In a wonderful book “The Upside of Irrationality”, the author, Dan Ariely, writes on many behavioral biases that cause ‘irrationality’. He writes in a chapter about the irrationality of revenge.

“To begin to understand how deeply the human desire for vengeance runs, I invite you to consider a study conducted by a group of Swiss researchers led by Ernst Fehr, who examined revenge using a version of an experimental game we call the Trust Game. Here are the rules, which are explained in detail to all participants.

You are paired with another participant. You are kept in separate rooms, and you will never know each other's identity. The experimenter gives each of you $10. You get to make the first move. You must decide whether to send your money over to the other participant or keep it for yourself. If you keep it, both of you get to keep your $10 and the game is over. However, if you send the other player your money, the experimenter quadruples the amount-so that the other player has their original $10 plus $40 (the $10 multiplied by four). The other player now has a choice: (a) to keep all the money, which means that they would get $50 and you would get nothing; or (b) to send half the money back to you, which means that each of you would end up with $25.

The question, of course, is whether you will trust the other person. Do you send them the money-potentially sacrificing your financial gain? And will the other person justify your trust and share the earnings with you? The prediction of rational economics is very simple: no one would ever give back half of their $50, and, since this behavior is so glaringly predictable from a rational economic perspective, no one would ever send over their $10 in the first place. In this case, the simple economic theory is inaccurate: the good news is that people are more trusting and more reciprocating than rational economics would have us believe. Many people end up passing along their $10, and their partners often reciprocate by sending $25 back.

This is the basic trust game, but the Swiss version included another interesting step: if your partner chooses to keep all $50 for himself, you can use your own money to punish the bastard. For each dollar of your own hard-earned money that you give the experimenter, $2 will be extracted from your greedy partner. This means that if you decide to spend, say, $2 of your own money, your partner will lose $4, and if you decide to spend $25, your partner will lose all his winnings. If you were playing the game and the other person betrayed your trust, would you choose this costly revenge? Would you sacrifice your own money to make the other player suffer? How much would you spend?

The experiment showed that many of the people who had the opportunity to exact revenge on their partners did so, and they punished severely. Yet this finding was not the most interesting part of the study. While making their decisions, the participants' brains were being scanned by positron emission tomography (PET). This way, the experimenters could observe participants' brain activity while they were making their decisions. The results showed increased activity in the striatum, which is a part of the brain associated with the way we experience reward. In other words, according to the PET scan, it looked as though the decision to punish others was related to a feeling of pleasure. What's more, those who had a high level of striatum activation punished others to a greater degree.

All of this suggests that punishing betrayal, even when it costs us something, has biological underpinnings. And this behavior is, in fact, pleasurable (or at least elicits a reaction similar to pleasure)”.