Mind & Brain / Primates

It sounds like the falling housing market. People stay in because it could ultimately go up, but meanwhile, the whole thing might crash and burn. Where is the evolutionary advantage in this?
My guess is that the strategy worked in the kinds of unstable, variable environments in which we evolved. Say you’re foraging for nuts and you’re in a really bad way­—if you don’t get five you’re going to die. If you haven’t found five nuts yet, you need to take a lot of risk to get them because the alternative is really bad. As seen in the housing market, people take more risk when facing loss than they do when dealing with gain.

There was a famous experiment done by the psychologists Daniel Kahneman and Amos Tversky back in the 1980s involving a hypothetical Asian disease that is expected to kill 600 people. There are options for fighting the outbreak. In one scenario, 400 people will die for certain. In another scenario, there’s a one-third chance that no one will die and a two-thirds chance that everyone, all 600, will die. What do you pick? When framed that way, people will choose the riskier scenario because they want the slim possibility that there’s not going to be any loss. The housing market is another case in point. You’ll risk an even bigger loss just on the chance that you might not have any loss at all.

You’ve also studied another kind of bias: the tendency to devalue what we don’t have, a “sour grapes” attitude.
Human experiments suggest that when forced to decide between two things we like equally, we end up devaluing the object we chose against. The Aesop’s fable of the sour grapes is a prime example of wanting something but when you find out you can’t have it, you decide you never liked it anyway.

The way we tested this in monkeys was to offer different-colored M&Ms, which the monkeys liked equally. We gave the monkeys a choice between, say, red and blue, and they picked blue. Then the question was, what happened to the M&Ms that the monkeys didn’t choose? Did they irrationally dislike those more than they had before? To get at that, we then gave them a choice between red—the one they chose against—and a novel color, like green.

The prediction was that the monkeys would pick more of the green. And that’s just what they did, which suggests they’ve irrationally devalued red M&Ms because they’d had to make an arbitrary choice about those a few seconds earlier.

The guy selling Jell-O is offering two cubes for the price of one. And lo and behold, the monkeys buy more Jell-O.

And how might this bias provide an evolutionary advantage?
Often we make a decision and then change our beliefs to fit the decision. When things don’t match up, we may try to resolve the dissonance through rationalization. It might be the case that your cognitive system strives for consistency in a way that leads to correct inferences about the world later on.

You’ve said that monkeys make the same reasoning errors humans do. Do you think monkeys can reason?
I do. They obviously can’t verbally reason things out, but their choices are extremely similar to the ones humans make. So in that sense, I think they have to be reasoning.

Why do you study lemurs when other primates are far more like us, both genetically and behaviorally?
Lemurs are important to study because they’re so distantly related to humans. If we see the same cognition in lemurs that we also see in macaques, capuchins, and chimpanzees, it allows us to learn more about how old that kind of cognition is. The other thing is that the lemurs developed in such a different environment on the island of Madagascar. So if we see the same kind of abilities in them, those abilities are probably widespread across primates.

Sort of like behavioral carbon dating?
Yes. That is the logic we use. If we find the same capacity in lemur species and in our capuchin monkeys at Yale and in the macaques we’ve been working with in Puerto Rico—if all these branches at the top of the tree have this capacity—then it probably didn’t evolve separately, and it is probably something that we all shared back in the day.

For instance, we found lemurs are as good as a six-month-old human baby at adding and subtracting objects. We showed the lemurs one lemon entering a box, and then a screen came up so they couldn’t see what was in the box anymore. Then we showed them another lemon being placed in the box. Then we would remove the screen and reveal either the correct outcome of two objects in the box or incorrect outcomes of three objects or one object.

Our measure of whether the lemurs expected this was based on how long they looked at it. The assumption was that if it was what they expected, they’d be bored and turn away. But if it was something that surprised them or that they found unexpected, they’d look longer. And that’s just what they did. All the lemur species looked longer at events where we showed them the incorrect outcome.

You also discovered that lemurs can use tools, even though they don’t use them in the wild.

One of the working hypotheses is that we have a set of simple physical and social capacities that help us function and make sense of how things work. And that provides the scaffolding for learning that can take place when you’re in an environment that has lots of tools. These are basic building blocks for all the stuff we learn later.

This set of capacities seems to be shared broadly across the primate order and seems to come online in humans early in their development. Whether you grow up as a hunter-gatherer in Africa or a modern-day human in Sarasota or a baby lemur in the forest, you seem to have these simple capacities.

Will we eventually discover that nonhuman primates have a complicated inner life?
How much they feel is still a big open question. Do they feel loved? Do they feel emotional pain? We still don’t have good measures. But in terms of how they think, we use the same measures that we use in humans, and our studies suggest primates are reasoning about things they can’t see and interpreting mental states. Just a few years ago, we didn’t think primates could do this. Animals’ mental lives are richer than we used to think.

Fifty years from now, are we going to look back on the way we handled other primates and think of it as barbaric?
The hope is that seeing more of ourselves in them will help us treat them more humanely and work to conserve them and their habitats in future generations. Our experiments suggest that these primates can understand our intentions. Can they morally reason about what we do? Can they understand someone’s intent to hurt them? When we get the full package of how they pay attention to the world, it might end up informing how we treat them.