Another attack on the resource-based model of willpower, Michael Inzlicht, Brandon J. Schmeichel and C. Neil Macrae have a paper called "Why Self-Control Seems (but may not be) Limited" in press in Trends in Cognitive Sciences. Ungated version here.

Some of the most interesting points:

• Over 100 studies appear to be consistent with self-control being a limited resource, but generally these studies do not observe resource depletion directly, but infer it from whether or not people's performance declines in a second self-control task.
• The only attempts to directly measure the loss or gain of a resource have been studies measuring blood glucose, but these studies have serious limitations, the most important being an inability to replicate evidence of mental effort actually affecting the level of glucose in the blood.
• Self-control also seems to replenish by things such as "watching a favorite television program, affirming some core value, or even praying", which would seem to conflict with the hypothesis inherent resource limitations. The resource-based model also seems evolutionarily implausible.

The authors offer their own theory of self-control. One-sentence summary (my formulation, not from the paper): "Our brains don't want to only work, because by doing some play on the side, we may come to discover things that will allow us to do even more valuable work."

• Ultimately, self-control limitations are proposed to be an exploration-exploitation tradeoff, "regulating the extent to which the control system favors task engagement (exploitation) versus task disengagement and sampling of other opportunities (exploration)".
• Research suggests that cognitive effort is inherently aversive, and that after humans have worked on some task for a while, "ever more resources are needed to counteract the aversiveness of work, or else people will gravitate toward inherently rewarding leisure instead". According to the model proposed by the authors, this allows the organism to both focus on activities that will provide it with rewards (exploitation), but also to disengage from them and seek activities which may be even more rewarding (exploration). Feelings such as boredom function to stop the organism from getting too fixated on individual tasks, and allow us to spend some time on tasks which might turn out to be even more valuable.

The explanation of the actual proposed psychological mechanism is good enough that it deserves to be quoted in full:

Based on the tradeoffs identified above, we propose that initial acts of control lead to shifts in motivation away from “have-to” or “ought-to” goals and toward “want-to” goals (see Figure 2). “Have-to” tasks are carried out through a sense of duty or contractual obligation, while “want-to” tasks are carried out because they are personally enjoyable and meaningful [41]; as such, “want-to” tasks feel easy to perform and to maintain in focal attention [41]. The distinction between “have-to” and “want-to,” however, is not always clear cut, with some “want-to” goals (e.g., wanting to lose weight) being more introjected and feeling more like “have-to” goals because they are adopted out of a sense of duty, societal conformity, or guilt instead of anticipated pleasure [53].

According to decades of research on self-determination theory [54], the quality of motivation that people apply to a situation ranges from extrinsic motivation, whereby behavior is performed because of external demand or reward, to intrinsic motivation, whereby behavior is performed because it is inherently enjoyable and rewarding. Thus, when we suggest that depletion leads to a shift from “have-to” to “want-to” goals, we are suggesting that prior acts of cognitive effort lead people to prefer activities that they deem enjoyable or gratifying over activities that they feel they ought to do because it corresponds to some external pressure or introjected goal. For example, after initial cognitive exertion, restrained eaters prefer to indulge their sweet tooth rather than adhere to their strict views of what is appropriate to eat [55]. Crucially, this shift from “have-to” to “want-to” can be offset when people become (internally or externally) motivated to perform a “have-to” task [49]. Thus, it is not that people cannot control themselves on some externally mandated task (e.g., name colors, do not read words); it is that they do not feel like controlling themselves, preferring to indulge instead in more inherently enjoyable and easier pursuits (e.g., read words). Like fatigue, the effect is driven by reluctance and not incapability [41] (see Box 2).

Research is consistent with this motivational viewpoint. Although working hard at Time 1 tends to lead to less control on “have-to” tasks at Time 2, this effect is attenuated when participants are motivated to perform the Time 2 task [32], personally invested in the Time 2 task [56], or when they enjoy the Time 1 task [57]. Similarly, although performance tends to falter after continuously performing a task for a long period, it returns to baseline when participants are rewarded for their efforts [58]; and remains stable for participants who have some control over and are thus engaged with the task [59]. Motivation, in short, moderates depletion [60]. We suggest that changes in task motivation also mediate depletion [61].

Depletion, however, is not simply less motivation overall. Rather, it is produced by lower motivation to engage in “have-to” tasks, yet higher motivation to engage in “want-to” tasks. Depletion stokes desire [62]. Thus, working hard at Time 1 increases approach motivation, as indexed by self-reported states, impulsive responding, and sensitivity to inherently-rewarding, appetitive stimuli [63]. This shift in motivational priorities from “have-to” to “want-to” means that depletion can increase the reward value of inherently-rewarding stimuli. For example, when depleted dieters see food cues, they show more activity in the orbitofrontal cortex, a brain area associated with coding reward value, compared to non-depleted dieters [64].

See also: Kurzban et al. on opportunity cost models of mental fatigue and resource-based models of willpower; Deregulating Distraction, Moving Towards the Goal, and Level Hopping.