choosing A too often made B a better bet. When A’s payoffs dropped, some players noticed right away and quickly switched to choosing B more often. But others stuck with A, gambling that it would return to its previous high-payoff rate. It appeared that some brains are more inclined to take risks than others—some players play conservatively; others are risk-takers. (Actually, Montague said, more accurate labels for the two types of players would be “matchers” and “optimizers.” “I call them conservative and risky because you can make good jokes about that,” he said.)

To me, it sounds more like they should be called “switchers” and “stickers.” But the labels don’t really matter. The most intriguing result from this experiment is the revelations from the brain scans. Sure enough, patterns of brain activity differed in the two groups, particularly in a small clump of brain cells called the nucleus accumbens. It’s a brain region implicated in drug addiction, and it’s more active in the “risk-taking” game players (the stickers).

The neatest thing, though, is that you can tell who the risk takers and play-it-safers are from their brain scans just after the very beginning of the game, even while their behaviors are still identical. This is the sort of evidence that destroys the old behaviorist position that behavior is the only thing that matters (or that you can know). Early in the game, two players can behave identically, making exactly the same choices. Yet by looking into their brains you can see differences that allow you to predict how they will play later, when the payoff rate changes.

“The people that ended up on average being risky are different from these people right away—nobody even jumps categories,” Montague told me. Even more intriguing, there appears to be a genetic difference between the two groups as well.

So neuroeconomics thus offers economists a tool they had not possessed before, giving hope that by getting inside people’s heads, science might really be on the road to finding the Code of Nature that governs human behavior.

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