Revenge feels sweet and Swiss researchers are saying they have brain scans to prove it.
In a study investigators said might help explain how social norms arose and regulate behaviour, brain centres linked to enjoyment and satisfaction lit up in young men who punished others for cheating them.
Dominique de Quervain of the University of Zurich and colleagues tested 15 male students, telling them they were doing an economic study.
The men all sat in positron electron tomography, or PET scanners, that recorded brain activity.
In the study published in the journal Science, they paired the men in an exchange of cash. Player A could give all or some of his money to player B, who could then give some or none of it back.
If the first player gave all his money, the amount was quadrupled and the player B could share the reward with player A. This scenario would obviously benefit both the most, so player A had an incentive to share.
If player B declined to share, player A could punish him by taking away imaginary points or taking away money.
"We scanned the subjects' brains while they learned about the defector's abuse of trust and determine the punishment," the researchers wrote.
The PET scans showed a clear pattern of activity in the brain's dorsal striatum, involved in experiencing satisfaction, when one player penalised the other for selfishness.
This was the case even when player A had to use some of his own money to inflict the punishment.
"Instead of cold, calculated, reason, it is passion that may plant the seeds of revenge," commented psychologist Brian Knutson of Stanford University in California.
He likened the feeling to a driver refusing to let another he considers a cheater squeeze in front of him in traffic.
"After squeezing back the intruder, you can't help but notice a smile creep onto your face," Knutson wrote in a commentary.