emotions, such as anger and disgust) was common in players who were more likely to reject low offers. Another brain structure—the anterior cingulate cortex—also showed increased activity in those who rejected unfair offers. That region is known to be involved in monitoring conflict—in this case, the conflict between the choice of punishing a cheapskate or turning away money. “Unfair treatment … can lead people to sacrifice sometimes considerable financial gain in order to punish their partner for the slight,” Sanfey and his collaborators reported in Science.10

In a commentary on that paper, Colin Camerer noted that it showed how the tenets of basic game theory do not always hold— people do not always act totally in their own self-interest (that is, maximizing their money), and all the players in a “game” therefore are not always trying to do the best they can do, as assumed in the underlying basis for a Nash equilibrium. But behavioral game theory, Camerer noted, can relax these assumptions and still learn a lot about human behavior. The neuroeconomics enterprise, in other words, is a powerful tool for developing behavioral game theory insights into how real people make choices.

Montague’s subjects at Baylor, for instance, play similar behavioral games that reveal the quirks of human economic behavior. In one such game—a task for testing trust—Player 1 is given $20 and is allowed to keep some of it and put the rest in a virtual pot, where the amount is then tripled. If Player 1 keeps $10 and donates $10, the sum in the pot becomes $30. Player 2 then gets to split the pot with Player 1—or take it all.

“If you split it 15-15, then in a sense you’ve repaid the trust,” said Montague. But if you take $29 and leave $1, Player 1 is not likely to offer much in the next round of the game. At any point in the game, one player or the other could decide to keep all the money, so the logical move is to take it all as soon as possible, before the other player does. But in fact, players typically trust each other not to be so selfish—although some are more trusting, and some more selfish, than others.

Traditional economists were not surprised at the results of such games. In the 1980s, game theory had fueled the rise of “experi-



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