Kurzban et al. argue that mental effort experienced during task engagement is a function of opportunity costs – that is, the degree to which the mental processes allocated to a target task or goal are not available for other tasks or goals. We consider this idea intriguing and concur that feelings of effort, like other sensations, are most likely the outputs of mechanisms designed to produce inputs to decision-making systems (Bloom Reference Bloom2010; Thornhill Reference Thornhill, Crawford and Krebs1998). We doubt, however, an additional assumption of the model, namely, that the experience of effort itself changes as a function of the presence of competing goals. We propose a potential alternative account of subjective feelings of effort and their adaptive value. Accordingly, subjective effort emerges from cost computations associated exclusively with the currently pursued goal and is a function of the means invested into the goal at hand and the means available for pursuing it. In our conceptualization, alternative goals do not enter this computation through opportunity costs. The presence of competing goals may, however, draw more attention to the resources and, in turn, the effort invested into pursuing the target goal.

Our account of mental effort differs from Kurzban et al.'s account regarding the effect of competing outcomes. Kurzban et al. predict that competing goals increase mental effort through opportunity costs, that is, the degree to which resources invested into the target goal are not available for achieving competing goals. In our view, subjective effort is a function of the resources a person perceives to invest into the pursuit of the target goal in relation to the subjectively available goal-relevant resources (e.g., when time is crucial for pursuing the target goal, perceived effort is mainly based on how much of one's available time is spent for its pursuit). Based on the definition of goals as cognitive representations linking means to desired outcomes (e.g., Freund et al. Reference Freund, Hennecke, Mustafic and Ryan2012; Kruglanski et al. Reference Kruglanski, Shah, Fishbach, Friedman, Chun and Sleeth-Keppler2002), the subjective perception of effort should be related to the means of goal pursuit (i.e., how many resources does one have to invest to attain the outcome?). Kurzban et al. posit that mental effort is related to potential outcomes or alternative goals, but it remains unclear how people gauge the effort if they do not do so on the basis of how many resources they invest relative to the resources they have available for the pursuit of a target goal. By reflecting these relative costs, subjective feelings of effort provide an informational basis for further cost-benefit computations that determine whether an individual decides to continue investments into the goal at hand or turn away from it and towards other goals. As costs during task engagement accumulate over time, the feeling of mental effort during task engagement increases simultaneously (cf. Boksem & Tops Reference Boksem and Tops2008). In agreement with Kurzban et al., and contra to previous accounts of mental effort (Kahneman Reference Kahneman1973), we consider mental effort not a finite capacity but a metacognitive phenomenon that signals the ratio of the finite amount of available resources to the subjectively invested ones.

How, then, is the subjective experience of effort influenced by potential alternative goals? We posit that perceiving alternative desired outcomes draws attention to the means of goal pursuit to determine whether such outcomes require the same means. The presence of competing outcomes should make people become more aware that their resources are finite and have to be spent carefully, that is, that they are best invested in means that yield the most valuable outcome (e.g., Ebner et al. Reference Ebner, Freund and Baltes2006).

Each alternative goal has an (expected) cost/benefit ratio. In order to compare multiple goals, a person needs to make a rough overall estimate as to how many resources will be needed and are available to attain the goals. Hence, a person should compare the experienced cost/benefit ratio of the ongoing goal against the expected cost/benefit ratio of additional or alternative goals. In this way, the mental effort invested into goal A enters cost-benefit computations that compare target goal A to the alternative goal B (note that goal B might also be to pause the pursuit of a target goal in order to conserve resources; Boksem & Tops Reference Boksem and Tops2008; Hennecke & Freund, in press). The presence of alternative goals creates a reference against which the cost/benefit ratio of the ongoing goal-pursuit is compared. By triggering this comparison, alternative goals draw attention to the effort – as a subjective representation of the costs – invested into the ongoing goal. Without changing subjective effort directly, alternative goals might thus change the perception of effort by putting it into the center of attention (Kool et al. Reference Kool, McGuire, Rosen and Botvinick2010). By providing information about the means/resources a person currently invests into the pursuit of a goal, subjective effort allows a rough estimate of how many resources are available for the pursuit of additional or alternative goals. It may thereby serve as an important metacognitive cue as to whether to continue with the current goal or to switch to an alternative goal. This function would be undermined if subjective effort were affected by the presence of alternative goals. Note that this conceptualization of subjective effort does not imply that it is a veridical reflection of the actual costs and resources a goal requires. A person might very well underestimate goal requirements and/or available resources, which might then lead to such phenomena as the planning fallacy (Buehler et al. Reference Buehler, Griffin and Ross1994).

Important in the current context, however, is that our conceptualization of subjective effort should be unaffected by the presence of competing outcomes. Rather than being an output of computations that consider (potential) costs and benefits of the current and the competing goal, effort enters these computations as a subjective representation of the costs of the current goal. As proposed by Kurzban et al., experiments that assess and compare perceived mental effort under different concurrent task conditions are needed to compare both accounts.