Interest in the iterated prisoner's dilemma (IPD) was kindled by Robert Axelrod in his book "The Evolution of Cooperation" (1984). In it he reports on a tournament he organized of the "N" step prisoner's dilemma (with "N" fixed) in which participants have to choose their mutual strategy again and again, and have memory of their previous encounters. Axelrod invited academic colleagues all over the world to devise computer strategies to compete in an IPD tournament. The programs that were entered varied widely in algorithmic complexity, initial hostility, capacity for forgiveness, and so forth.