Cardiff academic recreates Robert Axelrod’s highly influential Prisoner’s Dilemma tournament
27 November 2025
Professor Vincent Knight’s preprint summarises the first ever systematic reproduction of historic computer science tournament
The Prisoner’s Dilemma is a model of direct reciprocity in game theory, with applications across sociology, political science, economics, biology and international relations, among others.
In 1980, Professor Robert Axelrod of the University of Michigan staged two tournaments where strategies for the Prisoner’s Dilemma were played against each other.
The key finding from these tournaments was that the strategy ‘Tit for Tat’ performed best. Axelrod’s subsequent book The Evolution of Cooperation (1984) has been highly influential, having been cited tens of thousands of times.
However, Professor Knight’s reproduced preprint concludes that generalising from Axelrod’s original tournament is perhaps a mistake.
Professor Knight said: “By reviving the remaining code from the original tournament we were able reproduce the main conclusions: Tit for Tat again performed best, and the same core principles of cooperation emerged. However, the exact results do not reproduce which is a common problem in computational scientific research.”
He added: “We proceeded to run the largest tournament ever run with all strategies available in the open-source Python library named after Robert Axelrod. Unsurprisingly, Tit For Tat did not win. In fact, the highest performer from Axelrod’s second tournament ranked only 16th in our results. Almost all of the top fifteen strategies were sophisticated ones trained using reinforcement learning techniques, having learned to behave in contextually beneficial ways. This highlights the robustness of the original tournament: the original conclusions do not necessarily generalise to any environment. In fact our result shows that sophisticated behavior can be beneficial which is often a point overlooked based on Axelrod’s original work.”
Professor Knight worked on the tournament and preprint with Owen Campbell, Marc Harper, T.J. Gaffney and Nikoleta Glynatsi.
