The Altruistic Strategy of Early Christians

In 1980, game theorist Robert Axelrod ran a famous Iterated Prisoner’s Dilemma Tournament.

He asked other game theorists to send in their best strategies in the form of “bots”, short pieces of code that took an opponent’s actions as input and returned one of the classic Prisoner’s Dilemma outputs of COOPERATE or DEFECT. For example, you might have a bot that COOPERATES a random 80% of the time, but DEFECTS against another bot that plays DEFECT more than 20% of the time, except on the last round, where it always DEFECTS, or if its opponent plays DEFECT in response to COOPERATE.

In the “tournament”, each bot “encountered” other bots at random for a hundred rounds of Prisoners’ Dilemma; after all the bots had finished their matches, the strategy with the highest total utility won.

To everyone’s surprise, the winner was a super-simple strategy called TIT-FOR-TAT:

https://readscottalexander.com/posts/acx-the-early-christian-strategy