Robust Social Planning

This paper analyzes a society composed of individuals who have diverse sets of beliefs (or models) and diverse tastes (or utility functions). It characterizes the model selection process of a social planner who wishes to aggregate individuals' beliefs and tastes but is concerned that their beliefs are misspecified (or distorted). A novel impossibility result emerges: a utilitarian social planner who seeks robustness to misspecification never aggregates individuals' beliefs but instead behaves systematically as a dictator by selecting a single individual's belief. This tension between robustness and aggregation exists because aggregation yields policy-contingent beliefs, which are very sensitive to policy outcomes. Restoring possibility of belief aggregation requires individuals to have heterogeneous tastes and some common beliefs. This analysis reveals that misspecification has significant economic implications for welfare aggregation. These implications are illustrated in treatment choice, asset pricing, and dynamic macroeconomics.