Reward contrast in delay and probability discounting

In this study, we examined whether reward contrast influences choice between delayed and probabilistic outcomes. Specifically, we predicted that the subjective value of an intermediate reward would seem relatively larger or smaller, respectively, if it followed choices involving a smaller or larger reward and would produce corresponding changes in rates of delay and probability discounting. In Experiment 1, subjects made choices about hypothetical $5,000 or $50 outcomes and then made choices about $500 outcomes. Delay-discounting rates for the $500 outcome were larger for Group $5,000 than for Group $50, whereas the opposite result was obtained for probability-discounting rates. In Experiment 2, we used a design that allowed for contrast effects to be assessed within subjects. Two groups made choices about delayed or probabilistic rewards. After completing question blocks in which the amount was $5,000 or $50, subjects responded to questions with an intermediate amount ($475/$525). For Group Delay, the present value of the intermediate reward was greater after the $50 block than after the $5,000 block, whereas the opposite was obtained for Group Probability. The results from both experiments confirmed the predictions of reward contrast and suggested that the subjective value of a monetary reward varies inversely with the prior reward amount.