Scans show brain hard-wired for sweet revenge

By putting players into PET scanners, scientists find brain's reward centre lights up when planning revenge against a cheater.

Planning revenge really does feel sweet, according to a new study of brain scans.

Researchers in Switzerland said the brain centres linked to enjoyment "lit up" in young men who punished those who cheated them.

It's as if punishing cheaters is satisfying enough to motivate getting even, even if revenge is costly.

The Swiss research shows the seemingly irrational behaviour can happen.

Dominique de Quervain of the University of Zurich and colleagues tested 15 male students as they sat in PET scanners that recorded brain activity, to determine what motivates revenge.

The students were told they were participating in an economic study.

In the study, 14 players were paired up after being told if they trusted each other and co-operated, both men would earn money.

If one double-crossed the other, the cheater could keep an unfair share.

The rules of the game forced some cheating. The slighted player could retaliate by fining the cheater, although he sometimes had to spend his own money to enforce the fine.

"We scanned the subjects' brains while they learned about the defector's abuse of trust and determined the punishment," the researchers wrote.

The PET scans showed increased activity in the brain's dorsal striatum, an enjoyment centre, when a player punished a cheater, even if he had to use his own money to inflict punishment.

In a commentary on the research, psychologist Brian Knutson of Stanford University in California likened the findings to a driver who refuses to yield to a perceived cheater during traffic.

"After squeezing back the intruder, you can't help but notice a smile creep onto your face," Knutson wrote.

The researchers said the instinct likely evolved to ensure cheaters were punished during the thousands of years that human societies lacked law enforcement institutions like impartial police and judges.