A brain region that curbs our natural self interest has been identified. The studies could explain how we control fairness in our society, researchers say.

Humans are the only animals to act spitefully or to mete out "justice", dishing out punishment to people seen to be behaving unfairly – even if it is not in the punisher's own best interests. This tendency has been hard to explain in evolutionary terms, because it has no obvious reproductive advantage and punishing unfairness can actually lead to the punisher being harmed.

Now, using a tool called the “ultimatum game”, researchers have identified the part of the brain responsible for punishing unfairness. Subjects were put into anonymous pairs, and one person in each pair was given $20 and asked to share it with the other. They could choose to offer any amount – if the second partner accepted it, they both got to keep their share.

In purely economic terms, the second partner should never reject an offer, even a really low one, such as $1, as they are still $1 better off than if they rejected it. Most people offered half of the money. But in cases where only a very small share was offered, the vast majority of "receivers" spitefully rejected the offer, ensuring that neither partner got paid.

Fehr's fair

Previous brain imaging studies have revealed that part of the frontal lobes known as the dorsolateral prefrontal cortex, or DLPFC, becomes active when people face an unfair offer and have to decide what to do. Researchers had suggested this was because the region somehow suppresses our judgement of fairness.

But now, Ernst Fehr, an economist at the University of Zurich, and colleagues have come to the opposite conclusion – that the region suppresses our natural tendency to act in our own self interest.

They used a burst of magnetic pulses called transcranial magnetic stimulation (TMS) – produced by coils held over the scalp – to temporarily shut off activity in the DLPFC. Now, when faced with the opportunity to spitefully reject a cheeky low cash offer, subjects were actually more likely to take the money.

The researchers found that the DLPFC region's activity on the right side of the brain, but not the left, is vital for people to be able to dish out such punishment.

"The DLPFC is really causal in this decision. Its activity is crucial for overriding self interest," says Fehr. When the region is not working, people still know the offer is unfair, he says, but they do not act to punish the unfairness.