Here’s another neuroscience study, this time trying to explain the odd results of the Ultimatum Game.

The ultimatum game is a one turn game. Player 1 is given $100 and told to divide it with Player 2. If Player 2 rejects the offer, then neither player gets anything. After that turn, the players never interact again. Player 2 cannot hold out for a bigger offer.

According to pure logic in game theory, Player 1 will take $99 and give Player 2 $1. Since $1 is greater than $0, player 2 will accept. In real life, people are spiteful. A 56-44 split is more common.

The result is irrational within the context of the Ultimatum Game. A person will choose $0 over $1? Why? Because they want to harm another person more than they want to benefit.

NewScientist reports on a new study that identifies spitefulness and its connection with fair offers.

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.

Here’s how they found it:

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.

Spite is a unique human emotion.

The results of the Ultimatum Game actually are rational, but the game takes us out of our evolutionary context and puts us in an artificial environment.

People are spiteful and really do want to punish perceived unfairness. Men and women lose empathy when they see cheaters get punished, and men enjoy revenge. Spitefulness is linked to this sense of justice.

In the Ultimatum Game men are more spiteful than women too. Men with higher levels of testosterone are most likely to reject offers. Men are even meaner than women so they are offered a larger portion of the money in ultimatum games.

Spitefulness is an evolutionary punishment strategy. It’s an emotional reaction designed to maximize utility over repeated interactions rather than single games. Spite pays off. Everyone wants to maximize their games, so they must counter the other person’s mean spitefulness. Offers in Ultimatum Games are significantly higher than $1 as a result, often in the $40 range and nearing 50-50 splits.

Player 1 knows that Player 2 is not purely self-interested and is actually quite mean and would like to hurt Player 1. So 1 minimizes his harm by offering the second player more.

Our instincts for spite and revenge form our sense of justice. We uphold this justice by harming cheaters. Our mean spiritedness looks like the root of cooperation in society.