Kurzban et al. offer a careful and clever analysis that contains truths, but also points of confusion pertaining to interrelations among and roles of core constructs of effort, subjective effort, fatigue, and performance. The points of confusion could be informed by consideration of the sizable and expanding body of work derived from Brehm's seminal motivation intensity theory (Brehm & Self Reference Brehm and Self1989), which the authors somehow overlooked in their literature review. Particularly relevant is an elaboration from motivation intensity theory concerned with fatigue influence on effort and associated cardiovascular responses in people confronted with performance challenges (Wright & Stewart Reference Wright, Stewart, Wright and Gendolla2012; see also Wright & Kirby Reference Wright, Kirby and Zanna2001).

The elaboration takes as a working “given” a venerable hypothesis in cardiovascular psychophysiology that beta-adrenergic influence on the heart and vasculature is proportional to effort (“active coping”) in action circumstances (Obrist Reference Obrist1981). It also takes as a given the common understanding that difficulty appraisals increase with fatigue within relevant performance systems, that is, the depletion of resources in active performance structures (Fairclough Reference Fairclough, Hancock and Desmond2001). With these givens in place, the elaboration applies motivation intensity theory to derive interactional implications regarding fatigue influence, assuming – like motivation intensity theory – that effort is a mechanism through which energy is mobilized and that effort processes are designed to maximize energy efficiency, that is, to make the best use of energy stores.

Core propositions of motivation intensity theory are that effort (motivation intensity) should be (1) proportional to the perceived difficulty of a performance challenge so long as success is viewed as possible and worthwhile, and (2) low when success is viewed as impossibly difficult or excessively difficult, given the importance of meeting the challenge (i.e., the value of the benefit that can be accrued). In combination with the elaboration givens, this implies that fatigue should augment, retard, or leave unaffected effort and associated cardiovascular responses, depending on the difficulty of the challenge at hand and the importance of meeting it. In theory, fatigue should augment effort and cardiovascular responsiveness when it leaves unchanged a perception that success is possible and worthwhile, generating compensatory striving (i.e., effort exertion: Fig. 1, sect. A). By contrast, fatigue should retard effort and cardiovascular responsiveness when it causes success to appear impossible or excessively difficult, leading performers to withhold effort (Fig. 1, sect. B). By further contrast, fatigue should have no effect on effort and cardiovascular responsiveness when it reinforces a perception that success is impossible or excessively difficult, confirming performers' intention not to try (Fig. 1, sect. C).

Figure 1. Relation between challenge difficulty and effort for fatigued and rested performers (from Figure 1 in Stewart et al. Reference Stewart, Wright, Hui and Simmons2009).

Cardiovascular implications above have been confirmed repeatedly in fatigue studies involving a range of procedures and conducted in different laboratories (e.g., Marcora et al. Reference Marcora, Bosio and de Morree2008; Schmidt et al. Reference Schmidt, Richter, Gendolla and Van der Linden2010; Wright et al. Reference Wright, Martin and Bland2003; Reference Wright, Shim, Hogan, Duncan and Thomas2012). Moreover, they can be profitably brought to bear with respect to inhibition, a topic to which Kurzban et al. devote considerable attention. The implications can be brought to bear assuming (1) that behavioral restraint (a particular type of performance challenge) requires a degree of effort determined by the strength of the relevant behavioral impulse, and (2) that inhibitory performance systems can in fact become fatigued (weakened through the depletion of resources). Insofar as these assumptions are warranted, the suggestion is that inhibitory system fatigue should augment effort when it leaves unchanged a perception that inhibitory success is possible and worthwhile; retard effort when it causes inhibitory success to appear impossible or excessively difficult; and have no effect on effort when it reinforces a perception that inhibitory success is impossible or excessively difficult.

Importantly, although relevant cardiovascular responses in fatigue studies referenced above have consistently comported with effort expectations based on the elaborated fatigue analysis, subjective effort and performance outcomes have not. Disparities between cardiovascular outcomes, on the one hand, and subjective effort and performance outcomes, on the other, might be taken as evidence contrary to an effort interpretation of the cardiovascular results. However, they should not be so taken, because effort reports and performance outcomes have long been recognized as highly fallible indices of actual engagement levels. Regarding effort reports, there is reason to believe that performers sometimes over-report effort in order to please (e.g., experimental) observers and sometimes under-report effort to protect self-esteem in the event of failure. Further, it is possible that performers are not always aware of how engaged they are in goal pursuits (e.g., in the midst of “flow”) and that effort appraisals are sometimes impacted by outcomes other than effort itself, including opportunity costs (Kanfer Reference Kanfer and Ackerman2011). Regarding performance outcomes, depending on a variety of considerations, improved effort might or might not result in their improvement. Indeed, improved effort has potential for producing performance decrements (Harkins Reference Harkins2006).

Potential lessons are multifold. Effort is a mechanism involved in energy mobilization, that is, the process of converting energy stores into energy. It arguably is multifaceted, with physical and phenomenological components that might or might not correspond with one another. Fatigue is distinct from – and bears an interactional relation to – effort. Fatigue can serve a “stop” function, but also a “go” function insofar as it produces compensatory striving in certain performance circumstances. Fatigue can leave effort unaffected as well, in which case one might say it serves a “stay the course” function. Improved effort can, but will not necessarily, improve performance outcomes, which calls into question the use of such outcomes in making effort inferences. The authors' thesis that opportunity costs might systematically influence effort appraisals is reasonable in some respects, and testable, and could account for some empirical (e.g., performance) outcomes. However, we struggle to see how the thesis can explain effort, fatigue, and performance processes in general. Considering the function of effort, it seems that effort qualia are more likely to index that which is being lost (expended), than that which might be gained by altering the direction of behavior.