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Understanding the way people police use of shared resources — such as fishing stocks or forests — could offer clues to rescuing these resources when they come under threat. Manfred Milinski, an evolutionary ecologist at the Max Planck Institute of Limnology in Plön, Germany, teamed up with economist Bettina Rockenbach of the University of Erfurt to determine ways to encourage sharing. They used an interactive monetary game to see how players reward or punish each other, depending on their behaviour (see page 718).

How did an economist and an evolutionary biologist come together for this work?

Using iterative games in which subjects contribute to a common pool of money, empirical economists recently found that exacting punishment — by forcing non-cooperators to contribute more during the course of the game — enhanced cooperation. Meanwhile, evolutionary biologists found that building a solid reputation as a giver encouraged contributions from all. Bettina and I met at a workshop and decided to join forces.

You thought that the subjects would give up punishing behaviour. Was that the case?

In successive rounds of the experiment, we expected people to give up punishing behaviour because punishment has higher costs than withholding support from members who had accumulated a bad social reputation. We found that the subjects chose not to impose punishment that often, but they didn't want to get rid of it completely. We also found that combining punishment and reputation-building as a giver boosted cooperative efficiency.

It seems from your work that humans have a particular disdain for 'free riders'. Why?

As soon as a free rider enters the game, the others give up contributing to the public pool. The introduction of one free rider causes breakdown of cooperation. People have strong emotions against free riders, who are often seen as parasites.

Are humans gluttons for punishment?

When people were given a choice, they preferred to have both reputation and punishment available as tools to promote cooperation. Even though they used punishment at a low level, they preferred having a last resort. If reputation doesn't help, punishment provides the teeth necessary to discipline the worst guys.

What are the implications of your study?

Introducing publicity might help cure a 'public goods' dilemma. If people are seen overusing a public resource, they might stop.