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." Chen is a hyperverbal, sharp-dressing 29-year-old with spiky hair. The son of Chinese immigrants, he had an itinerant upbringing in the rural Midwest. As a Stanford undergraduate, he was a de facto Marxist before being seduced, quite accidentally, by economics. He may be the only economist conducting monkey experiments, which puts him at slight odds with his psychologist collaborators (who are more interested in behavior itself than in the incentives that produce the behavior) as well as with certain economist colleagues. "I love interest rates, and I'm willing to talk about their kind of stuff all the time," he says, speaking of his fellow economists. "But I can tell that they're biting their tongues when I tell them what I'm working on."

It is sometimes unclear, even to Chen himself, exactly what he is working on. When he and Santos, his psychologist collaborator, began to teach the Yale capuchins to use money, he had no pressing research theme. The essential idea was to give a monkey a dollar and see what it did with it. The currency Chen settled on was a silver disc, one inch in diameter, with a hole in the middle -- "kind of like Chinese money," he says. It took several months of rudimentary repetition to teach the monkeys that these tokens were valuable as a means of exchange for a treat and would be similarly valuable the next day. Having gained that understanding, a capuchin would then be presented with 12 tokens on a tray and have to decide how many to surrender for, say, Jell-O cubes versus grapes. This first step allowed each capuchin to reveal its preferences and to grasp the concept of budgeting.

Then Chen introduced price shocks and wealth shocks. If, for instance, the price of Jell-O fell (two cubes instead of one per token), would the capuchin buy more Jell-O and fewer grapes? The capuchins responded rationally to tests like this -- that is, they responded the way most readers of The Times would respond. In economist-speak, the capuchins adhered to the rules of utility maximization and price theory: when the price of something falls, people tend to buy more of it.

Chen next introduced a pair of gambling games and set out to determine which one the monkeys preferred. In the first game, the capuchin was given one grape and, dependent on a coin flip, either retained the original grape or won a bonus grape. In the second game, the capuchin started out owning the bonus grape and, once again dependent on a coin flip, either kept the two grapes or lost one. These two games are in fact the same gamble, with identical odds, but one is framed as a potential win and the other as a potential loss.

How did the capuchins react? They far preferred to take a gamble on the potential gain than the potential loss. This is not what an economics textbook would predict. The laws of economics state that these two gambles, because they represent such small stakes, should be treated equally.