chemical dopamine was the brain’s currency for gauging the relative payoffs of potential behaviors. The paper noted various lines of evidence supporting the idea that a circuit of activity linking two parts of the brain—one at the front, behind the forehead, and another deep in the brain’s middle—helps govern choice making by producing more or less dopamine. Dopamine levels predict the likely reward associated with different choices, the evidence indicated.

Dopamine had long been known as the brain’s chief pleasure molecule, linked to behavior that produces pleasant feelings. But it’s not merely pleasure that drives dopamine production. Actually, the brain’s dopamine currency seems tuned to the expectation of pleasure (or reward of some sort). Some of the brain’s dopamine-producing nerve cells are programmed to monitor the difference between expected and actual reward, Montague and Berns showed. If a choice produces precisely the predicted reward, the dopamine cells maintain a constant level of activity. When pleasure exceeds expectations, the cells squirt out dopamine like crazy. If the reward disappoints, dopamine production is curtailed. This monitoring system also takes timing into account—if dinner is delayed, dopamine is diminished. When the anticipated rewards aren’t realized, the dopamine monitoring system tells the brain to change its behavior. In this way the expectation of reward can guide a brain’s decisions.

A critical point, noted by Montague and Berns, is that all brains are not alike. One person’s dream reward might be another’s horrific nightmare. Some people make a risky choice only when expecting a huge reward; others gamble for the fun of it. Part of the promise of neuroeconomics is its ability to identify such individual differences with brain scanning.

In one experiment described by Montague and Berns, people chose either A or B on a computer screen and then watched a bar on the screen to see whether their choice earned a reward. (The bar recorded accumulated reward “points” as the game progressed.) As the game went on, the computer adjusted the rewards, based on the player’s choices. At first, choosing A raised the bar more, but

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