Monkey Business
By STEPHEN J. DUBNER and STEVEN D. LEVITT

Keith Chen's Monkey Research

Adam Smith, the founder of classical economics, was certain that humankind's knack for monetary exchange belonged to humankind alone. ''Nobody ever saw a dog make a fair and deliberate exchange of one bone for another with another dog,'' he wrote. ''Nobody ever saw one animal by its gestures and natural cries signify to another, this is mine, that yours; I am willing to give this for that.'' But in a clean and spacious laboratory at Yale-New Haven Hospital, seven capuchin monkeys have been taught to use money, and a comparison of capuchin behavior and human behavior will either surprise you very much or not at all, depending on your view of humans.

The capuchin is a New World monkey, brown and cute, the size of a scrawny year-old human baby plus a long tail. ''The capuchin has a small brain, and it's pretty much focused on food and sex,'' says Keith Chen, a Yale economist who, along with Laurie Santos, a psychologist, is exploiting these natural desires -- well, the desire for food at least -- to teach the capuchins to buy grapes, apples and Jell-O. ''You should really think of a capuchin as a bottomless stomach of want,'' Chen says. ''You can feed them marshmallows all day, they'll throw up and then come back for more.''

When most people think of economics, they probably conjure images of inflation charts or currency rates rather than monkeys and marshmallows. But economics is increasingly being recognized as a science whose statistical tools can be put to work on nearly any aspect of modern life. That's because economics is in essence the study of incentives, and how people -- perhaps even monkeys -- respond to those incentives. A quick scan of the current literature reveals that top economists are studying subjects like prostitution, rock 'n' roll, baseball cards and media bias.

Chen proudly calls himself a behavioral economist, a member of a growing subtribe whose research crosses over into psychology, neuroscience and evolutionary biology. He began his monkey work as a Harvard graduate student, in concert with Marc Hauser, a psychologist. The Harvard monkeys were cotton-top tamarins, and the experiments with them concerned altruism. Two monkeys faced each other in adjoining cages, each equipped with a lever that would release a marshmallow into the other monkey's cage. The only way for one monkey to get a marshmallow was for the other monkey to pull its lever. So pulling the lever was to some degree an act of altruism, or at least of strategic cooperation.

The tamarins were fairly cooperative but still showed a healthy amount of self-interest: over repeated encounters with fellow monkeys, the typical tamarin pulled the lever about 40 percent of the time. Then Hauser and Chen heightened the drama. They conditioned one tamarin to always pull the lever (thus creating an altruistic stooge) and another to never pull the lever (thus creating a...

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