How do you go about exploring monkey awareness of human perception?
The simplest study uses two experimenters standing on either side of a platform holding two portions of food. One of the experimenters can see the food and the other can’t because they are facing in opposite directions. They get into position near a monkey who is all by himself, and the monkey gets one trial to steal the food. And the question is, who does he steal the food from? Consistently the monkeys steal the food from the guy who’s facing away. Even without training, they realize that when eyes are pointed at them, it’s probably not a good idea to do it. But as soon as eyes are turned away, all bets are off.
Other studies suggest that monkeys pick up on what we can hear. We have a single experimenter who has two boxes. One is covered in jingle bells, so when it moves it makes a lot of noise. The other one looks the same because it’s covered in jingle bells, but we’ve taken the balls out of all the bells, so the monkey can hear that when the experimenter puts food in the box it’s not making any sound. So when the experimenter freezes and looks away, the question is, which box does the monkey steal from? What we find is that they sneak over and take food out of the quiet box and avoid stealing things from the box they know is noisy.
You’ve also begun researching economic behavior in capuchin monkeys. How did this study come about?
Some of the capuchins in our colony at Yale came from a lab where the males had already learned how to trade tokens for food. It was funny when the monkeys first arrived because one of the caretakers said we had “genius” monkeys. “They hand their orange peels back to us so we don’t have to clean the cages,” he said. And then I realized it was because they had been trained in token-trading behavior.
So they were using every available scrap to get more food?
Yes, they were pretty crafty—trying to negotiate even before we set up market experiments of our own.
How did you establish a barter system among the capuchins?
We set the monkeys up with different human traders who offered things at different rates. When I started at Yale, I was lucky enough to have a colleague, Keith Chen, who is an economist and had already been collaborating with Marc Hauser at Harvard. We got together for drinks one night and started talking about ways to set up these experiments. We started with a series of studies examining whether capuchins pay attention to the “price” of food and whether they switch their purchasing behavior when items go “on sale”—if they buy more of the foods when they’re on sale to maximize the amount of food they’re getting. The easiest way to do these studies is to find two objects the monkeys like about equally, like apples and Jell-O cubes. The monkeys have their little wallet of 12 tokens, and they can spend them any way they like. There are two “salesmen,” one selling apples and the other selling Jell-O. What we found is the monkeys spend half their budget on apples and half on Jell-O. The next day, one experimenter has the regular-size apples but the guy selling Jell-O is offering two cubes for the price of one. And lo and behold, just like a human would, they buy more Jell-O. We showed that the monkeys’ market sense is very similar to our own.
Physical and social capacities shared across the primate order help us function and provide the scaffolding for learning.
But you also began to focus on the monkeys’ mistakes.
I was interested in the errors that people make and looking for ways to study those errors in the monkeys.
Why focus on human error?
Humans do a lot of systematically dumb things. The way we categorize things is biased and leads to stereotypes. Our judgments are biased when we deal with finances, but why? We’ve hypothesized that maybe some of these errors were built in from the beginning, which is why they’re so hard to get rid of. And if that’s the case, then these kinds of biases might be shared with other primates, which leads to the question of whether they are really biases in the first place. Maybe they’re smart for something that is just wrong for our modern environment.
What kinds of errors are we talking about?
One source of error is an innate aversion to loss. Basically it’s the same experimental setup: There are two salespeople. One of them looks like he’s offering one piece of apple, but when the monkey pays him, that experimenter delivers a bonus—an extra piece of apple. So the monkeys get two, which they think is pretty good. The second salesman looks like he’s going to give the monkey three pieces of apple because he’s holding three pieces. But when the monkeys pay him, he takes one of these pieces away, so the monkeys see a loss of one piece of apple.
The important thing is that, on average, the monkeys get two pieces of apple from both experimenters. So in practice they shouldn’t care and should trade randomly, but that’s not what we see. The monkeys prefer to trade with the experimenter who’s giving them the bonuses and avoid the experimenter who’s giving them the losses.
What does this tell you about irrational economic behavior?
Economists assume that our decisions are based on what’s going to give us the most wealth or happiness. But the monkeys violate the assumption that all they care about is the amount of food they get. They also seem to care about how the amount of what they get differs from what they expected.
Have the monkeys surprised you in any other ways?
We did a similar study in a risky context, where both experimenters start out offering three pieces of something. The first experimenter appears to offer three items but each time ultimately gives the monkey two, so the monkey gets a loss, but it’s a safe, consistent loss. The second experimenter starts out offering three but introduces more risk: Sometimes the monkey gets all three, but sometimes it gets only one. We find that the monkeys prefer to go with the second experimenter. They prefer to risk losing more because there is also a chance they will have no loss at all. That is just what humans do.
