Playing Against the Machine: Cooperation, Communication, and Strategy Heterogeneity in Repeated Prisoner's Dilemma

Abstract

We study whether communication sustains cooperation when the partner is an artificial intelligence. In a laboratory experiment (n = 234), human participants play indefinitely repeated Prisoner's Dilemma games against an AI chatbot (GPT-5.2) under one of three conditions: no communication, free-form chat before the first round of each supergame, or free-form chat before every round. A fourth treatment replaces the AI with another participant under the every-round protocol. Communication is the main determinant of cooperation with the AI, and a single exchange is enough: pre-play chat raises cooperation from 49% to 84%, while chatting before every round adds less than one percentage point, in contrast to the timing effect found between humans. Once communication is available, cooperation with the AI is no lower than with a human partner, so we find no evidence of the 'machine penalty' documented in the psychology literature. The partner's identity nonetheless shifts strategies: under identical communication, participants tend to reciprocate against a human but to cooperate unconditionally against the AI. Participants also keep their promises to cooperate while carrying out their threats to punish only about half the time. The AI opens cooperatively and rarely defects first, so these differences originate on the human side.