Draft:Axelrod's Tournaments

Axelrod's tournaments were a set of computer tournaments in the iterated Prisoner's Dilemma in which players (or "strategies") would play each other strategy, and a copy of itself repeatedly. The strategies were submitted by well-known game theorists that were invited to do so by political scientist Robert Axelrod.

Tournament process

The iterated prisoner's dilemma in the first tournament worked in a payoff system:

	Player 1 cooperates	Player 1 defects
Player 2 cooperates	3,3	5,0
Player 2 defects	0,5	1,1

Strategies

Many strategies were submitted. When the tournament was run, the winning player was the Tit for Tat (TFT) strategy, which cooperated on the first move and subsequently matched the opponent's move on the last round; it was comparatively simple but produced the highest result against a variety of far more complicated programs.

Some of the other strategies included:

• Go-by-majority, which saved every past round and looked at whether the opponent had defected or cooperated a majority of times. If the opponent had defected a majority of rounds, the program responded by defecting, and vice versa.
• Poor-trusting-fool, which unconditionally cooperated.
• All-defect, which unconditionally defected.
• Random, which randomly defected or cooperated with a 50% chance of either. In the original tournament, this placed at the bottom.
• Tit for two tats, a variation on Tit-for-tat that cooperated unless the opponent had defected in the previous two matches.

Conclusion

In his book The Evolution of Cooperation, Axelrod details four requirements for a successful strategy; that it not be the first to defect, that it is provokable, that it is not envious of the other player's success, and that it is not too complex so that the opponent cannot understand it.