Irrational Animals: a neuroscience take on the news

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The science of getting your own back

Martha Gill's "Irrational Animals" column.

Actress Emily Vancamp from the US drama Revenge. Photograph: Getty Images

There’s a fantastic scene at the end of Quentin Tarantino’s film Death Proof. Three young girls are being hunted by Kurt Russell, a psychopath with a stunt car and a foot fetish. We have watched him torture and kill his way through the film, and, as he wends his way towards this group, their naivete is used as a foil to his ever increasing menace. In the last three minutes they beat him to death with an iron pole. It’s a great ending.

Outside Hollywood, the idea of revenge is often pushed into the background, especially when explaining our motives. We call it “justice” or “righting a wrong” or “balancing the scales”, but these notions don’t do much for us biologically, and don’t show up in an obvious way in the brain (it’s a grey area). Revenge, on the other hand, has a very clear neural signal, and that signal is pleasure.

A classic psychology scenario - the prisoner's dilemma - can be combined with neuroimaging to show this quite clearly. In the dilemma, two people have committed a crime and are being held in separate cells. They can either confess and get a reasonable sentence or blame the other, and get a light one. If both blame the other, though, they get the longest sentence of all.

In a study published in 2006, by Tania Singer at UCL, researchers asked two “prisoners” to play this out in front of an audience. Then members of the audience were then put inside fMRI machines while they witnessed each prisoner receiving electric shocks to his hands. As they watched, they showed increased activity in neural pain areas – evidence of empathy. Surprisingly, though, this empathy was present only when watching one of the “silent partners”. If a “confessor” was punished, the activity died down considerably. It looked like the brain cared much less about the pain of those who had betrayed their partner.

There was another finding that Singer didn’t expect. Watching “bad” prisoners get punished gave members of the audience pleasure: there was activation in reward-related areas of the brain, such as the ventral striatum and the nucleus accumbens. (This was limited to male subjects.)

So, we - or at least the men among us - get pleasure from revenge. According to similar studies, we also get activity in the left prefrontal cortex of the brain, which relates to goal planning. Revenge, then, seems to be not only a passing delight, but a craving, something we need, that we plan for. The desire increases when we are mistreated in front of others – we need to show people we’re not to be pushed around – and decreases when we’ve got more to lose by exacting it. In other words, the emotion has a logic to it.

Does it have an evolutionary function? Literature is littered with sayings about revenge and how it never pays, or is better expressed by forgiving the other person, or is exacted by “living well” (with gritted teeth). Yet our neurobiology feels otherwise. Which is right?

Well, let’s return to the prisoner’s dilemma. The dilemma itself is whether or not to trust your partner, but this issue figures only in the beginning. As the game plays out, round after round, prisoners seek petty revenge on each other, punishing the other for betraying them. Eventually, burned out, the two settle on a compromise. And this may be the lesson. Fear of retribution keeps us behaving fairly to others, making co-operation possible. This is why we love watching people get their comeuppance in Singer's test, and why we love watching Quentin Tarantino's films. Revenge is a sweet necessity.