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

A laboratory experiment involving 234 subjects explores the impact of communication on cooperative behavior when humans play the infinitely repeated prisoner's dilemma against the AI chatbot GPT-5.2. The experiment sets up three AI opponent conditions: no communication, free chat before each supergame, and free chat before each round, plus a human opponent control group with chat before each round. A single pre-game communication increases the cooperation rate with AI from 49% to 84%, while chat before each round only adds less than 1 percentage point. With communication, the cooperation rate with AI is no lower than that with humans, and no "machine punishment" effect mentioned in psychological literature is observed.