When rats, pigeons, and people choose between immediate and delayed rewards, the subjective value of the delayed reward decreases as time to its receipt increases. This discounting of the delayed reward is well described in all three species by a hyperboloid function. Interestingly, we have observed a magnitude effect (larger delayed rewards are discounted less steeply than smaller delayed rewards) with humans but not with rats or pigeons. In addition, in humans, if an additional waiting period is added prior to both rewards, thus creating a delay common to both alternatives, rate of discounting decreases as the common delay increases. We examined the effect of adding a common delay on discounting in pigeons. When the signals for the time to the sooner and later alternatives were different, the pigeons (in contrast to humans) showed increases in discounting rate with increases in the common delay. When the signal for the common delay was the same for both alternatives, however, rate of discounting decreased as the common delay increased, a result consistent with that obtained with humans. Taken together, our findings demonstrate profound similarities between delay discounting in humans and pigeons, arguing for the importance of conducting both human and nonhuman research.

Review Leonard Green’s biographical statement.