Domain independence and stability in young and older adults’ discounting of delayed rewards

Abstract

Individual discounting rates for different types of delayed reward are typically assumed to reflect a single, underlying trait of impulsivity. Recently, we showed that discounting rates are orders of magnitude steeper for directly consumable liquid rewards than for monetary rewards (Jimura et al., 2009), raising the question of whether discounting rates for different types of reward covary at the individual level. Accordingly, the present study examined the relation between discounting of hypothetical money and real liquid rewards in young adults (Experiment 1) and older adults (Experiment 2). At the group level, young adults discounted monetary rewards more steeply than the older adults, but there was no significant age difference with respect to liquid rewards. At the individual level, the rates at which young and older participants discounted each reward type were stable over a two- to fifteen-week interval (rs > 70), but there was no significant correlation between the rates at which they discounted the two reward types. These results suggest that although similar decision-making processes may underlie the discounting of different types of rewards, the rates at which individuals discount money and directly consumable rewards may reflect separate, stable traits, rather than a single trait of impulsivity.

Introduction

Individuals often have to choose between alternatives that differ both in the amount of reward and in how soon it can be received. In such situations, the tendency for some individuals to steeply discount the value of delayed rewards has frequently been assumed to reflect an underlying trait of impulsivity (for a review, see Perry and Carroll, 2008). According to this view, individuals who steeply discount one type of reward (e.g., money) also would be expected to steeply discount rewards from other domains. Consistent with this view, substance abusers have been shown to discount both monetary and non-monetary rewards more steeply than controls (for a review, see Yi et al., 2010), consistent with the hypothesis of a unitary impulsivity trait that substance abusers possess to a greater degree than non-abusers. However, Chapman (1996) has reported that rates of discounting monetary and health rewards are uncorrelated, a finding she termed domain independence.

The present study revisits the issue of domain independence with respect to normal individuals discounting very different types of non-abused rewards. Recently, we showed that discounting rates are orders of magnitude steeper for directly consumable liquid rewards than for monetary rewards (Jimura et al., 2009), raising the question of whether discounting rates for different types of reward covary at the individual level. In the present study, we examine the correlation between individuals’ discounting of delayed monetary rewards and real liquid rewards. If individuals are characterized by a unitary trait of impulsivity then the degree to which individuals discount these two types of rewards should be highly correlated. If there is domain independence, however, then discounting of the two reward types should be uncorrelated.

Correlations between different measures are difficult to interpret, however, if the reliabilities of the separate measures are not known. Therefore, in addition to assessing the degree of domain independence, the current study examines the stability (i.e., test–retest reliability) of discounting rates for these two reward types. Individual rates of discounting delayed monetary rewards have been shown to be stable over test–retest intervals of up to 6 years (Audrain-McGovern et al., 2009, Kirby, 2009); however, the stability of individual discounting rates for directly consumable rewards is not known, and the present study provides estimates of test–retest reliability for both types of reward.

The present study also addresses the issue of domain independence at the group level. If two groups are compared, and Group A discounts a particular type of reward (e.g., money) more steeply than Group B, the unitary trait hypothesis predicts that, other things being equal, Group A will also discount other types of reward more steeply than Group B. Young adults have been reported to discount delayed monetary rewards more steeply than older adults (Green et al., 1994), but it is not known whether this finding generalizes to other types of rewards. Therefore, we compared the discounting of monetary and liquid rewards by young adults and older adults in order to determine whether young adults are generally steeper discounters than older adults and also to examine whether, within each group, individuals who discount one type of reward more steeply than their peers also discount the other type of reward more steeply.

Two experiments are reported: one with young adults (Experiment 1) and one with older adults (Experiment 2). In each experiment, two sessions were conducted at least two weeks apart, and in each session, participants made a series of choices between immediate and delayed monetary rewards as well as a series of choices between immediate and delayed liquid rewards. For each group, individual-level analyses examined the test–retest reliability of discounting rates for each reward type separately, as well as the correlations between discounting rates for the two reward types. Finally, group-level analyses compared the rates at which young and older adults discounted the two types of reward.