This experiment investigated how the personal choice of task characteristics influences resource mobilization assessed as effort-related cardiac response during a task of clearly low vs. unclear (but also low) difficulty. We expected that the personal choice of the color of memory task stimuli would justify higher effort during task performance than external color assignment. Applying the logic of motivational intensity theory (MIT; Brehm et al., 1983; Brehm & Self, 1989), we further predicted that the personal choice of the stimuli’s color would directly lead to higher effort intensity than external color assignment when task difficulty was unclear but not when the task difficulty was clear. When task difficulty was low and clear, we expected actual effort to be low in general, because high resources are not necessary for a clearly easy task. Results were as expected: when task difficulty was unclear, participants who had personally chosen the stimuli’s color showed significantly stronger cardiac pre-ejection period reactivity, reflecting higher effort, than those in the other three conditions. These findings provide first evidence that personal choice justifies relatively high effort and further support the principles of MIT regarding the critical role of task difficulty for resource mobilization.