Photo: Photonica Traditional explanations, such as kinship and reciprocal altruism, rely on genetic relationships or self-interest. These work for animals, but fail for humans because people cooperate with strangers they may never meet again, and when the pay-off is not obvious.
Such cooperation can be explained if punishment of freeloaders or "free-riders" - those who do not contribute to a group but benefit from it - is taken into account. However, in real life, punishment is rarely without cost to the punisher. So why should someone punish a free-rider? Because of emotionally driven altruism, says Ernst Fehr, an economist at the University of Zurich in Switzerland.
All for one
To test this "altruistic punishment" hypothesis, Fehr and his colleagues played an experimental game with six groups of four students each, in which real money was at stake.
Each member was given 20 monetary units (MUs) to keep or invest in a group project. For every MU invested, the return for the group was 1.6 MUs, which was divided equally among the four members. So if only one person chose to invest, putting in 1 MU, she got back only 0.4 MU. But if everyone invested the full 20 MUs, they each ended up with 32 MUs, making total cooperation worthwhile.
Investment, therefore, was always in the interests of the group, but never in the interest of the individual doing the investing. A free-rider would benefit from not investing. She could just gain from the money invested by others.
After a series of six games, in which members' investments were anonymous and everyone invested simultaneously, Fehr found that members contributed an average of 10 MUs in the first game. But cooperation quickly unravelled, says Fehr. Contributions dropped to 4 MUs by the sixth game.
So Fehr decided to allow members to punish free-riders in their group, but at a cost. If a member punished another, it cost the punisher 1 MU and the punished 3 MUs. In six such games the average investment was always higher than in those without punishment, increasing to over 16 MUs. The threat of punishment sustained cooperation.
Crucially, the punishment was an altruistic act, as the punisher would never encounter the same free-rider again. To understand the motive behind altruistic punishment, the researchers questioned the students about their emotions.
They found that anger appeared to be the cause. "At the end of the experiment, people told us that they were very angry about the free-riders," says Fehr. "Our hypothesis is that negative emotions are the driving force behind the punishment."
"It's a great experiment," says Herb Gintis, an expert on human cooperative behaviour at the University of Massachusetts in Amherst. Social policies which do not provide an outlet for such emotions will fail, he says. In the 1980s, for instance, people revolted against the welfare state in the US because they felt that perceived freeloaders were not being taken to task.
Journal reference: Nature (vol 415, p 137)