Green, L., Myerson, J., Holt, D. D., Slevin, J. R., & Estle, S.
J. (2004).
Discounting of delayed food rewards in pigeons and rats: Is there a
magnitude effect?
Journal of Experimental Analysis of Behavior,
81, 39-50.
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
1 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.
Key words: discounting, delay, magnitude effect, adjusting-amount,
choice, key peck, lever press, pigeons, rats