Michael Marshall in New Scientist:
In the prisoner's dilemma, if both players keep quiet, each gets a brief sentence. But if one betrays the other, the snitch gets off scot-free while their partner suffers a long sentence. If both players betray each other, each gets a medium sentence. As a united pair, players do better if they both keep shtum. But crucially, if criminal A thinks B won't blab, it is in A's best interest to snitch, as he will then walk free – at B's expense.
The dilemma has obsessed economists for over 50 years because it helps to explain why individuals sometimes don't cooperate even when it is in their combined best interests to do so. Even climate change negotiations can be thought of as a prisoner's dilemma: no country wants to pay the cost of cutting emissions (keep shtum) if everyone else is going to keep on emitting (snitch).
The game becomes interesting when the same two partners play it over and over again. The way to minimise jail time under these conditions is usually to “play nice”: don't snitch, on the assumption that your partner won't either, and if they betray you then snitch on them in the next round, as a warning. So essentially, the best strategy is to collaborate.
Now, William Press at the University of Texas at Austin says he has uncovered a strategy to win that is not collaborative. And players who adopt his strategy end up spending much less time in prison than their opponents – in collaborative games, both players end up spending roughly equal time in prison.