The temporal stability of delay-discount rates for monetary rewards was assessed using a monetary choice questionnaire (Kirby & Marakovic, 1996). Of 100 undergraduate participants who completed the questionnaire at the initial session, 81 returned 5 weeks later and 46 returned 57 weeks later for subsequent sessions. The 5-week test-retest stability of discount rates was .77 (95% confidence interval = .67-.85), the 1-year stability was .71 (.50-.84), and the 57-week stability was .63 (.41-.77). Thus, at least when similar testing situations are reinstated, discount rates as individual differences have 1-year stabilities in the range that is typically obtained for personality traits. Discount rates index an attribute of the person that is relatively stable over time but that is moderated by aspects of the situation, such as reward type and deprivational state.
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People differ in how they value the future consequences of their actions. Some people value future consequences very highly, which, according to the discounting model of impulsive choice (Ainslie, 1975; Rachlin, 2000), tends to make them more self-controlled. Others appear to place little value on future consequences, which tends to make them more impulsive. The primary question addressed in this article is whether such individual differences in the way we value future consequences are stable over time.
Delay discounting refers to the decrease in the present value of a future outcome as the delay to that outcome increases. That is, future outcomes are discounted, in part, because of their delays. (Because the research below used only positive outcomes, I will henceforth refer to outcomes as rewards.) Figure 1 illustrates discount curves for two hypothetical individuals. Time moves left to right, and, thus, the delays to future rewards increase from right to left. The height of the vertical line at delay = 0 indicates the present value of a reward of amount A when it is available immediately. The curves show how the present value of the reward declines as the delay to the reward increases. The person represented by the solid curve has a lower discount rate than does the person represented by the dashed curve and, thus, tends to value future rewards more highly.
Previous research (for a review, see Green & Myerson, 2004) has shown that discount curves are well fit by the hyperbolic function (Mazur, 1987)
where V is the present value of reward A available at delay D, and k is the discount rate parameter. Although k determines the rate of discounting, it does not correspond to a fixed proportional rate of decrease. Thus, to get an intuitive sense of the value of k, it can be helpful to convert it to a half-life-that is, the delay at which the delayed reward has a present value equal to half its full amount (Kirby & Marakovic, 1996). For Equation 1, the half-life is equal to the inverse of k. For example, the hypothetical person represented by the dashed curve in Figure 1 has k = 0.02, and, thus, the reward A has half of its value when delayed by 50 days. In contrast, the hypothetical person represented by the solid curve in Figure 1 has k = 0.01, and so reward A has half of its value when delayed by 100 days.
According to the discounting model of impulsive choice (Ainslie, 1975; Rachlin, 2000), as discount rates increase, people are increasingly inclined to choose more immediate over more delayed rewards. Thus, k serves as an index of impulsivity and has acquired respectable external and criterion validity as such an index. Estimates of k correlate positively with self-reported impulsivity (Kirby & Finch, 2008; Kirby & Petry, 2004) and behaviors such as impulse buying (Dittmar, 2001). They correlate negatively with college grade point average (Kirby, Winston, & Santiesteban, 2005) and adolescent academic performance, which they predict better than does IQ (Duckworth & Seligman, 2005). …