra ones in a risky way. So the objective outcome was exactly the same. All that varied was the risk.

And what we found was that the monkeys selectively shopped at the person who gave them the safe bet. They, like humans, avoided risk when they were in a situation that seemed framed like a gain. But all that changed when we made the situation more about losses.

(SOUNDBITE OF MUSIC)

SANTOS: And so you can experience this yourself. You know, imagine I'm going to start you with $2,000, and your choices are how to lose this money. One option is that you can lose it with certainty, so I just take $500 away. But another option is that you can take a risk. I'm going to flip a coin. If it comes up heads, that's bad. You lose a thousand dollars. Tails, you lose nothing.

And so, objectively, the amounts are the same. On average, you'll get $1,500. But in practice, now that loss seems kind of yucky. Most people, actually, when given that choice, avoid the loss. And what we found when we gave the monkeys qualitatively similar economic situations is that they also avoided the loss. And that's what's irrational. They're switching their choice about risk depending on how a problem is arbitrarily framed.

VEDANTAM: In other words, they took the safe option when it was framed as a gain. They took the risky option when it was framed as a loss.

SANTOS: That's exactly right. And what's surprising is that's exactly the same bias that we see in housing markets. That's the exact same bias that we see on the golf course. People actually putt differently when they're putting, and they know they're going to go over par versus under par. It seems to be a basic bias we have in how we as humans frame the world. And what this study showed was that monkeys seem to have exactly that same bias.

VEDANTAM: So researchers like Danny Kahneman have also studied something called the endowment effect. What is the endowment effect? And do we see the same thing among monkeys?

SANTOS: Yeah. The endowment effect is a bias where we overvalue things that we own - you know, that old couch that you've had in your house. You know, if somebody were to, you know, offer you some money on eBay, they might think it's kind of - you know, a little ratty. But you think, because you own it, it's amazing. This is a bias we see playing out in all different parts of the market in human cultures. And we wanted to see whether monkeys had that bias too.

What we basically did was to give monkeys different food items, so now they're trading foods rather than tokens. And we wanted to see whether monkeys would give up their piece of food for an equivalent piece of food. And what we found was that they didn't seem to want to do that even when we upped the ante such that the food we were giving them was many, many, many times as valuable as the food that they had.

At one level, we were so interested in seeing if we could get the monkeys to trade back their food that we offered them, in contrast to, like, a small piece of grape, what we called the fluff taco, which is a Fruit Roll-Up filled with marshmallow fluff...

VEDANTAM: (Laughter).

SANTOS: ...It's about the sweetest thing you can offer a monkey in a small go. And even in that case, they still wouldn't trade their piece of food that they were endowed with for a different piece of food.

VEDANTAM: So humans and capuchin monkeys shared a common ancestor tens of millions of years ago. What does it tell us that we see loss aversion or the endowment effect in monkeys as well as humans?

SANTOS: I mean, I think it tells us that whatever cognitive strategies we're using, you know, when we go to the grocery store to buy our food, those mechanisms aren't just mechanisms that are built for using money. Like, those mechanisms are mechanisms that we've been using to frame decisions for millennia, right? The mechanisms that we use when we're making quick purchases online are the same as the mechanisms monkeys must be using when they're foraging and so on.

So I think it tells us that the strategies we're using are really, really old. And that gives us some hints about what those strategies might be for and whether or not we should really consider them irrational.

(SOUNDBITE OF MUSIC)

VEDANTAM: I was going to ask you that question. I mean, we think of these as being examples of bias or error. But the fact that they're really ancient suggests that perhaps they played a useful role, at least in our evolutionary past, if not the present.

SANTOS: Yeah. I mean, you'd think if they were that harmful, they would have been selected out. You know, monkeys who use loss aversion versus some other bias, you know, might not do as well kind of in, you know, real monkey markets out there in the evolutionary world. I think that's one possibility. I think, you know, when you think about what could a bias like that be useful for, it turns out that it's hard to think in objective terms in the world. Sometimes there's strategy where you think relative - like, relative to someone else's option or relative to a reference point. Those strategies can be better.

(SOUNDBITE OF MONKEY SCREECHING)

SANTOS: So imagine the following scenario - you're instantly a capuchin monkey. And you have to figure out how much food to get today. You know, you're foraging for nuts. How many nuts should you go for? And when should you stop? You know, it's a hard problem. But if you see that I'm walking around with three nuts, and Joe (ph) the monkey's walking around with three nuts, then maybe shooting for three nuts is pretty good. Maybe if you only get two, you should work a little harder and feel bad. But if you get four, I shouldn't - you know, you don't need to celebrate. That was just good.

And so it might be that strategies like loss aversion, where you really are comparing against other people, and then you have mechanisms to motivate you not to go under what other people have, those might have been more useful than we think. It's just when we throw them into stock markets that they kind of look a little quirky.

VEDANTAM: You know, we had Keith Payne, the psychologist, on HIDDEN BRAIN recently talking about how inequality works and the psychology of inequality. And in some ways, you and others have found that monkeys exhibit some of the same behavior. They pay very, very close attention, as in the analogy you just gave me, to what other monkeys have and whether what they have in relative terms is fair.

SANTOS: I think that's right. I mean, there's some classic work by the researcher Sarah Brosnan and Frans de Waal that find that monkeys don't just pay attention to what other individuals have. When they feel like they're getting a raw deal - you know, if you're a monkey who's getting kind of a yucky cucumber when another monkey's getting a grape, you'll actually reject that offer. So it's not just that monkeys sort of experience things like, hey, that's unfair. They actually act on it in ways that reduce the number of resources they have.

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VEDANTAM: When we come back, experiments that show what other animals do differently than us and the psychological quirks that make us uniquely human.

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VEDANTAM: In the 1800s, a British inventor named William Henry Talbot was so caught up in his work that he and his wife had to delay their honeymoon. Eventually, they did go, winding their way through Europe to Italy. He was mesmerized by Lake Como with its wishbone shape and its backdrop of the Alps. He was neither the first nor the last to be enchanted by the view.

SANTOS: It was even a lake that George Lucas used in one of the "Star Wars" movies when Padme and Anakin were getting together.

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NATALIE PORTMAN: (As Padme) We used to lie out on the sand and the sun dry us and try to guess the names of the birds singing.

HAYDEN CHRISTENSEN: (As Anakin Skywalker) I don't like sand. It's coarse...

SANTOS: And he allegedly didn't use any CGI on that scene, so it really shows how beautiful this lake is.

VEDANTAM: William Henry Talbot wanted to capture the moment and share it with others, but it was the 19th century. Cameras as we know them didn't exist. What he had was something called a camera lucida.

SANTOS: Which is like, you know, a little device that you can use to kind of sketch things. It's kind of like the iPhone 11 of his time. And he made an amazing sketch, one that's very famous. Because he was very disappointed with his sketch, he allegedly said it was a melancholy to behold, which is not what you want to say about your honeymoon pictures.

VEDANTAM: (Laughter).

SANTOS: And it caused him to start thinking about what it would be like - what technology could actually let images that he saw in his version of a camera to stick on paper. And it led him to be one of the first developers of a lot of the photo technology that we use in real cameras today.

VEDANTAM: William Henry Talbot's pioneering techniques in photography tell us something about how humans think. We have an impulse to share with others what is going on in our heads - our feelings, our experiences, our perspectives. Laurie thinks this might be an important area of difference with other animals.

SANTOS: I think even if animals had photographs, they wouldn't use them. You know, if they had their iPhone 11, they wouldn't take pictures. And it's so funny when you realize that because, you know, that urge to share stuff is such a very human urge. You know, it's one that researchers like Mike Tomasello and colleagues find emerges, like, within the first year of life. Babies are already starting to point at things and trying to get their parents to look. And so it's a very, very fundamental urge. And remarkably, it seems to be one that's unique to humans. And you can see this so strikingly when you watch the behavior of, you know, mother primates and their kids. Like, they just don't tend to show each other stuff. Like, there's just not, like, active teaching or even active, like, hey, Mom, look at this; look at me. They kind of just don't care.

VEDANTAM: How do these differences reveal themselves in experiments, especially when it comes to certain mistakes that perhaps monkeys, chimps and dogs don't make that humans do make?

SANTOS: Well, I think one of the consequences of being a species that shares a lot of stuff is that we also are probably a species that believes a lot of stuff. You know, we want to share 'cause it affects what others learn. And that suggests that we might be a species that kind of can overlearn based on when we're getting information from others. In other words, we really are liable to believe the stuff that we're hearing from other individuals. You know, that's why the sharing feels so powerful. But given that sharing is kind of in beta testing, it suggests that the learning might be in beta testing, too. And so the way that researchers have tried to test this is to ask whether or not humans are prone to overlearn when they get bad information from others. It's a tendency that researchers call overimitation, the tendency to imitate too much.

And so here's the study. You bring human children and chimpanzees into the lab - this is work done by Vicky Horner and her colleagues - and you give them access to a little puzzle box. In some cases, the puzzle box is opaque. It's hard to figure out how it opens up. But they get to see a human experimenter who helps them. And the human experimenter does all these different actions on the box - you have to tap a lever at the top and stick a tool in, and it's very, very elaborate. What you find is that when kids and chimpanzees see that, they copy pretty slavishly everything the human does. And that kind of copying makes sense because they don't know how to open the box. But in a different condition, all that changes. Now you give kids and chimpanzees a transparent puzzle box. And because it's transparent, it's really obvious how to open it. There's just a door and you open the door. There's nothing else in the box. The subjects also get to see this experimenter painstakingly trying to open the box, you know, using all these levers, you know, using tools inside it and so on. But you can see those actions aren't doing anything.

What you find is that the chimpanzees are pretty rational on this task. They completely ignore what the human did on the transparent box and just solve it in the really easy way. But sadly, perhaps, for our species, human children don't do that. They slavishly follow everything they see the human doing. They overimitate these actions, suggesting that when you see another individual doing something, especially intentionally, it messes up the way you think about the world. In the case of the children, it messes up the way they think the box works.

VEDANTAM: There's also been some work, I think, looking at a sort of child development, showing that when children are shown how a certain toy works a certain way, they become - it's really difficult for them to actually explore the toy and actually play with it in a different way, that they actually believe, OK, now I've been shown this is how the toy works, this is how I have to play with it.

SANTOS: That's exactly right. This is some lovely work by Liz Bonawitz, which has shown that when children are taught something, it limits their exploration. So if they're given a puzzle box with lots of different knobs and interesting things and a teacher comes and says, hey, here, this is how it works, and just presses one of the buttons, when children interact with that box, they'll just press that button. Even though all these other levers and cool things also make the toy do interesting stuff, kids never find it. And so it seems like teaching, we often think of as this wonderful thing - and of course it is. You know, of course this is the way we learn so much about our culture and how to do things in the world. But there's a downside to it, which is that we don't have mechanisms to be critical about the things we're hearing from other humans.

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VEDANTAM: There's something humans do that's so intuitive it's hard to even recognize that we're doing it. We can believe one thing but can understand another person might believe something entirely different. Imagine that you ask a friend what day it is. She tells you it's the 21. You check your calendar and realize it's actually the 20. You're capable of understanding that it is the 20 but that your friend believes something else.

SANTOS: We can constantly think about these cases where other people have misinformation. You know, otherwise, you know, "Romeo And Juliet" would make no sense. You know, Juliet didn't know that Romeo was alive and all these things that I forget from my high school English class. Right?

But this is the kind of thing we do all the time. We constantly can think about that other people have a different belief than us. We can think about past us and know that we had different beliefs than we do have now. We can think about future us and think, you know, I might have a different belief when I'm old and gray than what I do now.

And so this is - it seems so easy because it comes so fluidly to us. But cognitively, this is really tricky. It means I have to maintain this, like, representation, this information about the world that I know is really true versus, at the same time, knowing that somebody else thinks something different. It's kind of a hard task when you think about it.

VEDANTAM: You know, when you think about the movies for example, the movies wouldn't work if we didn't have this skill. I'm thinking about the famous shower scene in "Psycho." You know, as a member of the audience, you know that, you know, the murderer is about to kill the woman in the shower. But the woman in the shower doesn't know this. And of course, that increases your fear and anxiety about what's happening not just because you know a murder is about to take place but because you know the victim doesn't know that a murder is about to take place.

SANTOS: That's exactly right. And beyond that, you know, another feature of this ability that we have is, you know, it allows us to do fiction at all. You know? Like, you know that there's a murder about to take place, but it's not in your house. Like, you can represent that there's a separation between you and what you know in the moment and other people's representations, other people's beliefs.

So it's a really fundamental capacity. And that's one of the reasons that researchers have been so interested in whether or not nonhuman animals can do this. You know, how do we solve this hard computational problem? Like, do we need language to solve it? You know, is this the kind of thing other species can look at? Testing this, though, is pretty tricky because you have to be able to ask whether or not a monkey knows that some other person has a false belief. And it's hard because they can't tell you.

Luckily, we have different tricks we can use to test it. And one of those tricks is kind of using the same type of study we used to look at what monkeys know about other perspectives, these kinds of studies where monkeys see people acting in different ways based on what they know. But in this case, we can ask how monkeys expect people to behave based on their belief.

You know, so imagine I think it's raining out in New Haven. You might have certain predictions about my behavior. You might expect me to take an umbrella and be surprised, you know, if I went out in, like, you know, shoes that weren't appropriate for the rain. That would surprise you because you know something about my belief even though that belief is false. That was basically the logic we used to set up a study with monkeys. Monkeys get to watch as a person is looking for an object that she really wants in the world. And the monkeys get to watch this object rolling back and forth between two boxes, a left box and a right box.

So at this point, the monkey should be thinking - oh, the person's going to reach into the left box. But then all of a sudden, the person gets covered up. She's not paying attention anymore. And the monkey gets his own private access to the fact that the object that the person wants then rolls into the right-side box. So the monkey knows where it's really. But the question is whether or not the monkey recognizes that the person shouldn't know where it is, that the person should have a false belief.

If the monkey gets that, he should be surprised if the person reaches to where the object really is because she shouldn't know that. She should have a false belief. But interestingly, that's not what we find. What we find is that the monkey seemed to make no prediction in that case.

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SANTOS: And what this tells us, which is so cool, is it tells us that the monkeys realize the person's perspective is different from their own, but they can't simulate what it is. In other words, they know - well, she doesn't know what I know. Like, that's why they don't really have a prediction about where she should act. They don't think she should reach for the object, but they can't really simulate this other belief.

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VEDANTAM: So this is so fascinating because when you think about what the monkeys were doing on the island in Puerto Rico - and they're basically saying, this person has her back turned to me, so it's going to be easier to steal a fruit from this person than the person who is facing me. At some level, the monkey is able to say, I have a sense that the person whose back is toward me knows less than the person whose face is toward me.

So they have some sense of representation of what's happening inside the experimenter's mind or the experimenter's knowledge. But that doesn't translate in this experiment into what you think about are the beliefs of the experimenter. What's the distinction between those two scenarios?

SANTOS: Yeah. The distinction is between, like, a case where the monkeys simply think - oh, that person doesn't know anything - versus - that person has a very specific belief. Right? And when you get back to, you know, the example you used, the kind of - the shower example, the really scary example, the monkeys might have the same intuition and the same reaction to that scene that we do. You know, they would probably know the person doesn't know that, you know, a killer is coming.

(SOUNDBITE OF TV SHOW, "PSYCHO")

JANET LEIGH: (As Marion Crane) (Screaming).

SANTOS: But what they might not be able to represent is what the person themselves does believe. You know, they believe they're safe in the shower - you know, that nothing's going to happen, that they're going to be in there fine. The monkeys seem to not be able to simulate all the things that the person knows but that the monkey himself doesn't know. They know reality in the here and now, but they can't kind of simulate things that go beyond reality - you know, someone else's belief that's not true of reality now, or even, you know, what we're going to think in the future or what we thought in the past. Even counterfactual situations - things that aren't true in the here and now but, in theory, could be true - those aren't situations about the world that the monkeys seem to be able to simulate. That's not the kind of thing they seem to be able to represent. But if you think about how critical that is - not just to know someone has a different perspective but to know what that perspective is - it really is the crux of human cognition in so many ways. So it's fascinating that other animals seem not to have this.

VEDANTAM: There's one last area of difference I want to explore, which has to do with how price shapes the perceptions of humans and other animals. How do humans and other animals respond when you raise the price of an object?

SANTOS: Yeah. Well, this was another spot where we - we actually assumed we would see similarity between monkeys and humans in this respect. But it was one of the few spots in the economic domain where we saw these big differences. You know, humans have all kinds of theories about price, but one of the theories we have is that more expensive things are better. You know, this is why, you know, when a new iPhone comes out, it's often more expensive than the old iPhone and that kind of makes people think it's a lot more valuable. You know, it's one of the reasons that very, very expensive wine, people are all excited to try it not necessarily because they - you know, it's necessarily better, but people assume it's going to taste better. In fact, there's some neuroscience work that you can change the pattern of firing in the reward areas of people's brains merely by giving them the same wine and lying to them and telling them it was more expensive. You know, so this seems to be a basic way that we approach price. We assume that more expensive things have to be better.

And so we had this great way to test that in our monkey market. We could change the price of goods merely by altering the amount of food we gave monkeys. You know, so we could give them a really, really huge piece of one kind of food for their token dollar, which would kind of be like, you know, getting a brand of chocolate that's, like, really enormous for a dollar. Or we could give them a really, really tiny piece of food. You know, it's like the teeny, tiny truffle of expensive chocolate that you get for $1, right?

VEDANTAM: (Laughter).

SANTOS: And so we could train them on this and then give them a situation where they got to just decide which of those they wanted without having to pay. And so that's kind of the model. You know, imagine you learn, you know, for $1, you get Brand A of chocolate and it's this huge, enormous piece, you know, probably not that good. And then for another dollar, you get Brand B of chocolate and that is a teeny, teeny, tiny piece of chocolate. Now you go to a reception, and both of those kinds of chocolates are out there, you know, sitting for free. You know, which do you go after? That was basically what we gave the monkeys. They had a free feeding condition where they could grab whatever they wanted. And to our surprise, they just didn't seem to care, which was shocking because in all our other studies they showed us that they cared about price a lot. You know, they were these wonderful bargain shoppers. But they didn't fall prey to this bias that we show about price. They didn't irrationally think that higher-priced goods were going to taste better.

VEDANTAM: So in other words, what I'm hearing you say is that you could set up a Costco in the world of monkeys but probably not a three-Michelin star restaurant?

SANTOS: That's exactly right, especially a three-Michelin star restaurant that didn't actually have good food.

VEDANTAM: So many of the differences you've just identified between humans and other animals seem to be connected to the way we relate to our social worlds, how we want to share ideas with other people, how we intuit what's happening inside someone else's mind, how we care intensely about what others think of us. It's so interesting that these differences might be responsible for some of the best things that humans do but also some of the worst things that humans do.

SANTOS: Yeah, that's one of the things I find most fascinating about these findings - is that the things that make us different not only allow humans to do some of the smartest things we do - you know, set up these five-star restaurants, you know, understand rich fiction and movies - but they also make us do some downright dumb stuff. You know, I think that the biases we've identified are the same biases that result in fake news being spread so easily. They also are the same kinds of things that cause us to, like, spend lots of money on stuff that's probably not that good, you know? So it's remarkable that the very things that make us so special and so smart can also, in some context, make us seem kind of dumb.

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VEDANTAM: Laurie Santos is a professor of psychology and cognitive science at Yale University. She is the host of the podcast "The Happiness Lab."

Laurie, thanks for joining us today on HIDDEN BRAIN.

SANTOS: Thanks so much.

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VEDANTAM: This episode was produced by Rhaina Cohen and edited by Tara Boyle and Jenny Schmidt. Our team includes Parth Shah, Thomas Lu and Laura Kwerel. Our unsung hero this week is Andy Huether. Andy is an audio engineer at NPR. He's been helping us make the transition to a new audio editing software system. When you're as skilled at audio editing as Andy is, it's easy to fall prey to the curse of knowledge, to forget what it's like to not know something. But Andy understands exactly how to explain concepts to people who are starting from square one. He's patient and always willing to make time to answer our questions. Thanks, Andy.

For more HIDDEN BRAIN, you can follow us on Facebook and Twitter. If you enjoyed the show, please tell one friend about an idea in the episode that you found eye-opening. I'm Shankar Vedantam. And this is NPR.

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