Reward magnitude-specific delay discounting differentiates mania versus depression risk

Mania/hypomania, the pathognomonic feature of bipolar disorder (BD), is characterized by elevated impulsivity, often assessed via delay discounting-the preference for smaller, immediate versus larger, delayed rewards. It remains unclear whether delay discounting differentiates BD from non-BD individuals or serves as an objective behavioral marker of mania/hypomania versus depression risk. Bipolar disorder (n = 40) and non-BD (n = 187) individuals were recruited, with the latter encompassing a range of mania/hypomania and depression risk and stratified into mania/hypomania and depression risk tertiles. Kruskal-Wallis and Dunn's tests evaluated delay discounting rates (k values), assessed via the 27-Item Monetary Choice Questionnaire, across both risk groups compared to the BD group. Significant group effects were found for overall and geomean k values in both mania/hypomania (overall k: χ²(3) = 8.15, p = 0.043; geomean k: χ²(3) = 8.40, p = 0.038) and depression risk groups (overall k: χ²(3) = 8.30, p = 0.04; geomean k: χ²(3) = 8.75, p = 0.033). Only k values for medium reward magnitudes were significant for both mood risk stratifications (corrected α = 0.05/3 = 0.0167). Bipolar disorder had significantly higher k versus low-risk mania/hypomania individuals (adjusted p = 0.012), as did high-risk versus low-risk mania/hypomania individuals (adjusted p = 0.039). Bipolar disorder had higher k versus high-risk depression individuals (adjusted p = 0.005), as did low-risk versus high-risk depression individuals (adjusted p = 0.029). Bipolar disorder had significantly higher k for medium reward magnitudes versus high-risk depression-only (W = 398, p < 0.001), but not versus high-risk mania/hypomania-only (W = 587.5, p = 0.368) individuals. Delay discounting for medium reward magnitudes differentiates BD from non-BD individuals and distinguishes heightened mania/hypomania risk from depression risk, supporting its potential as an objective behavioral marker for mania/hypomania risk detection.