Participants with their right dorsolateral prefrontal cortex inhibited were more likely to make selfish decisions.

Credit: iStockphoto

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SYDNEY: Ever held back from taking that last piece of chocolate, offered your bus seat to a stranger or given a busker all your change? Scientists have located the region of your brain that made you do it, according to a new international study.

"Across species, humans have been spectacularly successful in limiting the impact of self-interest even in interactions between genetically unrelated strangers," the researchers say. Until now, however, little has been known about how the human brain restrains selfish impulses and prioritises fair behaviour.

The study, published in the U.S. journal Science, involved 52 male volunteers in a game that required players to make decisions relating to fairness. Some of the volunteers underwent low-frequency transcranial magnetic stimulation (rTMS) during the game, to inhibit a brain region known as the dorsolateral prefrontal cortex (DLPFC).

Previous research has identified the DLPFC as active during the 'Ultimatum Game'. In this game, two people - a proposer and a responder - must agree on how to split a sum of money.

The proposer makes a suggestion, which the responder can either accept or reject. If they accept, the money is divided according to the offer. If they reject, neither player wins anything.

Responders don't always accept smaller cuts, as would be expected if only the selfish notion that 'something is better than nothing' were at play. Responders sometimes reject smaller cuts out of a sense of what scientists call 'reciprocal fairness', which rewards fair acts and punishes unfair acts, even against the interests of the punisher.

Previous research has found that the left and right DLPFCs are active in responders making decisions about reciprocal fairness. However the role of these regions in directing behaviour has remained a mystery.

In this new study, volunteers were assigned the role of responder to 20 different anonymous proposers in the Ultimatum Game. Each responder had either a magnetic inhibition to the right or left DLPFC, or underwent a placebo experience, mimicking the rTMS.

While the acceptance rate for unfair offers was about 10 per cent with left DLPFC inhibition or the placebo, the acceptance rate (the 'selfish' choice) increased to 45 percent with right DLPFC inhibition. In other words, switching off the right dorsolateral prefrontal cortex allowed selfish impulses to direct behaviour.

The researchers conclude that the right DLPFC (though not the left) is crucial for our ability to override selfish impulses. So when you leave that last piece of chocolate for someone else it's your right DLPFC that's to blame.

Since the left DLPFC is also activated during fairness decisions, this raises "exciting questions about its exact role and the possible interplay of the left and right hemispheres in ... fairness related behaviours," according to the researchers.