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

TL;DR

This study examines how human interactions with AI in repeated Prisoner's Dilemma games influence cooperation, revealing high initial cooperation with AI that plateaus and differs from human-human dynamics, with implications for trust and strategy development.

Contribution

It provides novel insights into human-AI cooperation dynamics, highlighting differences in strategy and communication compared to human-human interactions in repeated games.

Findings

Cooperation against AI is initially high but plateaus below full cooperation.

Repeated communication increases cooperation in human-human, but not human-AI settings.

Humans favor Grim Trigger with AI, unlike Tit-for-Tat in human interactions.

Abstract

This paper investigates how natural language communication with an AI agent affects human cooperative behaviour in indefinitely repeated Prisoner's Dilemma games. We conduct a laboratory experiment (n = 126) with two between-subjects treatments varying whether human participants chat with an AI chatbot (GPT-5.2) before every round or only before the first round of each supergame, and benchmark against human-human data from Dvorak and Fehrler (2024) (n = 108). We find four main results. First, cooperation against the AI is high and initially comparable to human-human levels, but unlike in the human-human setting, where cooperation converges to near-complete levels, cooperation against the AI plateaus and never reaches full cooperation. Second, repeated communication, which substantially increases cooperation in human-human interactions, has no detectable effect in the human-AI setting. Third, strategy estimation reveals that human-AI subjects favour Grim Trigger under pre-play communication and remain dispersed under repeated communication, whereas human-human subjects converge to Tit-for-Tat and unconditional cooperation respectively. Fourth, human-AI conversations contain more explicit strategy commitments but fewer emotional and social messages. These results suggest that humans cooperate with AI at high rates but do not develop the trust observed in human-human interactions. Cooperation in the human-AI setting is sustained through conditional rules rather than through the social bonds and mutual understanding that characterise human-human cooperation.