I should point out that I've already been into the answer to this in some detail in the "others" section so if you don't want it spoiled, stay away from there!
Two Masters of Logic sit down for a friendly or not so friendly game of Iterated Prisoners' Dilemma. The rules of the game are quite simple:
Each player has 2 cards, marked "COOPERATE" or "DEFECT".
In each round of the game, each player chooses one card and plays it face down, then both cards are revealed.
If both players played "