Our sense of justice is built into the brain

London: Our sense of justice is built into the brain, with in-built mechanisms that trigger an automatic reaction to someone who acts unfairly.

In a new study from the Karolinska Institutet, Sweden, the sense of justice among subjects was challenged in a two-player money-based fairness game, while their brain activity was registered by an MR scanner.

When bidders made unfair suggestions as to how to share the money, they were often punished by their partners even if it cost them. A drug that inhibits amygdala activity subdued this reaction to unfairness, the journal Public Library of Science-Biology reports.

The study, a collaboration between Karolinska Institutet and the Stockholm School of Economics, is based on the universal human behaviour to react aggressively when another person behaves unfairly or unethically, according to a Karolinska statement.

Researchers had 35 subjects play a money-based fairness game (the Ultimate Game), whereby one player suggests to another how a fixed sum of money is to be shared between them.

The other player can then either accept the suggestion and take the money, or reject it, in which case neither player receives anything.

"If the sum to be shared is 100 SEK kronor and the suggestion is 50 each, everyone accepts it as it is seen as fair," says Katarina Gospic, who led the study at Karolinska.

"But if the suggestion is that you get 20 and I take 80, it's seen as unfair. In roughly half the cases it ends up with the player receiving the smaller share rejecting the suggestion, even though it costs them 20 SEK," adds Gospic.

By subjecting the participants' brain activity to an MR scanner during play, researchers were able to see that the brain area controlling these financial decisions was located in the amygdala, a primitive part of the brain that controls feelings of anger and fear.

The subjects were either given the anti-anxiety tranquilliser Oxazepam or a sugar pill (placebo) while playing the Ultimate Game.

Those who had received the drug showed lower amygdala activity and a stronger tendency to accept an unfair distribution of the money, this despite the fact that when asked, they still considered the suggestion unfair.

Men responded more aggressively to unfair suggestions than women and showed a correspondingly higher rate of amygdalic activity. This gender difference was not found in the group that received Oxazepam.