Journal
JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR
Volume 81, Issue 1, Pages 39-50Publisher
SOC EXP ANALYSIS BEHAVIOR INC
DOI: 10.1901/jeab.2004.81-39
Keywords
discounting; delay; magnitude effect; adjusting-amount; choice; key peck; lever press; pigeons; rats
Funding
- NATIONAL INSTITUTE OF MENTAL HEALTH [R01MH055308] Funding Source: NIH RePORTER
- NIMH NIH HHS [R01 MH055308, MH55308] Funding Source: Medline
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Temporal discounting refers to the decrease in the present, subjective value of a reward as the time to its receipt increases. Results from humans have shown that a hyperbola-like function describes the form of the discounting function when choices involve hypothetical monetary rewards. In addition, magnitude effects have been reported in which smaller reward amounts are discounted more steeply than larger amounts. The present research examines the cross-species generality of these findings using real rewards, namely food pellets, with both pigeons and rats. As with humans, an adjusting amount procedure was used to estimate the amount of immediate reward judged equal in value to a delayed reward. Different amounts of delayed food rewards (ranging from 5 to 32 pellets in pigeons and from 5 to 20 pellets in rats) were studied at delays,varying from I s to 32 s. A simple hyperbola, similar to the hyperbola-like mathematical function that describes the discounting of hypothetical monetary rewards in humans, described the discounting of food rewards in both pigeons and rats. These results extend the generality of the mathematical model of discounting. Rates of discounting delayed food rewards were higher for pigeons than for rats. Unlike humans, however, neither pigeons nor rats showed a reliable magnitude effect: Rate of discounting did not vary systematically as a function of the amount of the delayed reward.
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