Relative concave utility for risk and ambiguity

This paper presents a general technique for comparing the concavity of different utility functions when probabilities need not be known. It generalizes: (a) Yaari's comparisons of risk aversion by not requiring identical beliefs; (b) Kreps and Porteus' information-timing preference by not requiring known probabilities; (c) Klibanoff, Marinacci, and Mukerji's smooth ambiguity aversion by not using subjective probabilities (which are not directly observable) and by not committing to (violations of) dynamic decision principles; (d) comparative smooth ambiguity aversion by not requiring identical second-order subjective probabilities. Our technique completely isolates the empirical meaning of utility. It thus sheds new light on the descriptive appropriateness of utility to model risk and ambiguity attitudes.