How many impulsivities? A discounting perspective

Abstract

People discount the value of delayed and uncertain outcomes, and how steeply individuals discount is thought to reflect how impulsive they are. From this perspective, steep discounting of delayed outcomes (which fails to maximize long-term welfare) and shallow discounting of probabilistic outcomes (which fails to adequately take risk into account) reflect the same trait of impulsivity. Despite the fact that a hyperboloid function describes the discounting of both delayed and probabilistic outcomes, there is considerable evidence that the two kinds of discounting involve different processes as well as separate impulsivity traits. Several manipulations differentially affect delay and probability discounting, and correlational studies show that how steeply one discounts delayed rewards is relatively independent of how steeply one discounts probabilistic rewards. Moreover, people's discounting of delayed money and health outcomes are uncorrelated as are discounting of real, consumable rewards and hypothetical money. These results suggest that even within delay discounting, there may be multiple 'impulsivities,' each of which may be important for understanding a different aspect of decision making. Taken together, the pattern of findings reviewed here argues for a more nuanced view of impulsivity than that which is usually assumed in discounting research.

Fig. 1

Relative subjective value as a function of the delay until (upper panel) and the odds against (lower panel) receiving a smaller ($200) and a larger ($40,000) reward. Curves represent the best-fitting hyperboloid discounting functions (Equation 1). Data are from Experiments 1 and 2 of “Differential Effects of Amount on Temporal and Probability Discounting of Gains and Losses,” by S. J. Estle, L. Green, J. Myerson, and D. D. Holt, Memory & Cognition, 34, p. 926. Copyright 2006 by the Psychonomic Society, Inc. Reprinted with permission.

Fig. 2

Probability discounting as a function of delay discounting. Each data point represents how steeply a particular individual drawn from a hypothetical population distribution discounts delayed and probabilistic rewards. The solid squares represent a group of individuals who steeply discount both delayed and probabilistic rewards, reflecting both impatience and risk aversion, respectively; the solid circles represent a group of individuals who steeply discount delayed rewards but shallowly discount probabilistic rewards, reflecting impatience and risk taking, respectively.

Fig. 3

Relative subjective value as a function of the delay until (left panel) and the odds against (right panel) receiving different types of rewards. Curves represent the best-fitting hyperboloid discounting functions (Equation 1). Data are from the large amount condition of “Discounting of Monetary and Directly Consumable Rewards,” by S. J. Estle, L. Green, J. Myerson, and D. D. Holt, Psychological Science, 18, p. 60. Copyright 2007 by Blackwell Publishers, Ltd. Reprinted with permission.