If you had the chance to confront a thief who stole $10 from you, what would you do? Would you just ask for your $10 back or would you seek revenge?

Psychologists revealed that your decision to punish the cheat might depend on whether the thief ended up being richer than you, according to a new study.

Most economic theorist believe that self-interest is what motivates human behavior, but previous studies show that people consistently sacrifice their own welfare to punish cheats.

For instance, a classic economic experiment called the "ultimatum game," which involves two players and only allows one person hold a certain number of dollars and to give to the second player who is allowed to reject the offer resulting in the first player losing everything. 

Researchers had found that rather than accepting the any offer given to them by the first player, the second player will consistently reject low offers, preferring to receive nothing than to receive something less than the first player.

"Punishment is a costly behavior which is often aimed at individuals that cheat during social interactions," said study author Dr. Nichola Raihani of the University of London.

"Several previous studies have shown that punishment is motivated by negative emotions. However we wanted to know what precisely makes people want to punish cheats," said Raihani. "Is punishment motivated purely by a desire for revenge, or do individuals judge whether cheats end up better off than them before deciding whether to punish?"

Raihani and Katherine McAuliffe of Harvard University recruited 560 volunteers through an online labor market to play a simple game over the Internet. Subjects were paired off, with one person assigned to be the cheater and the other the victim.  

Researchers put the pairs in three scenarios.  In the first scenario, the cheating partner started out with a lot less money than the non-cheating partner and the cheater will choose to "steal" 20 cents but will still not have more money than the victim.  In the second scenario the cheater would also steal 20 cents and will end up having the same amount of money as the victim.  In the third scenario, the cheater will steal 20 cents and end up richer than the victim.

When researchers gave victims a chance to pay 10 cents to punish the cheaters, roughly the same proportion of non-cheating partners paid to punish the cheats in the first two scenarios, but when the cheating partner ended up having more money than the victim, punishment rates more than doubled.

Researchers said that while negative emotions motivate punishment, they are unsure as to why these negative emotions are produced when confronting cheats.

However they suggest that victims of cheats experience negative emotions because cheats infringe cooperative norms within society, and impose losses on cooperative partners.  They explain that punishment may be used to promote fair behavior rather than to satisfy the desire to dish out payback.

The study was published in Biology Letters.