Fifty-six heroin addicts and 60 age-matched controls were offered choices between monetary rewards ($11-$80) available immediately and larger rewards ($25-$85) available after delays ranging from 1 week to 6 months. Participants had a 1-in-6 chance of winning a reward that they chose on one randomly selected trial. Delay-discounting rates were estimated from the pattern of participants' choices. The discounting model of impulsiveness (Ainslie, 1975) implies that delay-discounting rates are positively correlated with impulsiveness. On average, heroin addicts' discount rates were twice those of controls (p = .004), and discount rates were positively correlated with impulsivity as measured by self-report questionnaires (p < .05). The results lend external validity to the delay-discounting rate as a measure of impulsiveness, a characteristic associated with substance abuse.

The independence of delay-discounting rate and monetary reward size was tested by offering subjects (N = 621) a series of choices between immediate rewards and larger, delayed rewards. In contrast to previous studies, in which hypothetical rewards have typically been employed, subjects in the present study were entered into a lottery in which they had a chance of actually receiving one of their choices. The delayed rewards were grouped into small ($30-$35), medium ($55-$65), and large amounts ($70-$85). Using a novel parameter estimation procedure, we estimated discounting rates for all three reward sizes for each subject on the basis of his/her pattern of choices. The data indicated that the discounting rate is a decreasing function of the size of the delayed reward (p < .0001), whether hyperbolic or exponential discounting functions are assumed. In addition, a reliable gender difference was found (p = .005), with males discounting at higher rates than females, on average.

I thank Jonathan Baron for thoughtful comments on the article, David Laibson for introducing me to second-bid auctions, and Todd Poret for conducting the auctions and assisting with data

These data suggest that discount rates vary with the preferred drug of abuse, and that high discount rates should be considered in the development of substance abuse prevention and treatment efforts.

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 457 © 2009 The Psychonomic Society, Inc. BRIEF REPORTSOne-year temporal stability of delay-discount rates KRIS N. KIRBY Williams College, Williamstown, MassachusettsThe 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 r...