Certainty Independence and the Separation of Utility and Beliefs

Economists often operate under an implicit assumption that the tastes of a decision maker are constant, while his beliefs change with the availability of new information. It is therefore customary to seek representations of preferences which cleanly separate the taste component, called ‘utility,’ from the beliefs component. We show that a complete separation of utility from the other components of the representation is possible only if the decision maker’s preferences satisfy a mild but not completely innocuous condition, called ‘certainty independence.’ We prove that the preferences that obtain such separation are a subset of the biseparable preferences.nonatomic probability measures, we extend some of these results to the case of individuals with decreasing marginal evaluations.