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Punishment helps discourage the dishonest from destroying the fabric of cooperative human societies. But that's not what you actually think about when you feel the urge to punish a rule-breaker. Scientists have long debated what motivates humans' deep-seated desire for retaliation, which we'll carry out even at great personal cost.

New research published Wednesday (July 18) in the journal Biology Letters suggests our motivations for punishing rule-breakers aren't really based on revenge, or the desire to inflict as much harm on them as they have inflicted on others. We just hate to see someone get ahead using unfair means. Cheating, specifically, only bothers us when it works.

First, what's the difference? Past studies have shown that misanthropic behavior, such as cheating, elicits strong negative emotions in people, which recede and give way to pleasure when we inflict punishment on the wrongdoer. Scientists had not previously managed to tease apart two distinct possibilities for why cheaters make us so angry. Do we dislike the fact that they've broken society's rules in an attempt to get ahead? Or does it bother us much more if they have gotten ahead?

In other words, "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?" said lead author Nichola Raihani of the University College London in a press release. [How Many People Cheat on Taxes?]

To find out, Raihani and her colleague, Katherine McAuliffe of Harvard University, grouped 560 volunteers into pairs of cheaters and noncheaters, and had the partners play a simple game over the Internet. For all the teams, cheaters could choose to "steal" 20 cents from their noncheating partners, and then the noncheaters had the option of paying 10 cents to "punish" the thief by reducing their partner's wealth by 30 cents.

However, in one-third of the teams, the cheater still had less money than his partner even after stealing 20 cents. In another third of the teams, stealing resulted in the partners having equal wealth. Only in the last group did stealing 20 cents make the cheater's wealth surpass that of the noncheater.

In the first two scenarios, roughly the same proportion of noncheaters paid to punish cheaters, and they did this regardless of whether or not the cheaters had actually chosen to cheat. (According to the researchers, this was the "baseline" of punishments — punishing wasn't actually correlated with cheating.) But among the third group, when the cheating partners' wealth surpassed the noncheaters', punishment more than doubled.

The results make a strong case for the idea that the decision to punish stems from our aversion to unfairness. We dislike it when cheating enables one person to get ahead of someone else who didn't cheat. But if the cheater's ploy didn't get them very far, we don't become nearly so angry.

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