is given a sum of money and must offer a share of it to the second player. The second player may either accept the offer (and the first player keeps the rest) or the second player may reject the offer, in which case all the money is returned and neither player gets anything.

By the time Henrich tried the game in Peru, it had been widely played with college students, who usually make offers averaging more than 40 percent of the pot. Such offers are routinely accepted. Sometimes lower amounts would be offered, but they would usually be rejected. Among the Machiguenga, though, Henrich observed that lower amounts were routinely offered—and usually accepted.

“We both expected the Machiguenga to do the same as everybody else,” UCLA anthropologist Robert Boyd told me. “It was so surprisingly different that I didn’t know what to expect anymore.”9

Could it be that the Machiguenga actually understood the rational-choice rules of game theory, while everybody else in the world let emotions diminish their payoffs? Or would other isolated cultures behave in the same way? Soon Henrich, Boyd, and others acquired funding from the MacArthur Foundation, and later the National Science Foundation, to repeat the games in 15 smallscale societies on four continents. The results were utterly baffling. From Fiji to Kenya, Mongolia to New Guinea, people played the ultimatum game not just the way college students did, or the way economic theory dictated, but any way they darn well pleased.

In some cultures, like the Machiguenga, low offers were typical and were often accepted. But in other cultures, low offers were frequently made but typically rejected. In a few cultures the offers would sometimes be extra generous—even more than half. But in some societies such generous offers were likely to be refused. Among other groups, rejections almost never occurred, regardless of the size of the offer.10

“It really makes you rethink the nature of human sociality,” Henrich, now at Emory University in Atlanta, told me. “There’s a lot of variation in human sociality. Whatever your theory is about human behavior, you have to account for that variation.”11

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