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Background: Impulsive choice can be defined as the selection of small immediate rewards rather than larger delayed rewards, or a predisposition to risk taking. A multiplicative hyperbolic model (Ho, Mobini, Chiang, Bradshaw, & Szabadi, 1999) proposes a quantitative methodology for analyzing inter-temporal choice has proved successful in describing impulsive and risky choice in rats. The present study aims to test the applicability of the model to data from human participants. Methods: Forty healthy volunteers underwent two experimental tasks of choosing between two alternatives (A and B) for monetary rewards. Each task consisted of several conditions based on the same design. On one task, alternative A produced a probabilistic reward after a short delay (d(subscript A)), and B a certain reward after longer delays, d(subscript B). d(subscript A) was manipulated across 5 conditions. There were 50 trials (10×5 trial type) in each condition. Indifference delays, d(subscript B(50)) (value of d(subscript B) yielding 50% choice of B) were estimated for each participant in each condition. On the other task, a similar procedure was employed, except that reward sizes for A and B were different, but with the same probabilities. Linear functions of d(subscript B(50)) versus d(subscript A) were fitted; the slopes and intercepts provided indices of sensitivity to reinforcement size, delay and probability. Results: d(subscript B(50)) increased linearly with d(subscript A) (r^2 ＞.99) in the two tasks. There was a significant correlation (r=.60, p ＜.001) between the intercepts (sensitivity to delay) but not between the slopes (sensitivity to size and probability) in the two tasks (r=.18, p =.269). Conclusion: These results show the indifference equations specified by the multiplicative model to account for effects of reward size, delay and probability on choice appear to be applicable to data from humans.
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