Implications, Qualifications, and Future Directions

The present results suggest that self-control involves blood

glucose and that the effects of an initial self-control task stem

partly from reduced levels of glucose. As our title indicates,

willpower is more than a metaphor. The nature of the resource that

gets depleted has been a puzzle since the earliest findings on

self-control depletion emerged (Baumeister et al., 1998). Some

researchers have thought that self-control depletion was primarily

a motivational deficit. This view may have been (somewhat un-

wittingly) encouraged by Muraven and Slessareva’s (2003) dem-

onstration that offering motivational incentives could counteract

depletion effects. However, the present results suggest that self-

control depletion involves a shortage of fuel for brain activities. As

Muraven and Slessareva showed, behavior during that state can

still respond to motivational incentives, but the state of self-control

depletion is not primarily a deficit in motivation. By analogy,

money may induce a person to continue working despite physical

exhaustion, but that does not mean that physical exhaustion is

essentially the result of a lack of money. Insofar as self-control

depletion is a deficit in fuel, it reflects an impairment in capacity

to perform not in desire (motivation) to perform, even though

stimulating desire can sometimes offset a reduced capacity.

Another implication of the present work is that individual dif-

ferences in glucose processing may contribute to different out-

comes in self-control. Consistent with that suggestion, juvenile

delinquents lack self-control (Gottfredson & Hirschi, 1990; Pratt &

Cullen, 2000), and there is some evidence that juvenile delinquents

process and tolerate glucose less effectively than comparable ad-

olescents who do not have legal troubles (Gans et al., 1990;

Matykiewicz, La Grange, Vance, Mu, & Reyes, 1997).

There is no reason to assume, however, that self-control is

unique in its reliance on blood glucose. As far as we understand,

all brain processes use some glucose, though some use more than

others and are therefore more susceptible to fluctuations of the

supply in the bloodstream. We would certainly expect that the

patterns we have observed for self-control would generalize to

other executive functioning or controlled processing. Evidence is

accumulating that these other executive processes rely on the same

energy source as self-control. Engaging in effortful choice and

decision making has been shown to lead to impairments in subse-

quent self-control (Baumeister et al., 1998; Vohs, Baumeister,

Twenge, Schmeichel, Tice, & Crocker, 2004). Along the same

lines, controlled thinking processes, such as logical reasoning and

extrapolation, are impaired among people who have performed

recent acts of self-control (Schmeichel et al., 2003). We regard the

self as the controller of controlled processes, and so its effortful

activities are likely to involve glucose dynamics just like self-

control. It is also plausible that there are some additional brain

activities that would not involve the self but would still depend on

the glucose in the same way, though we do not know just what

those might be. For now, the most reasonable and parsimonious

formulation would seem to be that the agentic, effortful activities

of the self use significant amounts of glucose and are impaired

when the supply of glucose has been depleted.

Still, it seems likely that relatively few psychological processes

are as expensive as self-control in terms of requiring large amounts

of glucose. There are two reasons for thinking that self-control is

unusual, even if not unique. First, what it accomplishes is rather

advanced and difficult. We assume that the psychological system

evolved to want and do things. To interrupt and override these

well-established responses, especially after they are already in

process, seems quite difficult. In plain terms, an animal may have

evolved to feel and act on strong desires, such as for food or sex,

and so self-control requires an inner mechanism strong enough to

counter those powerful responses. Second, the widespread occur-

rence of self-control failures is evidence that self-control is not

easy, and high metabolic cost would be one likely explanation for

this.

Not all self-control processes will be equally expensive, either.

The present results showed ample evidence of individual differ-

ences. Studies 2– 6 specifically showed that the same task depleted

some people more than others. These differences may reflect

differences in personality and values (as in Study 2) or in physi-

ological capabilities and response patterns. We also assume, but

have not shown, that different self-control tasks would be differ-

entially depleting even to the same person in the same circum-

stances. At its core, self-regulatory change involves overriding one

response in order to enable a different response. The stronger the

initial response or impulse, the more difficult the self-control task

will be—and, we would assume, the greater amount of energy in

the form of blood glucose the system would have to expend in

order to succeed.

Another implication is that self-control tasks that have a direct

impact on blood glucose may raise particular problems for self-

control. Most obviously, dieting essentially involves restricting

one’s caloric intake, and there may be an ironic conflict in which

the dietary restriction produces lower glucose, which, in turn,

undermines the willpower needed to refrain from eating. Further

research in self-control may explore how efforts to control some

behaviors paradoxically undercut the capacity for control by in-

terfering with the body’s glucose processes.

We do not wish to overstate the importance of glucose to

self-control. Self-control may have multiple physiological and

psychological links and processes. Sleep and rest, for example,

seem beneficial to self-control independently of caloric processes

(see Baumeister et al., 1994, for a review). Although it is possible

that their effects could be mediated by glucose, they may signal

that other factors can also be decisive. The present results none-

theless do point to glucose as important for self-control.

Moreover, despite our manipulations, we do not intend to ad-

vocate consuming large quantities of sugar as an ideal strategy for

improving self-control. Eating several candy bars, for instance,

may give one a boost of energy and better self-control, but these

benefits are likely to disappear when glucose levels eventually

drop. Protein or complex carbohydrates may be more effective for

sustained self-control. We used sugar in our studies because it is

fast-acting and convenient.

Concluding Remarks

It has long been known that action consumes energy. More

recent evidence has indicated that some brain and cognitive pro-

cesses likewise consume substantial amounts of energy—indeed,

some far more than others. The “last-in, first-out rule” states that

cognitive abilities that developed last ontogenetically are the first

to become impaired when cognitive and physiological resources

are compromised. Self-control, as a relatively advanced human

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