<p>Why do we form moral judgments? One influential answer is that the human capacity for moral judgment is an evolutionary adaptation. P. Kyle Stanford argues that morality is adaptive because it leads individuals to externalize norms, prompting them to evaluate both their own behavior and that of potential partners by a single standard. In this paper, we examine Stanford’s hypothesis using a game-theoretic model. Our results support and extend his proposal, demonstrating that – under the assumptions chosen – externalization is a plausible driver of the evolution of morality as it fosters the learning of cooperation in challenging novel contexts. We find that externalization allows agents to avoid the <i>exploitation trap</i>: by resisting unsustainable short-term gains through exploitation, externalizing agents secure the benefits of robust long-term cooperation. By modeling externalization as a constraint on strategy and partner choice, we avoid the common pitfall of game-theoretic approaches to represent morality as a mere disposition toward altruistic behavior.</p>