Agreeing to Disagree Politically

Aumann's agreement theorem and subsequent work shows that people who are rational in a certain Bayesian sense cannot agree to disagree on matters of fact. This result hinges on a type of epistemic impartiality: a rational person will not give extra weight to a piece of evidence simply because they themselves discovered it rather than someone else. We extend this argument by questioning the possibility of reasonable political disagreement for political liberals. On our reading, a “Good Liberal” must not give extra weight in public deliberations to their own preferences or values simply because they are their own. This political impartiality mirrors the epistemic impartiality of Aumann's theorem and we argue that disagreement on policy is impossible in a world of “Good Liberal Bayesians,” assuming common knowledge of both Bayesian rationality and Liberal reasonableness. The persistence and predictability of disagreement in the real world provides support for expressive accounts of political behaviour and points to the important role of epistemic trust in politics. This issue of epistemic trust provides insight into recent trends in political polarization in the United States and elsewhere.