Glucose and self control 1
The role of glucose in self-control: Another look at the evidence and an alternative conceptualization.
Christopher J. Beedie & Andrew M. Lane
University of Wolverhampton, UK
Running Header: Glucose and self-control
Revision submitted May 6th2011
Corresponding author:
Dr Chris Beedie, School of Sport, Performing Arts and Leisure
University of Wolverhampton
Gorway Road, Walsall, WS13BD, UK
Tel. 01902 32 2862
Fax. 01902 322894
Abstract
The strength model suggests that self-control relies upon a limited resource. One candidate for this resource is glucose. Counter to the proposals of the glucose hypothesis as presented by Gailliot and co-authors, we argue that the resource issue is one of allocation, not of limited supply. We address our argument from three perspectives;the evolution of mental processes at the species level, the adaptation of these same processes at the individual level, and the physiology of glucose transport. We argue that the brain has both sufficient resources and resource delivery mechanisms with which to support self-control, but that these resources are allocated in accordance with personal priorities. As an alternative to the limited resource model, we propose a resource-allocation model of self-control (RAMS) and present several testable hypotheses.
Key words: Ego-depletion, evolutionary psychology, motivation, physiological adaptation
The role of glucose in self-control: Another look at the evidence and an alternative conceptualization.
Self-control is fundamental to human interaction and arguably, in evolutionary terms, to human survival. In the former setting, it enables people to behave in a manner consistent with social norms. In the latter, it may have enabled individuals to overcome emotional responses such as fear and anger that, although generally adaptive, might have proved counter-adaptive in certain circumstances. The failure of individuals to exert self-control has been implicated in phenomena ranging from obesity and personal debt to disease and violent crime (Muraven & Baumeister, 2000). Self-control is therefore of interest to many areas of social science.
Baumeister, Heatherton, and Tice (1993) identified three distinct psychological approaches to the study of self-control. Firstly, traditional folk concepts such as willpower suggested that self-control relies upon a limited resource. Secondly, cognitive models suggested that self-control is a function of factors such as knowledge, decision-making, emotion regulation, and choice. Thirdly, developmental approaches treated self-control as a learned skill. Muraven, Tice, and Baumeister (1998) tested these models in experimental studies. They hypothesized that whereas a limited resource model would predict that self-control ability would decline over repeated attempts, cognitive and developmental models would predict no such decline (in fact, cognitive and developmental models might predict an improvement in self-control over time). Based on the findings of several studies, the authors concluded that the limited resource model best fit the evidence. The same research team subsequently proposed the ‘strength model’, positing that acts of self-control are ‘fuelled’ by a limited resource. Furthermore, repeated self-control attempts deplete this resource sufficiently to compromise subsequent attempts (Baumeister, 2003; Baumeister, Bratslavsky, Muraven & Tice, 1998; Muraven & Baumeister, 2000). These authors also proposed the term ‘ego-depletion’ to describe the lack of resources resulting from one or more attempts at self-control.
Muraven and Baumeister (2000) summarized the strength model in five key proposals. First, self-control strength is necessary for the executive component of the self. Second, this strength is limited, in the sense that a person has finite capacity for self-control. Third, all self-control operations draw on the same resource. Fourth, the success or failure of self-control depends on the person's level of self-control strength. Lastly, self-control strength is expended in the process of self-control. Muraven and Baumeister also made a clear distinction between a limited resource model such as that applied to self-control, and a limited capacity model, such as is often used to describe attention and working memory. This distinction was based on the proposal that although working memory appears to have a limited capacity, that capacity does not appear to remain "depleted” after use; the full capacity of attention or working memory is available as soon as an attention or memory task is completed. In contrast, the concept of a limited resource suggests that the resource is depleted by the mental process and must be replenished before that process is able to function fully again. Baumeister (2003) summarized the work from his laboratory to that point by stating, “All our findings suggest that it [self-control] operates like a muscle or a well of energy. It becomes depleted through use and takes time (and rest) to replenish itself” (p. 4).
Since 1998, almost 90 published studies have tested the strength model and ego-depletion (Hagger, Wood, Stiff, & Chatzisarantis, 2010). The experimental method typically adopted in this research has been the dual-task design. In this approach, experimental participants complete two consecutive tasks requiring self-control, whilst control participants complete one task not requiring self-control followed by one task that does.The hypothesis is that experimental participants will experience greater ego depletion than controls following the first experimental task, and will therefore perform worse than controls on the second experimental task. The majority of the published studies support of the strength model and lend support to the second, fourth, and fifth of Muraven and Baumeister’s (2000) proposals above. Furthermore, ego depletion effects have been observed in a wide range of experimental tasks, including resisting the temptation to eat chocolate (Baumeister et al., 1998, Study 1), the suppression of emotion (Muraven, Tice, & Baumeister, 1998, Study 1), the suppression of prejudice and stereotype (Gordijn, Hindriks, Koomen, Dijksterhuis, & Van Knippenberg, 2004), and thought suppression (Vohs, Baumeister, & Ciarocco, 2005, Study 5). These findings suggest a common self-control resource, thereby lending support to Muraven and Baumeister’s third proposal above. Collectively, the findings suggest not only that self-control depends on a limited resource, but that the same resource might underlie self-control across a wide range of situations.
In a recent quantitative synthesis of published research that has used the dual-task design to investigate the strength model, Hagger et al. (2010) meta-analyzed 83 studies with 193 independent effects. The authors reported an overall effect size of 0.62 (95CI [0.57, 67]), suggesting a moderate to large overall effect (Cohen, 1987). Only two of these effect sizes were in the opposite direction to that proposed in the strength model. Based on these findings, Hagger et al. concluded, “The strength model is a useful explanatory system with which to understand self-control” (p. 520) and that the findings “Corroborate the view that self-control draws from a single, global resource and depletion is not an artifact of specific spheres or tasks” (p. 515). They did however caution, “Further refinements may be necessary, particularly when it comes to the identification of mechanisms” (p. 520).
The issue of what mechanism or resource underlies self-control strength remains unanswered. It has been proposed that the ego depletion effect might be the result of individuals conserving their resource. For example, Muraven, Shmueli, and Burkley (2006) reported that participants who expected that they would undergo three self-control tasks tended to experience greater ego-depletion before the second task than participants who believed they only had to perform two. Alternatively, motivation has been posited as a key factor in self-control. For example, it has been demonstrated that individuals perform better at self-control tasks when presented with incentives (e.g., Muraven & Slessareva, 2003).
Recently, Gailliot, Baumeister and colleagues proposed that brain glucose is the resource that underlies self-control. Gailliot and Baumeister (2007) argued, “Findings fit an energy model because they suggest that the first act of self-control consumes or depletes some resource, thereby impairing self-regulation on a subsequent task” (p. 305). They continued, “Multiple findings have indicated that acts of self-control lead to low levels of glucose...there was also ample evidence that low glucose contributes to poor self-control” (p. 319).
In support of their proposals, Gailliot and Baumeister (2007) suggested two main hypotheses. Firstly, “effortful, controlled, or executive processes are quantitatively different from other processes in that they require more glucose” (p. 306). Secondly, “glucose can be consumed in the brain faster than it can be replenished however, and so cerebral activities can deplete the brain’s supply of glucose” (p. 306). In a series of nine experimental studies, Gailliot, Baumeister, DeWall, Maner et al. (2007) demonstrated experimentally that acts of self-control reduced blood glucose levels, and that these low levels of blood glucose predicted poor performance on a subsequent self-control task[1]. They also demonstrated that consuming a glucose drink eliminated observed ego depletion effects. In investigating alternative mechanisms, the authors stated that moods, emotions, and arousal did not relate to the dependent variable and therefore did not explain the link between glucose and self-control performance.
In the two years following the above papers, the same research laboratory presented further experimental data in support of the glucose hypothesis (De Wall, Baumeister, Gailliot, & Maner, 2008; Gailliot, Peruche, Plant, & Baumeister, 2009; Masicampo & Baumeister, 2008). In meta-analyzing the findings of these studies, Hagger et al. (2010) estimated the overall effect size for glucose supplementation in self-control to be large by Cohen’s (1987) criterion at 0.75 (95 CI [0.48, 1.03]). Hagger et al. concluded, “Supplementing people with glucose is associated with significantly better performance on self-control tasks among depleted people relative to controls provided with a sweet placebo” (p. 514).
Empirical support for both the strength model and the glucose hypothesis is growing. The hypothesis is groundbreaking and consistent with a trend towards greater interdisciplinary overlap between psychology and related scientific disciplines (American Psychological Association, 2010). However, we believe that there are problems with the hypothesis, specifically the position that glucose unavailability is a causal factor in the impairment of self-control. Rather than contest the validity of the evidence linking blood glucose levels and self-control as does Kurzban (2010), whose findings we briefly review below, in the present paper we question the mechanism proposed to explain glucose depletion. We argue that the resource issue is one of allocation, not of limited supply. Our explanation of this process is that the human organism is, in all but the most extreme situations or among populations with specific health conditions, able to supply sufficient glucose to the brain to fuel necessary mental processes. We argue that the redirection of glucose is governed by an appraisal of the situation in the context of available resources and immediate individual priorities. We present three arguments in support of this proposal—the first based on the evolution of mental processes at the species level, the second on the adaptation of these same processes at the individual level, and third on the physiology of glucose transport.Furthermore, we present a resource allocation model of self-control (RAMS) that explains self-control in terms of the allocation of resources in response to the perceived importance of the task. We also propose testable hypotheses.
Evolutionary Mechanisms at the Species Level
Evolutionary theory suggests that each characteristic of a species is present in a certain quantity because past members of the species possessing that characteristic in that quantity benefitted in survival or mating terms over those that did not (Tooby & Cosmides, 1992). On this basis, the genes that coded for that characteristic were more likely to be passed to future generations than were the genes that did not. This process, often termed natural selection, favors characteristics that helped our human ancestors survive or the species propagate. Our bipedal ancestors, for example, had a significant survival advantage over those who moved on four limbs (e.g., improved predator avoidance, greater thermoregulatory efficiency, wider dietary choice and availability, and freedom of the arms and hands). Genes that resulted in anatomical, neural, or physiological characteristics that facilitated bipedal locomotion were in turn more likely to propagate through the species than those that did not. Natural selection also appears to have built a degree of economy into evolved characteristics; they solved adaptive problems without extorting huge costs from the individual (Williams, 1966). For example, despite its complexity in coordination and control terms and relative instability in mechanical terms (Lewin & Foley, 2004), walking often seems effortless and automatic to the individual. It also has a relatively low energy cost in comparison with other forms of movement (Shephard & Åstrand, 2000).
The field of evolutionary psychology addresses the natural selection of mental characteristics. Generally, the same principles as outlined above apply; that is, an adaptive psychological characteristic is more likely to propagate through the species than a non-adaptive one (Tooby & Cosmides, 1992). Evolutionary psychologists consider the adaptive function of psychological processes and their implications for behavior. In common with the study of many psychological phenomena, such analyses are generally done with little reference to physiology or anatomy. However, in addressing the evolution of psychological characteristics, it is important to consider that these did not evolve independently of anatomical or physiological characteristics. All psychological phenomena are a function of the brain, and the brain relies on glucose to function. Despite its small size relative to other organs, the brain accounts for up to 20% of the body’s daily glucose consumption (Dunbar, 1998);at rest or in a fasted state it can consume up to 55% of body glucose (Wang & Mariman, 2008). Areas of the brain that are active at any one point in time use more energy and nutrients than do inactive areas at that point in time (Greenfield, 2001). Cerebral blood flow is redirected to those areas requiring nutrients and away from areas that do not (Peppiatt & Attwell, 2004). Psychological function therefore depends on physiological processes such as the ingestion and transport of nutrients. These physiological processes logically evolved either prior to, or alongside, the psychological phenomena in question.
Returning to the glucose hypothesis, given the reliance of the brain on glucose, the proposals that blood glucose is reduced after a self-control task and that low levels of glucose might result in a failed attempt at self-control, are logical. There is consequentlyintuitive appeal to the idea that glucose is the resource underlying self-control.In the evolutionary context however, the idea that a single attempt at self-control in an everyday situation would deplete brain glucose sufficiently to compromise further attempts suggests that the process of self-control is highly uneconomic. This in turn suggests that the body has no evolved mechanism to either store sufficient glucose or to replenish brain glucose quickly. Taken together, these factors suggest that a restricted capacity to exert self-control is adaptive. That is, on the basis that acquired psychological characteristics are a feature of the modern brain because they solved a problem in our ancestral past, a limited capacity for self-control may have provided our ancestors with a survival or procreation advantage. This presents the questions of what adaptive purpose would limited self-control serve?
Gailliot and Baumeister (2007) presented two evolution-based explanations for the high cost of self-control. Firstly, they argued that as an act of self-control often goes against hard-wired and well-evolved processes; it is necessarily more effortful and less economic than those same hard-wired processes. Secondly, they proposed that self-control evolved relatively recently, and cited Wilder’s (1948) “last in-first out” rule, suggesting that processes that developed more recently are the first to become impaired in the absence of sufficient resources.
The above arguments are seemingly logical. Like most other human psychological and biological processes, in the healthy individual, the human brain functions effectively and economically in the majority of situations. Effectiveness and economy of function are achieved in part via numerous automated, or hard-wired, mental processes (LeDoux, 1998). Hard-wired mental processes are a feature of the modern brain because they helped our forbearers to survive. Their survival in turn ensured that the genes that coded for the mental process in question (e.g., fear responses) were handed down to the next generation. A hard-wired process that helped ensure the survival of previous generations represents a valuable asset to the individual who currently possesses it. However, if it is an automatic process, that individual may be unaware of either the presence or the value of the process. He or she might therefore be unaware of the implications of selecting a different course of action to that hard-wired response, and do so to their cost (fear arguably represents a good example of a hard-wired and automatic process, whereas curiosity represents a course of action that might over-ride it). Thus, that natural selection might have resulted in one or more restraint mechanisms that would discourage an individual from overriding hard-wired processes. As suggested by Gailliot and Baumeister (2007), an excessively high glucose cost associated with overriding the hard-wired response would be an effective restraint mechanism. If such a physiological restraint existed, using self-control to overcome, for example, a legitimate fear response, would be something an individual would not choose to do very often.