Why do we so often fail to exert cognitive control, even though we are in principle able to do so? In this review, we begin to address this question by considering the contribution of the major ascending neuromodulators that are often implicated in cognitive control and motivation, in particular dopamine, noradrenaline and serotonin. Accumulating evidence indicates that cognitive control is subjectively costly and people generally choose to refrain from mentally effortful tasks, despite, at

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... s, devastating consequences. This tendency to avoid cognitive control tasks has been shown to be sensitive to catecholaminergic interventions in rodents and humans, where choices about cognitive control can be altered even in the absence of performance changes. Such effects might reflect modulation by dopamine and/or noradrenaline of a variety of mechanisms that contribute to our motivation for cognitive control. These likely include the calculation and integration into behavior of both the expected value (i.e. cost vs benefit), as well as outcome uncertainty of exerting cognitive control. In addition, serotonin might impact cognitive control avoidance by modulating specifically the computation of effort costs. Advancing our understanding of the distinct roles of the various chemical neuromodulators will help elucidate the computational mechanisms that contribute to our tendency to avoid difficult cognitive tasks.