In the simple game there are only two possible strategies, cooperate and defect. Iteration, however, allows lots of conceivable strategies, and it is by no means obvious which one is best. The following, for instance, is just one among thousands: ‘cooperate most of the time, but on a random 10 per cent of rounds throw in a defect’. Or strategies might be conditional upon the past history of the game. My ‘grudger’ is an example of this;
