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　　Fair punishment supports human altruism

　　The idea of fair punishment helps to maintain altruism in human groups, new experiments have shown. People playing an investing game with real money rapidly abandoned their altruistic behaviour if they felt the punishment given for selfish acts was unwarranted.

　　Human altruism intrigues researchers because evolutionary theory predicts that we should only be kind to others if there is something in it for us. But people frequently help out strangers even if they do not expect to meet them again, e.g. tipping waiters or giving to charity.

　　This unqualified generosity shows through the experiments of Ernst Fehr at the University of Zurich, Switzerland, and Bettina Rockenbach at the University of Erfurt in Germany.

　　But the important role of fair punishment was a surprise. It may be that groups of our ancestors who found the best strategies to promote altruism prospered, and bequeathed their behaviours to us.

　　Rate of return

　　The investing games were run anonymously between pairs of students. The "investor" can invest up to 10 monetary units (one MU equals 0.26 Euro) with a "trustee", and requests a specific rate of return. The trustee receives triple the investment put in by the investor and then can choose how much, if any, to return.

　　Even though anonymous trustees could have just kept the investment without being identified, 19 out of 24 of them returned some money. And the more money invested - implying faith in the trustee by the investor - the more they returned. Next, the researchers gave the investors the option of setting a punishment for the trustee in advance, if they did not return the amount asked for. A fine of four MUs would be deducted from the trustee's earnings if they paid back too little.

　　Choosing not to impose any punishment clearly went down well with the trustees - they gave back 50 per cent more money on average. "They feel an obligation to pay back, because they have been treated nicely by the investor," says Fehr.

　　Getting greedy

　　Women attracted to altruistic men, but not vice versa

　　But, crucially, trustees make a distinction between fair and unfair punishments. When the requested return is low, meaning the trustee will make more than the investor, then imposing a fine did not significantly change the payback given.

　　However, if the investor is more greedy and asks for a large return that leaves the trustee worse off, then an additional threat of punishment makes trustees slash their payouts by nearly two thirds.

　　"If people feel the punishment is fair, they respond by cooperating," says Herbert Gintis, an economist and expert in human altruism at the University of Massachusetts in Amherst, but they react very badly to an unfair punishment.

　　Journal reference: Nature (vol 422, p 137)