What discount rate should be applied to social investments? It is standard to use the market interest rate, thereby respecting private preferences. We show that application of this “revealed preference” criterion rests on faith, not on logic. It is justified only if preferences over all choices, including past choices, are time invariant. The conditions for this to be true are stringent. Under more reasonable conditions, policy makers should be more patient than private citizens, whose choices define the most short‐sighted Pareto optimum.