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All societies rely on individuals to police each other: if we think someone is behaving unfairly, we say so. Such rebukes rein in selfish behaviour and provide social glue.

But where does the desire to stop the cheats come from? A team of economists and neuroscientists has now identified a brain region that seems to play a critical role. As well as shedding light on how we cooperate, researchers say the finding could have implications for our understanding of economics and mental disorders.

The finding, published online in the journal Science¹, describes the results from an adaptation of a famous experiment called the ultimatum game. One participant in the game the proposer is given 20 Swiss francs and told to pick an amount to share with the other player. The catch is that if that second player the responder turns down the offer, then neither player gets any money.

The game is interesting because it tests the conflict between our willingness to punish selfish actions and rational economic behaviour. In a single round of the game, the most rational decision is to accept whatever is offered, because the alternative is to receive nothing at all. But players often view very low offers as insultingly unfair; most choose to punish the proposer and refuse the money if less than 5 Swiss francs is offered.

Daria Knoch of the University of Zurich in Switzerland and a team of neuroscientists and economists have now shown that inhibiting part of the brain can make a responder more likely to accept unfair offers.

Crime and punishment

The righthand part of a region towards the front of the brain called the dorsolateral prefrontal cortex is already known to be involved in processes such as those that guide the ultimatum game (see ' Driven to market'). Knoch's team applied a magnetic field to this chunk of cortex in order to inhibit its functioning.

The players' perceptions of the fairness of offers were unchanged by this: the responders still thought the proposer was being selfish if he or she offered less than a certain number of Swiss francs. But they became less likely to inflict the punishment of rejection.

The team concludes that this part of the cortex, at least when it is working properly, is involved in brain systems that promote fairness and help smother the rational desire to take whatever is on offer.

"It's a terrific piece of work," says Robert Boyd of the University of California, Los Angeles, who studies the evolution of culture. "It shows what talking across disciplines buys you."

The finding also boosts the theory that emotions - in this case the desire for fairness play an important role in economic decision-making, adds André Aleman, a neuroscientist at the University of Groningen in the Netherlands who is not part of the team. "It's common sense idea," he says. "But in economics the prevailing theory is that it's about cognitive decisions."

Friendly farmers

Boyd says he would next like to know more about the cognitive mechanisms underlying decisions about fairness. He notes that the ultimatum game gives different results when played by different cultures. One society tested a group of farmers from tropical South America would accept almost any offer, no matter how unfair.

A full understanding of that difference, says Boyd, requires a better explanation of how fairness decisions are made.

The finding could also help probe mental disorders, says Aleman. He notes that people with conditions such as autism and schizophrenia play the ultimatum game differently, maybe because they struggle to understand the emotions that their opponent may be feeling.

If the brain mechanisms that control game tactics can be unravelled, that might help shed light on the changes in the brain that underlie mental illnesses.

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