Courses/CS 461/Museum of unintended consequences/Meta signals in prisoner's dilemma

From CSWiki

From an article in Wired.com.

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 … 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.

If the program recognized that another player was not a Southampton entry, it would immediately defect to act as a spoiler for the non-Southampton player. The result is that Southampton had the top three performers -- but also a load of utter failures at the bottom of the table who sacrificed themselves for the good of the team.