Does the Brain Consume Additional Glucose during Self-Control Tasks?
A currently popular model of self-control posits that the exertion of self-control relies on a resource, which is expended by acts of self-control, resulting in less of this resource being available for subsequent acts of self-control. Recently, glucose has been proposed as the resource in question. For this model to be correct, it must be the case that A) performing a self-control task reduces glucose levels relative to a control task and B) performing a self-control task reduces glucose
... duces glucose relative to pre-task levels. Evidence from neurophysiology suggests that (A) is unlikely to be true, and the evidence surrounding (B) is mixed, and is unlikely to be true for subjects who have not recently fasted. From the standpoint of evolved function, glucose might better be thought of as an input to decision making systems rather than as a constraint on performance. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Background and Introduction A currently popular model of self-control posits that the exertion of self-control relies on a resource, which is expended by acts of self-control, resulting in less of this resource being available for subsequent acts of self-control (Baumeister, Vohs, and Tice, 2007; Muraven and Baumeister, 2000; Schmeichel and Baumeister, 2004; Vohs et al., 2008) . The nature of this resource was previously left unspecified, referred to using metaphorical terms such as "willpower," or with the notion that self-control was "like a muscle" (Muraven and Baumeister, 2000) . Recently, reported the results of nine studies designed to make this model more concrete, specifying glucose as the resource necessary for, and depleted by, acts of self-control (see also . Like much research in this literature, the studies in question used methods along the following lines. First, a "selfcontrol" task was performed by subjects, after which a second "self-control" task was performed. In this literature, it is frequently reported that compared to relevant controls, performance on the second task tends to be lower (see Hagger, Wood, Stiff, and Chatzisarantis, in press, for a recent meta-analysis); the "resource" explanation of such Glucose and self-control effects is that a resource was depleted by the first task, causing a reduction in performance on the second. For the glucose account to be able to explain such results, both of the following propositions must be true. Proposition 1. Performing a self-control task reduces glucose levels relative to a control task. Proposition 2. Performing a self-control task reduces glucose levels relative to glucose levels before the task. If Proposition 1 is not true, then the difference in performance observed in these studies cannot be due to a reduction in glucose. If Proposition 2 is not true, and glucose levels are as high or higher than they were before the task, then a reduction in glucose cannot be the cause of reductions in performance. The requirement for Proposition 2 can be stated more strongly. Specifically, for any subject in whom glucose does not go down relative to baseline, it cannot be the case that glucose is the resource in question. That is, because the model specifies that it is the reduction in the resource that causes the reduction in performance; if there is no reduction in the resource, then this cannot explain the reduction in performance. Note that the following two Propositions must also be true in order for the glucose model to be correct. Proposition 3. Performance on self-control tasks depends in some way on glucose. Proposition 4. Reduced performance on self-control tasks is due to reductions in glucose. Note, however, that these both represent conditions of logical necessity, not logical sufficiency. If either of these two additional Propositions is false, then the model must be false. However, if either or both of them are true, that does not logically entail that the model is true. This short piece is not intended as a complete review of the self-control literature (Baumeister, Schmeichel, and Vohs, 2007; . Instead, it is intended to consider the plausibility of the glucose model given what is known about brain metabolism, and, using this analysis, consider what is arguably the best evidence in the social psychological literature that self-control tasks reduce blood glucose, the work by . Note that the present paper reviews only five of the nine studies in , and presents some novel analyses of the data collected in this paper, but not reported originally. To be clear, the goal of this reanalysis is to investigate only Proposition 2, above. If Proposition 2 is false, then, because the truth of this Proposition is a necessary condition for the glucose model to be correct, then the glucose model of self-control cannot be correct, independent of whether the other Propositions are true. When and why does peripheral glucose go down? The glucose model of self-control is committed to the view that self-control tasks deplete glucose because of energy consumption by the brain (as opposed to organs in the