Why does revenge taste so sweet? Why do we feel the need to chant in the streets after the death of a hated man? The answer returns us to the brain, and to the fascinating ways in which those three pounds of meat mirror the ideals of game theory.

A few years ago, a team of researchers at University College London led by Tania Singer conducted a simple exploration of the Prisoner's Dilemma. The Dilemma considers the following scenario: You and your accomplice are both held prisoner, having been captured by the police. The prosecutor interrogates you separately and offers each of you a deal. If one of you (the defector) confesses and incriminates the other – while the partner remains silent – then the defector will be given a light sentence (1 year) while the silent partner will be jailed for 4 years. However, if you both remain silent and cooperate with each other, each of you will receive a 2 year sentence, since the police don't have enough evidence to convict you of the more serious crime. The last possibility is that you both confess, in which case you will receive a sentence of 3 years.

The scientists played this game in front of the experimental subjects, allowing people to form strong opinions of the two "prisoners." In many instances, people grew to strongly dislike those who defected, viewing them as untrustworthy cheaters. Then, while the subjects were stuck inside an fMRI machine, the scientists applied painful electrical shocks to the hands of the prisoners. Here's where the results get interesting: Although every subject showed an increase of activity in pain-related areas when the prisoners were shocked – they couldn't help but empathize with the hurt of others – this activity was slightly reduced when defectors were punished. In other words, their bad social behavior had diminished our sympathy, making us less interested in their pain.

The most striking finding, however, was limited to the minds of men. According to the data, when men (but not women) watched a defector get punished, they showed additional activation in reward related areas of the brain, such as the ventral striatum and nucleus accumbens. These are essential elements of the dopamine reward pathway, that same highway of nerves that also gets titillated by sex, drugs and rock n' roll. Apparently, we are engineered to get pleasure from punishing those who deserve to be punished. As the scientists note:

The findings of enhanced activation in ventral striatum to a signal indicating that a defector is receiving pain are in agreement with the hypothesis that humans derive satisfaction simply from seeing justice administered, even if the instrument of punishment is out of their control.

And this leads us back to the Prisoner's Dilemma, that simple exercise in game theory. When the game is simulated for thousands of sequential rounds, it turns out that the most effective basic strategy is an approach known as "tit for tat." The rules of tit for tat are incredibly simple: Unless provoked, the prisoners will cooperate (and not confess). However, once they are provoked, they will seek out revenge, Old Testament style. This help ensures that defection is discouraged, that people know their cheating has consequences. And this is why the brain, at least in young men, takes so much delight in the pain of bad people. An eye for an eye feels great.

P.S. John Pavlus has a great post on how the Obama administration deftly "killed the concept" of Osama:

Where there was once a kind of “bad Batman” out there, more than just a man, haunting our collective consciousness like a demon, inspiring others by example and, later, by simple nose-thumbing existence, now there is just a Nothing: no body, no image, no locus for more bloodlust or vengeance or worship or debate. Just a lacuna in the text, a literal dead end.

While such a disappearance might be less emotionally satisfying (at least for our dopamine neurons) than some bloody images of the revenge, I think it also helps to slow the downward spiral of tit for tat. As Gandhi famously said, "An eye for eye, and soon the whole world is blind."