[Subjective probability of reward receipt and the magnitude effect in probability discounting]

Previous research suggested that larger probabilistic rewards were discounted more steeply than smaller probabilistic rewards (the magnitude effect). This research tests the hypothesis that the magnitude effect reflects the extent to which individuals distrust the stated probability of receiving different amounts of rewards. The participants were 105 college students. Probability discounting of two different amounts of rewards (5 000 yen and 100 000 yen) and the subjective probability of reward receipt of the different amounts (5 000 yen, 100 000 yen and 1 000 000 yen) were measured. The probabilistic 100 000 yen was discounted more steeply than the probabilistic 5 000 yen. The subjective probability of reward receipt was higher in the 5 000 yen than in the 100 000 yen condition. The proportion of subjective probability of receiving 5 000 yen to that of receiving 100 000 yen was significantly correlated with the proportion of degree of probability discounting for 5 000 yen to that for 100 000 yen. These results were consistent with the hypothesis stated above.