Lessons from the Prisoner's Dilemma

1: Introducing the Game

The Prisoner's Dilemma is a game where you and the other player simultaneously choose either to Co-operate (C) or Defect (D). This gives four possible outcomes, each with a different payoff, as shown in the table (the small, superscripted numbers are your opponent's payoffs).

• Nobody wants to get the sucker payoff (0pts); to have their co-operation exploited by the other player.
• If you both defect, neither of you is a sucker but neither of you gains from co-operation (1pt).
• If you both co-operate, you both gain a reasonable amount (3pts). This option is best for the common good, but not for you.
• However, it's better for you still if you defect and make the other player the sucker: this gets you the 5 point Temptation.

The goal of the game is to maximise the number of points you win. If you aim to beat the other player, or aim to maximise the total number of points won, it becomes effectively a different game. The Prisoner's Dilemma is most interesting when it's a game between egotists. The 3 point payoff of co-operation may be nice, but don't those 5 points in the bottom-left of the table look very tempting?

To model situations in which people have repeated opportunities to co-operate with each other, we can play the game a number of times in succession; let's say 20 rounds. From the payoff table, it should be clear that the highest score you can get in twenty rounds of this game is 100 points, but unless the other player is a total and utter sucker that's just not going to happen.

On the next page, you'll play 20 rounds of the game against an opponent who plays randomly.

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