GENIUS FORUMS

Generous players: game theory explores the Golden Rule's place in biology

Charles Darwin's theory of natural selection seems to describe a brutal world in which creatures compete ruthlessly to promote their own survival. Yet biologists observe that animals and even lower organisms often behave altruistically. A vervet monkey who spots a leopard, for instance, warns his fellow monkeys, even though the call may attract the leopard's attention to the individual. A vampire bat that has hunted successfully shares nourishing blood with a fellow bat that failed to find prey.


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For decades, game theorists' basic paradigm for the puzzle of cooperation has been the scenario called the prisoner's dilemma, in which each player has a powerful incentive to exploit the other. The game is set up so that cooperation is best for the group, but each player individually does better by taking advantage of the other. A growing body of mathematical analysis and computer modeling now suggests that in many circumstances, cooperators can survive in the prisoner's dilemma. In the April 8 Nature, researchers argue that, under certain conditions, a cooperator can infiltrate, and eventually take over, a population of cheaters.


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Although the prisoner's dilemma scenario may seem artificial, many interactions of animals and other organisms may have a similar structure of rewards and penalties, some biologists and game theorists argue. In biological versions of the prisoner's dilemma, organisms are competing not for shorter prison sentences but for increased fitness and reproductive success.

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TIT FOR TAT Things look rosier for cooperation in situations where a participant plays the prisoner's dilemma repeatedly with the same opponent and learns from previous games. After all, it can be risky to exploit someone you know you're going to encounter again.

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Martin Nowak of Harvard University and his collaborators have now shown that for a wide range of game parameters and population sizes, a lone tit-for-tat player actually has a better chance of its descendants eventually taking over the entire population than any individual defector does.