Temporal Discounting of Hypothetical and Real Extra Credit Points Makenzie D. Williams, Heidi L. Dempsey, & David W. Dempsey Jacksonville State University Research on temporal discounting has primarily used hypothetical monetary rewards. For the college student population, course points serve as a powerful generalized reinforcer, much like money does for the general population. To date, few studies have examined the degree to which students discount points within a course. Silva and Gross (2004, Study 2) found that higher performing students were less likely to discount extra credit points than middle or lower performing students. However, this study focused on the effort to earn the reinforcer, not on the delay of the reinforcer. Williams and colleagues (2007) developed a computerized procedure to examine the rate at which students discounted extra credit points. They found that the majority of students did not discount course points and those who did showed rather shallow discounting curves. They posited that these results may have been due to a magnitude effect (10 course points for a maximum of 55 minutes) or too short of delay periods (ranging from 1 minute to 10 minutes). Further, participants might have been more willing to engage in self-control behaviors on the longer time delays due to prior commitment on the shorter time delays. The current study was designed to address some of the aforementioned limitations in Williams et al. (2007) by lengthening the delays, counterbalancing the presentation order, and looking for evidence of a magnitude effect. Method Participants 16 participants (4 males and 12 females) Each were enrolled in an undergraduate general psychology class Each earned a varying amount of extra credit, not exceeding 10 course points for participation Procedure and Materials Students were informed that the researchers were interested in the amount of time they would be willing to “study” (operationally defined as reading a specified psychology textbook out loud) in order to earn varying amounts of extra credit points. Next, students were instructed that they would make 18 choices (or 21 for a subset of 6 participants) representing different time-point combinations. After they made all of their choices, the researcher would pick 2 to 6 of these and have the student study for the amount of time s/he chose on each. After they finished studying, they would receive the corresponding amount of extra credit points. For each trial students were presented with choices in the form of a ruler with a continuum of point-time pairs. Each ruler had one of three maximum point values [0.5 (small magnitude), 2 (medium magnitude), or 4 points (large magnitude)] and one of 6 or 7 maximum delays (10, 20, 30, 40, 50, 60, and 90 minutes—the 90 minute delays were only included for 6 participants). These combinations of 3 point levels and 6 or 7 delays resulted in 18 or 21 discrete trials. Point-time combinations within each ruler began with 10% of the maximum points awarded “now” and ranged over 10 equally-spaced sub-intervals to the maximum amounts of points for the maximum time delay. At each time position, a corresponding proportional number of points was indicated. To try and control for a high- p sequence effect the point/delay choices were varied from trial to trial. Figure 1 – Median Indifference Points for 4-point, 2-point, and 0.5-point Rulers for the 60-minute and 90-minute delays The tables below represent the R 2 values for each of the point-values. In all cases, the Rachlin (2006) power function provided the best fit to the data. Furthermore, a magnitude effect was observed where the smallest amount of extra credit (0.5 points) was discounted more steeply than the higher amounts of extra credit (2 and 4 points). Discussion Overall, these data are broadly consistent with previous discounting research using hypothetical rewards and delays. However, the procedure used was not like the procedures typically used to examine the discounting function. One key difference was the fact that participants actually experienced real delays and often received real rewards. The second key difference was the use of course points as the delayed reward. The results showed, under these circumstances, Rachlin’s (2006) power function was the best fit for the data. Furthermore, a magnitude effect was found, where participants were less likely to discount higher rewards (Johnson & Bickel, 2002; Myerson & Green, 2004). Although Rachlin’s (2006) model proved to be the best fit for these data it was still not an exceptionally good fit. It may be the case that the discounting of course points may not be well represented by the current hyperbolic models that are used. When looking at the discounting of extra credit points and using real short-time delays (e.g., in minutes instead of months or years) it may be that the current models need to be modified. That is, when using short amounts of time, very little discounting is observed, but when the delays began to stretch into 60 and 90 minutes, the curves start to look more like a typical discounting curve. However, the problem with extending the delays even further so that we might see the typical hyperbolic-like curve is that the type of delays represented here are more typical of delays related to studying (that is, the majority of students study in small blocks, not segments greater than 90 minutes). One further issue to be considered is whether students discount course points in the same manner as they discount other commodities such as hypothetical monetary rewards. Thus our future research will center on examining whether discounting may be adequately represented by the current mathematical models when there are shorter time delays and high-value, real reinforcers such as course points. 60-minute delays 90-minute delays 4-point ruler 60-minute90-minute Mazur (1987) Myerson & Green (1995) Rachlin (2006) Exponential point ruler 60-minute90-minute Mazur (1987) Myerson & Green (1995) Rachlin (2006) Exponential point ruler 60-minute90-minute Mazur (1987) Myerson & Green (1995) Rachlin (2006) Exponential Results Of the 16 participants, 2 did not discount at all and 14 engaged in at least some degree of discounting; only the 14 who engaged in some degree of discounting are included in further analyses. In the graphs listed to the right, the median indifference points are plotted as well as curves corresponding to the Mazur (1987) hyperbolic discounting function, the Myerson & Green (1995) hyperbola-like discounting function, Rachlin (2006) power function, and a standard exponential function.