I contend that Kurzban et al.'s model is silent on three issues. First, the extent to which opportunity-cost computations are automatic or deliberative is unclear. Second, the role of individual differences in biasing opportunity-cost computations needs elucidating. Third, in the absence of “next-best” tasks, task persistence will be indefinite, which seems unfeasible, so perhaps integration with a limited-resource account is necessary.

We show that Kurzban et al.'s approach illuminates the relationship between religion and self-control. Whereas resource-depletion theorists suggest religion replenishes self-control resources (“strength”), we submit that religious cues make people feel observed, giving them “reason” to persevere, and we describe an experiment that supports our interpretation. Finally, we question the claim that subjective fatigue is a signal to redeploy resources.

I make three points. First, processors and depletable resources should not be regarded as alternative means of processing information: they are both necessary. Second, comparing a processor account with a rational allocation mechanism to a depletable-resources account without one is not a fair comparison. Third, depletable resources can act as signals as well as fuels.

Different to Kurzban et al., we conceptualize the experience of mental effort as the subjective costs of goal pursuit (i.e., the amount of invested resources relative to the amount of available resources). Rather than being an output of computations that compare costs and benefits of the target and competing goals, effort enters these computations as an input.

One shortcoming of Kurzban et al.'s model is that it is not clear how animals persist through subjectively effortful tasks, particularly over a long time course. We suggest that the anterior cingulate cortex plays a critical role by encoding the utility of an action, and signalling where efforts should be best directed based on previous and prospected experience.

Kurzban et al.'s framework may be extended in fruitful ways by treating effort also as a cost input that affects the utility computation of a given option (rather than only as the output of a utility comparison between options). The weight people assign to effort as a cost may vary dynamically as a function of situational and dispositional factors.

The target article highlights the role of the anterior cingulate cortex (ACC) in conflict monitoring, but ACC function may be better understood in terms of the hierarchical organization of behavior. This proposal suggests that the ACC selects extended goal-directed actions according to their learned costs and benefits and executes those behaviors subject to depleting resources.

The opportunity cost model (OCM) aims to explain various phenomena, among which the finding that performance degrades if executive functions are used repeatedly (“resource depletion”). We argue that an OCM account of resource depletion requires two unlikely assumptions, and we discuss an alternative that does not require these assumptions. This alternative model describes the interplay between executive function and motivation.

The opportunity cost model offers an ultimate explanation of ego depletion that helps to move the field beyond biologically improbable resource accounts. The model's more proximate explanation, however, falls short of accounting for much data and is based on an outdated view of human rationality. We suggest that our own process model offers a better proximate account of self-control fatigue.

We argue that prioritization, simultaneity, and mutual exclusion are mind-body integration functions that can't be addressed meaningfully at the psychological (computational) level alone. We describe the outlook for an integration between Kurzban et al.'s profound discussion of opportunity cost/benefit prioritization and decades of related development in biofunctional science.

Kurzban and colleagues carry forward an important contemporary movement in cognitive control research, tending away from resource-based models and toward a framework focusing on motivation or value. However, their specific proposal, centering on opportunity costs, appears problematic. We favor a simpler view, according to which the exertion of cognitive control carries intrinsic subjective costs.

An opportunity cost model of effort requires flexible integration of valuation and self-control systems. Reciprocal connections between these networks and brainstem neuromodulatory systems are likely to provide the signals that affect subsequent persistence or failure when faced with effort challenges. The interaction of these systems should be taken into account to strengthen a normative neural model of effort.

Kurzban and colleagues propose that experiences of effort alter motivations to persist during goal pursuit by highlighting costs of persistence. I expand this proposal by discussing how effort experiences (a) not only influence, but can be influenced by motivations to persist on a goal; and (b) not only highlight costs that undermine persistence, but can also signal progress and increase persistence.

Kurzban et al. make a convincing case against the idea that willpower is a depleting resource. However, they do not advance a positive account of willpower. Rather than treating “willpower” as a synonym of “executive function,” we argue that the term willpower should be designated for mechanisms individuals deploy to reduce dynamic inconsistency in their behavior.

Though the aversiveness of effort may indeed serve in selecting tasks for executive attention, the notion that it reflects opportunity costs is questionable: The potency of distractions in real-life situations is not regularly related with the potential benefit from attending to them.

Kurzban et al.'s opportunity cost model of mental effort relies heavily on counterfactual thinking. We suggest that a closer inspection of the role of counterfactual emotions, and particularly of action/inaction asymmetries in anticipated regret, may be important in understanding the role of opportunity costs in decisions to persist with a current task.

Kurzban et al. associate mental effort and fatigue with a hypothetical mechanism able to estimate the utilities of possible actions and then select the action with a maximal utility. However, this approach faces fundamental problems. In my opinion, mental effort and fatigue are results of a conflict between the monotony of long-term activities and the novelty-processing systems.

Kurzban et al.'s expectancy-value mechanism of effort allocation seems relevant in situations when familiar tasks are initiated. However, we think additional mechanisms are important when people continue with a task for a prolonged time. These mechanisms, which are particularly relevant for performance of novel or urgent tasks, involve neural systems that track performance costs and resources.

If cognitive effort indexes opportunity costs, it should be investigated like other cost factors including risk and delay. We discuss recent methodological advances in behavioral economics and neuroeconomics, highlighting our own work in measuring the subjective (economic) value of cognitive effort. We discuss the implications of Kurzban et al.'s proposal and how some of its predictions may be untestable without behavioral economic formalisms.

We address points of confusion pertaining to interrelations among and roles of core constructs involved in the production of performance outcomes. We do so informed by the body of work derived from Brehm's seminal motivation intensity theory – in particular an elaboration from the theory concerned with fatigue influence on effort and associated cardiovascular responses in people confronted with performance challenges.