An important advance along that road came in 2003 with the publication of a paper in the journal Science by researchers at Princeton University. In a study by Alan Sanfey and colleagues, participants in an experiment played the ultimatum game, one of the favorites of behavioral game theorists. It’s kind of like a TV game show contest in which you are given a lot of money, but you have to share your windfall with a stranger. Suppose you get $100. You then offer the stranger part of the money and keep the rest— unless the stranger refuses your offer. Then you have to give all the money back, and nobody wins anything.

In theory, the stranger should take any offer, no matter how small, in order to get something rather than nothing. Therefore, a game theorist might conclude, you should offer a low amount— $10, say, or even $1—so that you will then walk away with the most money possible. But in practice, most strangers reject low offers. If you offer $10, for instance, you’re much more likely to walk away with zero than $90, as the stranger will probably reject your offer just to punish you, even at personal expense. Consequently people typically share more generously—offering 40 to 50 percent of the prize, say—in anticipation of an angry rejection of an unfair offer.

So this is another case where naive game theory, in assuming that everybody will maximize their money, makes an incorrect prediction, as many economic experiments had already established. The Princeton study went further, though, by scanning the brains of the strangers who were considering whether to accept the offer from the other participant. In this case, the prize was only $10— science doesn’t have budgets like Who Wants to Be a Millionaire?— but the principle was the same. If the first player offered only $1 or $2, the offer was usually rejected. But not always. And you could tell who was likely to accept or reject a low offer by watching what went on inside their brains.

Stronger brain activity in the front part of a brain region known as the insula (an area known to be associated with negative

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