mental economics” in which such deviations from pure self-interest showed up regularly. What’s new in neuroeconomics is eavesdropping on the players’ brains via the MRI scanners while the games are in progress. Montague’s lab is particularly well equipped for this sort of thing, with a pair of scanners, one each in two rooms separated by the scientists’ observing station. The scientists watch as computers record the brain activity of players deciding how to move or how to react to another player’s move. “You can see what went on in the behavior. You can back up and look at their intent to act badly or their intent to invest more,” Montague said. “It allows us to cross-correlate what’s going on in the two brains. I think it’s cool. I think it’s an obvious way to study social interactions.”11

Neuroeconomics does not always require scanning, though. Paul Zak, director of the Center for Neuroeconomics Studies at Claremont Graduate University in California, sometimes uses blood tests instead of brain scans. He can relate variant economic behaviors to levels of certain hormones. In one of Zak’s versions of the trust game, players communicate via computer. One player, given $10, offers some of it to another player, who is paid triple the amount offered. (So if Player 1 offers $5, Player 2 gets $15). Player 2 then can take it all, or give part of it back to Player 1. But in this version of the experiment, the game ends after just one round. There’s no incentive to earn trust so as to get more money the next time around.

So standard game theory suggests that Player 2 would take all the money, having nothing to gain by giving some back. But Player 1, anticipating that move, should therefore offer none of the money to begin with. Nevertheless, many players defy naive game theory and show at least some trust that the other player will play fair. About half of the first-movers offer some money (suggesting that they are trusting souls), while three in four of the responders give some back (suggesting that they are trustworthy).

Once again, the intriguing thing about such games is finding out what’s behind the differences in individual behavior. It turns out that among the trustworthy players, blood tests revealed higher

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