Is our sense of justice hardwired?

New Scientist reports: [edited]

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.

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.

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.

"Self interest is one important motive in every human," says Fehr, "but there are also fairness concerns in most people."

"In other words, this is the part of the brain dealing with morality," says Herb Gintis, an economist at the University of Massachusetts in Amherst, US. "[It] is involved in comparing the costs and benefits of the material in terms of its fairness. It represses the basic instincts."

Psychologist Laurie Santos, at Yale University in Connecticut, US, comments: "This form of spite is an evolutionary puzzle. There are few examples in the animal kingdom." The new finding is really exciting, Santos says, as the DLPFC brain area is expanded only in humans, and it could explain why this type of behaviour exists only in humans.

Fehr says the research has implications for how we treat young offenders. "This region of the brain matures last, so if it is truly overriding our own self interest then adolescents are less endowed to comply with social norms than adults," he suggests. The criminal justice system takes into account differences for under-16s or under-18s, but this area only fully matures around the age of 20 or 22, he says.
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