OBJECTIVES: To study whether the minimally important differences (MIDs) values outcomes based on the behavioral economic theory. METHODS: We studied the behavior of individuals discriminating minimally important differences (MIDs), a method that identifies the change in a health measure necessary for a patient to discriminate an improvement. The behavioral theory predicts that discrimination of a quantity is governed by Weber’s Law:  If a quantity is increased by some factor, the threshold for a MID also increases by this factor constantly. And this leads to logarithmic function for quantifying outcomes. We examined a logarithmic functional form for MIDs in nine pairs, a combination of three health states extracted from EuroQol-5D (State A (21222), B (21122), and C (22222)) and three survival durations (10, 20, and 30 years). An online survey was conducted and 100 subjects were recruited. . One-way repeated measure analysis of variance and one-way within subjects analysis were applied. RESULTS: A total of 98 subjects completed the survey. In State A, the normalized mean of 10, 20, and 30 years were 0.63, 0.61, and 0.59, respectively. There were no significant difference in the means (p = 0.1102). For State B, the normalized means were 0.66, 0.68, and 0.66 for 10, 20, and 30 years, respectively, with no significant difference among three durations (p = 0.6496). Lastly, in State C, the normalized means for the starting years were 0.57, 0.55, and 0.57, respectively, and there was no statistical difference (p = 0.5404). Thus, the impact of changing durations was not significant in all three states. CONCLUSIONS: Our findings implies that the MID is constant over proportional change in duration, indicating that the utility function of the MID follows a logarithmic function. This violation of the normative decision model implies treatment decisions based on the MID may not represent our preferences for health.