The Effect of Symbolic, Linguistic Information on the Discounting of Delayed, Real Liquid Rewards

Povich, M., Lee, S., Hoopes, R., Myerson, J., & Green, L.

Delay discounting refers to the decrease in a reward’s subjective value as the time to its receipt increases. Unlike nonhuman animals, humans display a magnitude effect: Larger, delayed rewards are discounted less steeply than smaller, delayed rewards. To be noted, however, are differences in the ways that discounting experiments are conducted with humans and animals. Notably, human discounting experiments typically involve hypothetical, monetary rewards, and their amounts and delays are stated explicitly. In contrast, in animal discounting experiments, amounts and delays are experienced directly, and the outcome is a primary, consumable reinforcer. Thus, differences between humans and animals in the effect of amount on degree of discounting may be due to these experimental differences, rather than species differences. To evaluate this possibility, Experiment 1 studied the effect of symbolic, linguistic information on the degree to which humans discount the value of delayed, real liquid rewards. Thirty-two human participants were studied in two experimental groups: In one group, the participants were provided with symbolic, linguistic information as to the amount of (9.6 and 28.8 ml) and delay to (15 and 60 seconds) the liquid reward, and in the other group, no such information was presented. A computer-run, adjusting-amount procedure was used to estimate the relative subjective values of the rewards. A magnitude effect was observed in the symbolic group, but there was no differential effect of amount in the non-symbolic group. In addition, there was a significant effect of delay on degree of discounting in the symbolic, but not in the non-symbolic, group. Experiment 2 was conducted as a systematic replication of Experiment 1 in which a greater difference in the delays to the liquid reinforcer were studied (5 and 60 seconds). Consistent with the findings of Experiment 1, effects of amount and of delay were observed in the symbolic group but not in the non-symbolic group of Experiment 2. Taken together, these results are consistent with the hypothesis that differences in decision-making between human and nonhuman animals are due to differences in procedure, rather than in species.