Abstract
Studies of delay discounting typically have involved choices between smaller, immediate outcomes and larger, delayed outcomes. In a study of delay discounting in humans, Green et al. (2005) added a period of time prior to both outcomes, creating a delay common to both. They found that the subjective value of the more delayed reward was well described by a hyperboloid discounting function and that the degree to which that outcome was discounted decreased as the common delay increased. In two experiments, we examined the effect of adding a common delay on the discounting of food rewards in pigeons. In Experiment 1, an adjusting-amount procedure was used to establish discounting functions when the common delay was 0, 3, 5, and 10 s, and different stimuli signaled time to the smaller, sooner and larger, later rewards. In contrast to humans, the pigeons showed increases in the degree of discounting when a common delay was added. In Experiment 2, the delay common to both rewards and the delay unique to the larger, later reward were each specifically signaled. With this procedure, the degree of discounting decreased as the common delay increased, a result consistent with that obtained with humans (Green et al., 2005). These findings reveal fundamental similarities between pigeons’ and humans’ choice behavior, and provide strong interspecies support for the hypothesis that choice between delayed outcomes is based on comparison of their hyperbolically discounted present subjective values.
Highlights
People and other animals often have to choose between an immediate reward and another, larger reward of the same kind that is available only after a delay
This difference in preference is assumed to reflect the fact that the value of a delayed reward is discounted, with longer delays leading to greater discounting, and it is observed in both humans (Green et al, 1994; Kirby, 1997) and non-human species
When the common delay was increased, pigeons discounted the value of the larger, later reward much more steeply. This finding is clearly inconsistent with the predictions of Eq 3 and opposite to what has been observed when human subjects discount delayed hypothetical monetary rewards (Green et al, 2005); for humans, the degree of discounting decreased as the common delay increases
Summary
People and other animals often have to choose between an immediate reward and another, larger reward of the same kind that is available only after a delay. When the delay to the later reward is long, a small amount of immediate reward may be chosen over the delayed reward, but if the delay is brief, the choice may be to wait for the larger reward This difference in preference is assumed to reflect the fact that the value of a delayed reward is discounted, with longer delays leading to greater discounting, and it is observed in both humans (Green et al, 1994; Kirby, 1997) and non-human species (rat and pigeon, Richards et al, 1997; Mazur, 2000; Green et al, 2004; monkey, Freeman et al, 2009). Choice of the larger reward is more likely if the decision is made at an earlier point in time (e.g., at T1), whereas choice of the smaller reward is more likely if the decision is made later (e.g., at T2) Both humans and non-human animals show the preference reversals predicted by Figure 1 (e.g., Ainslie and Herrnstein, 1981; Green et al, 1981, 1994)
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