Objective but not subjective fatigue increases cognitive task avoidance

Mentally demanding tasks feel effortful and are usually avoided. Furthermore, prolonged cognitive engagement leads to mental fatigue, consisting of subjective feeling of exhaustion and decline in performance. Despite the intuitive characterization of fatigue as an increase in subjective effort perception, the effect of fatigue on effort cost has never been tested experimentally. To this end, sixty participants in 2 separate experiments underwent a forced-choice working memory task following either a fatigue-inducing (i.e. Stroop task) or a control manipulation. We measured subjective fatigue and effort as well as their objective behavioral signatures: performance decline and task avoidance, respectively. We found that fatigue-induced performance decline was correlated with task avoidance, while the feelings of fatigue and effort were unrelated to each other. Our findings highlight the discrepancy between subjective and objective manifestations of fatigue and effort, and provide valuable evidence feeding the ongoing theoretical debate on the nature of these constructs.