In the study of decision making, researchers typically compare the observed pattern of choice with a normative one, meaning the one that people would adopt if they were “rational.” The normative model for decision making under risk is expected utility theory, and for intertemporal choice it is*discounted utility*(DU) theory. In a simplified form, DU theory holds that a rational decision maker will discount the costs or benefits from all delayed events by a constant rate per unit of time (Samuelson 1937; Strotz 1955). This constant rate is analogous to a psychological interest rate, and the same equations are