The first Freakonomics column, Monkey Business, is up:
”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.”
Chen’s early work was actually with tamarins:
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 selfish jerk). The stooge and the jerk were then sent to play the game with the other tamarins. The stooge 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 dropped to 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.”
It took a while to train the capuchin monkeys that silver tokens were money, easily traded for food, but they picked up on the concept surprisingly well:
Perhaps the most distinguishing characteristic of money, after all, is its fungibility, the fact that it can be used to buy not just food but anything. During the chaos in the monkey cage, Chen saw something out of the corner of his eye that he would later try to play down but in his heart of hearts he knew to be true. What he witnessed was probably the first observed exchange of money for sex in the history of monkeykind. (Further proof that the monkeys truly understood money: the monkey who was paid for sex immediately traded the token in for a grape.)