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 Oct 14, 2004 - 10:30 AM - by Michael
* Tit for Tat defeated

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In the world of AI programming, a set of cooperative algorithms have defeated "Tit for Tat" as the reigning strategy for winning the Prisoners' Dilemma.

Before Southampton came along, a strategy called Tit for Tat had a consistent record of winning the game. Under that strategy, a player's first move is always to cooperate with other players. Afterward, the player echoes whatever the other players do. The strategy is similar to the one nuclear powers adopted during the Cold War, each promising not to use its weaponry so long as the other side refrained from doing so as well.

The 20th-anniversary competition was the brainchild of Graham Kendall, a lecturer in the University of Nottingham's School of Computer Science and Information Technology and a researcher in game theory, and was based on the original 1984 competition run by a University of Michigan political scientist, Robert Axelrod.

The Iterated Prisoner's Dilemma is a version of the game in which the choice is repeated over and over again and in which the players can remember their previous moves, allowing them to evolve a cooperative strategy. The 2004 competition had 223 entries, with each player playing all the other players in a round robin setup. Because Axelrod's original competition was run twice, Kendall will run a second competition in April 2005, for which he hopes to attract even more entries.

Teams could submit multiple strategies, or players, and the Southampton team submitted 60 programs. These, Jennings explained, were all slight variations on a theme and were designed to execute a known series of five to 10 moves by which they could recognize each other. Once two Southampton players recognized each other, they were designed to immediately assume "master and slave" roles -- one would sacrifice itself so the other could win repeatedly.
The real-life applications are dubious - you can't simply seed a real world field with known self-sacrificing pawns - but it was an interesting solution to winning the competition.

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