Mankiw

Monkey Business - New York Times

07/07/2005 03:38 PM

June 5, 2005

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 anotherdog,'' 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 teachthe 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 increasinglybeing 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 mediabias.
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 otherin 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: overrepeated 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 selfish jerk). The stooge and the jerk were then
sent to play the game with the other tamarins. Thestooge blithely pulled her lever over and over, never failing to dump a marshmallow into the
other monkey's cage. Initially, the other monkeys responded in kind, pulling their own levers 50 percent of the time. But once they figured out
that their partner was a pushover (like a parent who buys her kid a toy on every outing whether the kid is a saint or a devil), their rate of
reciprocation droppedto 30 percent - - lower than the original average rate. The selfish jerk, meanwhile, was punished even worse. Once her
reputation was established, whenever she was led into the experimenting chamber, the other tamarins ''would just go nuts,'' Chen recalls. ''They'd
throw their feces at the wall, walk into the corner and sit on their hands, kind of sulk.''
Chen is a hyperverbal, sharp-...