Reviving, reproducing, and revisiting Axelrod’s second tournament

Abstract

Abstract Direct reciprocity, typically studied using the Iterated Prisoner’s Dilemma ( IPD ), is central to explaining cooperation. In the 1980s Robert Axelrod ran two computer tournaments in which Tit for Tat ( TFT ) emerged as the winner. Yet the archival record is incomplete: for the first tournament only a report survives, and for the second the submitted Fortran strategies remain but not the final tournament code. This gap raises questions about the reproducibility of these historically influential results. We recreate the second tournament by restoring the surviving Fortran implementations to compile with modern compilers and by building an interface that calls the original strategy functions without modification. We reproduce Axelrod’s main findings: TFT prevails, and successful play tends to be cooperative, responsive to defection, and willing to forgive. Strategy rankings remain mostly unchanged. We then enlarge the field with additional strategies and one of the largest IPD tournaments to date, showing the original setting favored TFT and that several lesser-known submissions perform strongly in more diverse settings and under noise.