The Tragedy of the Commons occurs when a group’s individual incentives lead them to take actions which, in aggregate, lead to negative consequences for all group members. It is a multi-player version of the Prisoner’s Dilemma. In the version of the game from which it got its name, the players are two prisoners, held in separate cells. Each has to choose between “cooperating” with the other (keeping quiet) or “defecting” (giving evidence against the other). Each makes the choice without knowing what the other will do. If both prisoners keep quiet, they are each sentenced to one year in prison. If one rats on the other, he or she goes free and the other gets 10 years. If they both rat on each other, they each get 5 years. The problem arises because whatever your opponent does, defecting gives you a higher payoff than cooperating. In 1984 Robert Axelrod published a book called The Evolution of Cooperation, which contained a surprising reflection: if you play a Prisoner’s Dilemma game, not once, but repeatedly, then what you are likely to see emerging is cooperative, rather than mutually destructive, behaviour. He announced a competition, inviting the submission of computer programs to play Prisoners Dilemma, which he pitted against each other to see which strategies did best. Surprisingly, it was the simplest programme which won: called Tit-for-Tat, submitted by Professor Anatol Rapoport. Axelrod went on to boil the essence of Tit-for-Tat down to four simple rules:
- Be Nice. Start by cooperating, and never be the first to defect. Otherwise you have no chance of getting into the zone where you both cooperate repeatedly and rack up the best outcome over time.
- Be Retaliatory. If the other player defects, inflict a cost on him or her which is at least as severe – otherwise you open yourself to exploitation.
- Be Forgiving. If your opponent mends his ways after defecting, restore cooperation as quickly as possible, so that you can both get back to scoring highly on each round.
- Be Clear. Since there is no way to beat the Nice, Retaliatory and Forgiving strategy, if your opponent knows you are following it, there is no incentive for him or her to seek advantage – it will only destroy his or her score as well as yours.
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