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 change 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, the authors estimated discounting rates for all three reward sizes for each subject on the basis of his/her pattern of choice. The data indicated that the discounting rate is a decreasing function of the size of the delayed reward (p < .0001), whether hyperbolic or exponential functions are assumed. In addition, a reliable gender difference was found (p = .005), with males discounting at higher rates than females, on average.
Abstract