EXPONENTIAL VERSUS HYPERBOLIC DISCOUNTING OF DELAYED OUTCOMES - RISK AND WAITING TIME

Frequently, animals must choose between more immediate, smaller reward s and more delayed, but larger rewards. For example, they often must d ecide between accepting a smaller prey item versus continuing to searc h for a larger one, or between entering a leaner patch versus travelli ng to a richer patch that is further away. In both situations, choice of the more immediate, but smaller reward may be interpreted as implyi ng that the value of the later reward is discounted; that is, the valu e of the later reward decreases as the delay to its receipt increases. This decrease in value may occur because of the increased risk involv ed in waiting for rewards, or because of the decreased rate of reward associated with increased waiting time. The present research attempts to determine the form of the relation between value and delay, and exa mines implications of this relation for mechanisms underlying risk-sen sitive foraging. Two accounts of the relation between value and delay have been proposed to describe the decrease in value resulting from in creases in delay: an exponential model and a hyperbolic model. Our res earch demonstrates that, of the two, a hyperbola-like discounting mode l consistently explains more of the variance in temporal discounting d ata at the group level and, importantly, at the individual level as we ll. We show mathematically that the hyperbolic model shares fundamenta l features with models of prey and patch choice. In addition, the pres ent review highlights the implications of a psychological perspective for the behavioral biology of risk-sensitive foraging, as well as the implications of an ecological perspective for the behavioral psycholog y of risk-sensitive choice and decision-making.