An opportunity cost model of subjective effort and task performance (h/t lukeprog) is a very interesting paper on why we accumulate mental fatigue: Kurzban et al. suggest an opportunity cost model, where intense focus on a single task means that we become less capable of using our mental resources for anything else, and accumulating mental fatigue is part of a cost-benefit calculation that encourages us to shift our attention instead of monomaniacally concentrating on just one task which may not be the most rewarding possible. Correspondingly, the amount of boredom or mental fatigue we experience with a task should correspond with the perceived rewards from other tasks available at the moment. A task will feel more boring/effortful if there's something more rewarding that you could be doing instead (i.e. if the opportunity costs for pursuing your current task are higher), and if it requires exclusive use of cognitive resources that could also be used for something else.

This seems to make an amount of intuitive/introspective sense - I had a much easier time doing stuff without getting bored as a kid, when there simply wasn't much else that I could be doing instead. And it does roughly feel like I would get more quickly bored with things in situations where more engaging pursuits were available. I'm also reminded of the thing I noticed as a kid where, if I borrowed a single book from the library, I would likely get quickly engrossed in it, whereas if I had several alternatives it would be more likely that I'd end up looking at each for a bit but never really get around reading any of them.

An opportunity cost model also makes more sense than resource models of willpower which, as Kurzban quite persuasively argued in his earlier book, don't really fit together with the fact that the brain is an information-processing system. My computer doesn't need to use any more electricity in situations where it "decides" to do something as opposed to not doing something, but resource models of willpower have tried to postulate that we would need more of e.g. glucose in order to maintain willpower. (Rather, it makes more sense to presume that a low level of blood sugar would shift the cost-benefit calculations in a way that led to e.g. conservation of resources.)

This isn't just Kurzban et al's opinion - the paper was published in Behavioral and Brain Sciences, which invites diverse comments to all the papers that they publish. In this particular case, it was surprising how muted the defenses of the resource model were. As Kurzban et al point out in their response to responses:

As context for our expectations, consider the impact of one of the central ideas with which we were taking issue, the claim that “willpower” is a resource that is consumed when self-control is exerted. To give a sense of the reach of this idea, in the same month that our target article was accepted for publication Michael Lewis reported in Vanity Fair that no less a figure than President Barack Obama was aware of, endorsed, and based his decision- making process on the general idea that “the simple act of making decisions degrades one’s ability to make further decisions,” with Obama explaining: “I’m trying to pare down decisions. I don’t want to make decisions about what I’m eating or wearing. Because I have too many other decisions to make ” (Lewis 2012 ).

Add to this the fact that a book based on this idea became a New York Times bestseller (Baumeister & Tierney 2011 ), the fact that a central paper articulating the idea (Baumeister et al. 1998 ) has been cited more than 1,400 times, and, more broadly, the vast number of research programs using this idea as a foundation, and we can be forgiven for thinking that we would have kicked up something of a hornet’s nest in suggesting that the willpower-as-resource model was wrong. So we anticipated no small amount of stings from the large number of scholars involved in this research enterprise. These were our expectations before receiving the commentaries.

Our expectations were not met. Take, for example, the reaction to our claim that the glucose version of the resource argument is false (Kurzban 2010a ). Inzlicht & Schmeichel, scholars who have published widely in the willpower-as-resource literature, more or less casually bury the model with the remark in their commentary that the “mounting evidence points to the conclusion that blood glucose is not the proximate mechanism of depletion.” ( Malecek & Poldrack express a similar view.) Not a single voice has been raised to defend the glucose model, and, given the evidence that we advanced to support our view that this model is unlikely to be correct, we hope that researchers will take the fact that none of the impressive array of scholars submitting comments defended the view to be a good indication that perhaps the model is, in fact, indefensible. Even if the opportunity cost account of effort turns out not to be correct, we are pleased that the evidence from the commentaries – or the absence of evidence – will stand as an indication to audiences that it might be time to move to more profitable explanations of subjective effort.

While the silence on the glucose model is perhaps most obvious, we are similarly surprised by the remarkably light defense of the resource view more generally. As Kool & Botvinick put it, quite correctly in our perception: “Research on the dynamics of cognitive effort have been dominated, over recent decades, by accounts centering on the notion of a limited and depletable ‘resource’” (italics ours). It would seem to be quite surprising, then, that in the context of our critique of the dominant view, arguably the strongest pertinent remarks come from Carter & McCullough, who imply that the strength of the key phenomenon that underlies the resource model – two-task “ego-depletion” studies – might be considerably less than previously thought or perhaps even nonexistent. Despite the confidence voiced by Inzlicht & Schmeichel about the two-task findings, the strongest voices surrounding the model, then, are raised against it, rather than for it. (See also Monterosso & Luo , who are similarly skeptical of the resource account.)

Indeed, what defenses there are of the resource account are not nearly as adamant as we had expected. Hagger wonders if there is “still room for a ‘resource’ account,” given the evidence that cuts against it, conceding that “[t]he ego-depletion literature is problematic.” Further, he relies largely on the argument that the opportunity cost model we offer might be incomplete, thus “leaving room” for other ideas.

(I'm leaving out discussion of some commentaries which do attempt to defend resource models.)

Though the model still seems to be missing pieces - as one of the commentaries points out, it doesn't really address the fact that some tasks are more inherently boring than others. Some of it might be explained by the argument given in Shouts, Whispers, and the Myth of Willpower: A Recursive Guide to Efficacy (I quote the most relevant bit here), where the author suggests that "self-discipline" in some domain is really about sensitivity for feedback in that domain: a novice in some task doesn't really manage to notice the small nuances that have become so significant for an expert, so they receive little feedback for their actions and it ends up being a boring vigilance task. Whereas an expert will instantly notice the effects that their actions have on the system and get feedback of their progress, which in the opportunity cost model could be interpreted as raising the worthwhileness of the task they're working on. If we go with Kurzban et al.'s notion of us acquiring further information about the expected utility of the task we're working on as we continue working on it, then getting feedback from the task could possibly be read as a sign of the task being one in which we can expect to succeed in.

Another missing piece with the model is that it doesn't really seem to explain the way that one can come home after a long day at work and then feel too exhausted to do anything at all - it can't really be about opportunity costs if you end up so tired that you can't come up with ~any activity that you'd want to do.