Discounting of delayed rewards: (A)theoretical interpretation of the Kirby questionnaire

111 Participants, recruited from Amazon's MTurk worker pool, completed Kirby's (2009) monetary choice questionnaire, which involves choosing between immediate, but smaller rewards and delayed, but larger rewards. Individual participants' responses were scored in three ways: first, by calculating the proportion of choices of the delayed rewards; second; using the scoring procedure described by Kirby et al. (1999) to estimate discounting rate (i.e., the value of the k-parameter in a hyperbolic discounting function); and third, using logistic regression to estimate discounting rate (Wileyto et al., 2004). Individuals' scores calculated using the proportion measure and the logarithms of their estimated k values were very strongly correlated (rs > .97). In addition, the proportions of choices of small, medium, and large amounts of the delayed rewards were strongly correlated (rs > .80). Taken together, these results suggest that the relative ease of calculating the proportion measure does not require sacrificing reliability. Moreover, the proportion measure is atheoretical and very easy to calculate whereas estimating an individual's discounting rate requires assuming a theoretical model that may not be appropriate. Significant differences in the proportion of delayed reward choices were observed between the small, medium, and large delayed reward amounts, with smaller rewards being discounted more steeply than larger ones, replicating previous findings of magnitude effects. These results provide further validation of the proportion of delayed reward choices on the Kirby questionnaire as a measure of individual and group differences in discounting.