Abstract
<title>Abstract</title> <p>Willpower failures in real-life goal-directed conflicts are characterized by decisions to engage in low-effort immediate rewards over effortful actions with delayed benefits. While intertemporal choice paradigms have extensively examined how delays discount rewards, the influence of concurrent effort demands on decision-making remain unexplored. We developed a novel Effort-based Delay Discounting paradigm to assess the integrated influence of delay and cognitive effort on subjective value, and to derive a model-free behavioural index of this combined discounting. Sixty-two healthy adults completed an Effort-based Delay Discounting task combining three levels of cognitive load (n-back: 2-back, 3-back, 4-back) with six temporal delays (1–180 days), making 144 incentivised choices for real monetary rewards. Linear mixed modelling revealed a significant effort × delay interaction: higher cognitive effort demands systematically attenuated the gradient of delay discounting by 42%, compressing sensitivity to temporal distance. This pattern persists after controlling for task performance, indicating genuine motivational dynamics rather than cognitive constraints. Volume Under Surface (VUS), a model-free index of combined discounting, showed substantial individual variability and discriminant validity from trait willpower measures. These findings reframe willpower failures as failures of integrative cost weighting under concurrent demands, with implications for understanding goal-directed motivation and designing interventions targeting context-dependent decision-making vulnerabilities.</p>