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Self-Control Relies on Glucose as a Limited Energy Source: Willpower Is

More Than a Metaphor

Matthew T. Gailliot, Roy F. Baumeister,

C. Nathan DeWall, Jon K. Maner, E. Ashby Plant,

Dianne M. Tice, and Lauren E. Brewer

Florida State University

Brandon J. Schmeichel

Texas A&M University

The present work suggests that self-control relies on glucose as a limited energy source. Laboratory tests

of self-control (i.e., the Stroop task, thought suppression, emotion regulation, attention control) and of

social behaviors (i.e., helping behavior, coping with thoughts of death, stifling prejudice during an

interracial interaction) showed that (a) acts of self-control reduced blood glucose levels, (b) low levels

of blood glucose after an initial self-control task predicted poor performance on a subsequent self-control

task, and (c) initial acts of self-control impaired performance on subsequent self-control tasks, but

consuming a glucose drink eliminated these impairments. Self-control requires a certain amount of

glucose to operate unimpaired. A single act of self-control causes glucose to drop below optimal levels,

thereby impairing subsequent attempts at self-control.

Keywords: self-regulation, glucose, attention, emotion regulation, prejudice

Self-control (or self-regulation) is the ability to control or over-

ride one’s thoughts, emotions, urges, and behavior. Self-control

allows for the flexibility necessary for successful goal attainment,

and it greatly facilitates adherence to morals, laws, social norms,

and other rules and regulations. As such, it is one of the most

important and beneficial processes in the human personality struc-

ture. A burgeoning body of evidence has linked good self-control

to a broad range of desirable outcomes, including healthier inter-

personal relationships, greater popularity, better mental health,

more effective coping skills, reduced aggression, and superior

academic performance, as well as less susceptibility to drug and

alcohol abuse, criminality, and eating disorders (DeWall,

Baumeister, Stillman, & Gailliot, in press; Duckworth & Selig-

man, 2005; Finkel & Campbell, 2001; Gailliot, Schmeichel, &

Baumeister, 2006; Gottfredson & Hirschi, 1990; Kahan, Polivy, &

Herman, 2003; Pratt & Cullen, 2000; Shoda, Mischel, & Peake,

1990; Tangney, Baumeister, & Boone, 2004; Vohs & Heatherton,

2000).

Self-control seems to rely on a limited energy or strength, such

that engaging in a single act of self-control impairs subsequent

attempts at self-control, as if some sort of energy had been used up

during the initial act (for reviews, see Baumeister, Gailliot, De-

Wall, & Oaten, in press; Muraven & Baumeister, 2000). Although

viewing self-control as an energy resource has served as a highly

convenient metaphor that explains a broad range of empirical

findings, the precise nature of the energy source of self-control has

remained unspecified. In the present, we examined whether self-

control does indeed rely on an actual energy source, namely, blood

glucose.

Since Freud (1923/1961a, 1933/1961b), psychological theoriz-

ing about personality or the self has used energy models relatively

sparingly. Yet, the human body is undeniably an energy system,

and its very life depends on ingesting energy and then using it to

fuel its activities, including complex psychological processes. The

human brain consumes 20% of the body’s calories even though it

constitutes only 2% of the body’s mass (Dunbar, 1998). In order

for evolution to have selected in favor of such expensive psycho-

logical processes, those processes must have paid great adaptive

dividends to offset such a high cost in calories. The capacity for

self-control provides numerous benefits (e.g., Baumeister, 2005),

and so it is plausible that self-control may have been one psycho-

logical process that was immensely valuable despite being so

expensive in terms of caloric energy (glucose).

An accumulating amount of evidence is consistent with the

notion that self-control relies on some kind of energy. For instance,

after resisting the temptation to eat freshly baked cookies, partic-

ipants in one study quit sooner on a subsequent task requiring

effortful persistence, compared with participants who did not have

to resist eating the cookies (Baumeister, Bratslavsky, Muraven, &

Tice, 1998). Resisting the temptation to eat the cookies presum-

ably depleted an energy resource that could otherwise have been

used to persist on the subsequent task. A variety of other behaviors

have been found to rely on and deplete this energy source as well,

including managing one’s impression (Vohs, Baumeister, & Cia-

rocco, 2005), suppressing stereotypes and prejudice (Gordijn, Hin-

driks, Koomen, Dijksterhuis, & Van Knippenberg, 2004; Richeson

& Shelton, 2003; Richeson & Trawalter, 2005; Richeson,

Trawalter, & Shelton, 2005), coping with thoughts and fears of

dying (Gailliot et al., 2006), controlling one’s monetary spending

(Vohs & Faber, 2004), restraining aggression (DeWall et al., in

Matthew T. Gailliot, Roy F. Baumeister, C. Nathan DeWall, Jon K.

Maner, E. Ashby Plant, Dianne M. Tice, Lauren E. Brewer, Department of

Psychology, Florida State University; Brandon J. Schmeichel, Department

of Psychology, Texas A&M University.

Correspondence concerning this article should be addressed to Matthew

Gailliot or Roy Baumeister, Department of Psychology, Florida State

University, Tallahassee, FL 32306-1270. E-mail: gailliot@psy.fsu.edu or

baumeister@psy.fsu.edu

2007, Vol. 92, No. 2, 325–336 0022-3514/07/$12.00 DOI: 10.1037/0022-3514.92.2.325

325

press; Stucke & Baumeister, 2006), and managing one’s intake of

food and alcohol (Kahan et al., 2003; Muraven, Collins, & Nien-

haus, 2002; Muraven, Collins, Shiffman, & Paty, 2005; Vohs &

Heatherton, 2000).

Thus, there is ample evidence that self-control processes operate

as if they depend on some kind of limited energy resource. But

what might that energy resource actually be? Glucose may be one

facet of the energy dynamics of self-control.

Self-Control and Glucose

Glucose is one vital fuel for the brain. The brain’s activities rely

heavily on glucose for energy (e.g., Laughlin, 2004; Siesjo, 1978;

Weiss, 1986). The metabolization of glucose from the bloodstream

allows each brain region to carry out its given functions (e.g.,

McNay, McCarty, & Gold, 2001; Reivich & Alavi, 1983).

Even though nearly all of the brain’s activities consume some

glucose, most cognitive processes are relatively unaffected by

subtle or minor fluctuations in glucose levels within the normal or

healthy range. Controlled, effortful processes that rely on execu-

tive function, however, are unlike most other cognitive processes

in that they seem highly susceptible to normal fluctuations in

glucose. For instance, low glucose has been linked with impaired

performance on difficult (incongruent) but not easy (congruent)

trials of the Stroop color word interference task (Benton, Owens,

& Parker, 1994) and on complex but not simple reaction time tasks

(Owens & Benton, 1994). One study found that low glucose was

associated with poor performance on a driving simulation task, but

only toward the end of the task, when participants were fatigued

and the task was most demanding (as cited in Benton, 1990). Low

glucose, therefore, seems to impair controlled or effortful pro-

cesses but not the simpler or automatic processes, most likely

because controlled processes require more glucose than automatic

processes (Fairclough & Houston, 2004).

Self-control relies on controlled or executive processes in that

the self must effortfully override urges, thoughts, emotions, and

habitual or automatic response tendencies. Self-control, therefore,

may be highly susceptible to fluctuations in glucose. Indeed,

indirect evidence suggests that self-control failure may be more

likely when glucose is low or when glucose is not transported

effectively from the body to the brain. For instance, poor self-

control is one of the leading causes of criminal behavior (Gottfred-

son & Hirschi, 1990; Pratt & Cullen, 2000), and several studies

have linked criminal behavior to decrements in the processing of

glucose (e.g., Bolton, 1979; Virkkunen & Huttunen, 1982). Prob-

lems with glucose have been associated with increases in aggres-

sion and impulsivity (Donohoe & Benton, 1999; Lustman, Frank,

& McGill, 1991) and with decrements in concentration and emo-

tion regulation (Benton & Owens, 1993; Benton et al., 1994).

Alcohol impairs many forms of self-control (Baumeister, Heath-

erton, & Tice, 1994), and likewise, alcohol reduces levels of

glucose in the brain and body (Altura, Altura, Zhang, & Zakhari,

1996). Failures at self-control are more likely later in the evening

than during the day (Baumeister et al., 1994), and glucose is used

less effectively later in the evening than during the day (Van

Cauter, Polonsky, & Scheen, 1997). Glucose has also been found

to facilitate coping with stress (Simpson, Cox, & Rothschild, 1974)

and quitting smoking (West, 2001). These links between self-

control and glucose suggest that glucose may be an important

component of the energy source on which self-control relies.

Overview and Hypotheses of the Present Work

We used nine studies to test the hypothesis that decrements in

self-control are caused in part by low glucose. This relatively large

number of studies allowed us to provide converging multimethod

evidence that would demonstrate the effects of glucose on a broad

range of self-control behaviors and rule out potential alternative

explanations. We hypothesized that completing a self-control task

would use up a relatively large amount of glucose, compared with

completing a cognitive task that does not require self-control.

Because there exists an equilibrium between glucose in the blood-

stream and the brain (Lund-Anderson, 1979), low blood glucose

levels after an initial self-control task were then predicted to impair

performance on subsequent self-control tasks, insofar as available

quantities of glucose are too low for self-control to function

unimpaired, and possibly because the self starts to avoid effortful

activities in order to conserve its reduced remaining stock of

glucose (Muraven, Shmueli, & Burkley, 2006). Restoring glucose

to higher and optimal levels should replenish the ability to exert

self-control.

The first step in the present investigation was to show that acts

of self-control reduce the level of glucose in the bloodstream. In

Studies 1 and 2, we examined whether completing a task that

required self-control, as compared with a task that did not require

self-control, would cause a drop in levels of glucose in the blood.

Next, we sought to link reduced glucose to the behavioral signs of

prior self-control exertion. In Studies 3– 6, we tested the hypoth-

esis that low glucose after an initial self-control task would be

associated with greater self-control impairments, in the form of

poorer performance on a subsequent self-control task. Having

established that glucose correlated with greater impairments to

self-control, we then turned to showing a causal role for glucose.

More precisely, we aimed to show that experimental manipulations

of glucose levels (administration of glucose drinks) would coun-

teract the effects of prior exertions of self-control (see Studies

7–9). We predicted that participants who performed the initial

self-control task would exhibit the typical pattern of performing

worse than others on the second task but that receiving a glucose

drink would reduce this effect.

Two of the major goals of this investigation were (a) to establish

that blood glucose levels are reduced from before to after perform-

ing an initial self-control task and (b) to show that low levels of

glucose after a first self-control task predict behavioral deficits on

a second self-control task. In theory, we may have been able to do

both of these in the same study. In practice, methodological

complications rendered the two goals somewhat incompatible.

Fasting facilitates (a) but interferes with (b). If participants have

eaten recently, then some glucose may be entering the bloodstream

at unpredictable intervals (possibly even when we might be as-

sessing their glucose levels after the first self-control task), so the

level may be rising when our hypothesis would predict a decrease,

even if our hypothesis were completely correct. Hence, in Studies

1 and 2, we had participants fast prior to the experiment so their

blood glucose would be stable apart from the impact of our

manipulations. However, there is some evidence that hungry par-

326

GAILLIOT ET AL.

ticipants already perform poorly on self-control tasks,

which

made it much harder to find behavioral effects of a laboratory

manipulation of self-control depletion. Therefore, we did not re-

quire fasting in Studies 3– 6 in order to obtain better behavioral

data.

Study 1

Study 1 provided an initial test of the hypothesis that exerting

self-control uses up a relatively large amount of glucose. Partici-

pants completed a task that either did or did not require self-

control. At the end of this task, we assessed glucose levels. We

predicted that the self-control task would diminish glucose relative

to the task that did not require self-control.

Method

Participants. Participants were 110 undergraduates (69

women and 1 unreported) enrolled in an introductory psychology

course. Seven participants indicated having medical conditions

related to glucose (e.g., diabetes), and so their data were discarded

from all analyses. This left a final sample of 103 participants (64

women and 1 unreported). Participants in this and all subsequent

studies received credit toward fulfilling a course requirement.

Participants were instructed not to eat for 3 hr prior to arriving

at the experiment. Glucose levels fluctuate regularly throughout

the day as a result of eating (and for rather long and variable

intervals after eating). Requiring participants to refrain from eating

thus allowed glucose levels to stabilize, which greatly reduced

extraneous variance in glucose measurement.

Procedure. Participants were run individually and were told

the study was investigating physiological measures and task per-

formance. First, the experimenter assessed baseline blood glucose

levels. Blood samples were taken with single-use blood sampling

lancets. Blood glucose levels were measured (mg/dL) using an

Accu-Chek compact meter.

Next, participants completed the video task that served as the

manipulation of self-control exertion. Participants watched a 6-min

video (without sound) of a woman talking (modified from Gilbert,

Krull, & Pelham, 1988). In the bottom corner of the screen,

common one-syllable words (e.g., hair, hat, pulse) appeared indi-

vidually for 10 s. Participants randomly assigned to the attention

control condition were instructed to focus their attention only on

the woman’s face and to refrain from looking at the words. If they

happened to look at the words, then they were to refocus their

attention on the woman as quickly as possible. Participants ran-

domly assigned to the watch normally condition were instructed to

watch the video as they would normally. After completing this

task, the experimenter assessed blood glucose levels a second time.

In Studies 1– 8, we assessed current mood valence and arousal

after participants completed the initial self-control task or prior to

any subsequent self-control task. Participants completed either the

Brief Mood Introspection Scale (Mayer & Gaschke, 1988), the

Positive and Negative Affect Schedule (Watson, Clark, & Telle-

gen, 1988), or the UWIST Mood Adjective Checklist (Matthews,

Jones, & Chamberlain, 1990). None of the mood measures (in-

cluding the Valence and Arousal subscales) had a significant effect

on the dependent measure in any study. To save space, we do not

report all these null findings individually. In each study, partici-

pants were thanked and debriefed at the end of the experiment.

Results and Discussion

Analyses confirmed that the self-control task used up a rela-

tively large amount of glucose. A 2 ⫻ 2 mixed model analysis of

variance (ANOVA) indicated a significant interaction between

attention control condition and time of measurement, F(1, 100) ⫽

6.08, p ⬍ .05. Among participants in the attention control condi-

tion, glucose was lower after the video task (M ⫽ 101.22, SD ⫽

18.34) than before (M ⫽ 107.10, SD ⫽ 21.02), t(50) ⫽⫺2.57, p ⬍

.05. Among participants in the watch normally condition, glucose

levels did not differ from before (M ⫽ 102.24, SD ⫽ 21.20) to

after (M ⫽ 103.24, SD ⫽ 18.71) the video task (t ⬍ 1). Thus, all

participants watched the same video, but glucose levels dropped

only among participants who had to exert self-control while watch-

ing.

These results are consistent with the notion that exerting self-

control uses up a relatively large amount of glucose. Blood glucose

levels were lower after participants regulated their attention while

watching a video—lower than their own levels before the video

and lower than those of participants who had just watched the

same video without controlling attention.

Study 2

The purpose of Study 2 was to provide additional evidence that

self-control uses enough glucose to partially deplete the supply in

the bloodstream. Suppressing stereotypes or prejudice, such as

during the course of an interracial interaction, has been shown to

deplete self-control strength (Gordijn et al., 2004; Richeson &

Shelton, 2003; Richeson & Trawalter, 2005; Richeson et al., 2005;

see also Richeson et al., 2003). Therefore, we examined in Study

2 whether an interracial interaction would deplete glucose.

Restraining prejudice may be more difficult for some people

than for others. Therefore, Study 2 measured individual differ-

ences in internal motivation to respond without prejudice (Plant &

In support of the notion that hunger obscures the effects of self-control

exertion on subsequent behavior, a pilot study found that hunger impaired

future self-control to the same extent as did prior self-control exertion.

Specifically, participants (N ⫽ 27) were instructed to arrive hungry to the

laboratory and completed the same video task used in Study 1. They were

randomly assigned to one of three conditions: hunger/attention control,

hunger/watch normally, or no hunger/watch normally. Participants in the

no-hunger/watch normally condition received an orange juice drink and a

muffin bar to eat after the video task. Participants in the other two

conditions proceeded immediately to the next part of the experiment.

Next, participants completed the Stroop task for 3 min. The number of

trials completed (i.e., speed) on this task served as the dependent measure

of Stroop performance.

A one-way ANOVA indicated a main effect of condition, F(1, 24) ⫽

4.22, p ⬍ .05. Participants in the no-hunger/watch normally condition

(M ⫽ 198.88, SD ⫽ 8.72) completed more Stroop trials than participants

in the hunger/watch normally (M ⫽ 168.60, SD ⫽ 7.80) and no-hunger/

attention control (M ⫽ 168.78, SD ⫽ 8.22) conditions. The difference

between the hunger/watch normally and no-hunger/attention control con-

ditions was not significant (t ⬍ 1). Thus, the Stroop measure suggested that

self-regulatory performance was impaired to about the same degree by

prior self-regulation as by hunger. These results are also consistent with the

hypothesis that poor self-control is caused by low glucose, insofar as

hunger is associated with low glucose (e.g., Cox, Eickhoff, Gonder-

Frederick, & Clarke, 1993).

327

SELF-CONTROL AND GLUCOSE

Devine, 1998), which were predicted to moderate the consumption

of glucose during an interracial interaction. Internal motivation

reflects the desire to respond without prejudice because of the

personal importance of endorsing nonprejudiced beliefs. Internally

motivated people are less likely than others to respond with racial

bias across all sorts of situations. For them, suppressing prejudice

should be a well practiced and hence presumably automatic way of

acting, and therefore it should be easier for them to avoid express-

ing bias in our laboratory (Plant, 2004). In contrast, people who are

low in internal motivation to avoid prejudice prefer to avoid

interracial interactions than to struggle to suppress their views

while in them (Plant, 2004), and so when they find themselves

having to suppress prejudice in a sensitive situation, they may have

to exert considerable self-control to speak and act appropriately.

For them, unlike the others, stifling prejudicial thoughts may

require effortful self-control rather than falling back on a habitual

pattern. We therefore expected that an interracial interaction might

reduce glucose primarily among participants low in internal mo-

tivation to respond without prejudice.

Method

Participants. The final sample included 38 White college un-

dergraduates (29 women). We excluded from all analyses 1 par-

ticipant because he was diabetic. Participants were randomly as-

signed to interact with either a Black or a White experimenter.

Participants were instructed not to eat for 3 hr prior to arriving at

the experiment.

Procedure. Upon arrival at the laboratory, participants were

greeted by a Hispanic female experimenter. Participants were told

that the study was examining factors related to different tasks and

physiological measures.

First, we assessed initial glucose levels. Next, participants had a

5-min conversation with either a Black or a White female exper-

imenter. After introducing themselves, participants were asked to

state their opinions on affirmative action and criminal profiling (in

counterbalanced order) and were given 2 min to discuss each topic.

These topics were chosen because they involve unequal treatment

of individuals on the basis of race and would therefore be likely to

activate racial stereotypes.

Glucose levels then were assessed a second time. Last, partici-

pants completed a questionnaire on which they indicated the extent

to which they felt like they exerted effort during the interaction so

as to avoid saying anything negative, using a 9-point scale ranging

from 1 (very little)to9(a lot). The questionnaire also contained

the Internal Motivation to Respond Without Prejudice scale (IMS;

Plant & Devine, 1998), which contains five items (e.g., “Because

of my personal values, I believe that using stereotypes about

Blacks is wrong”).

Results and Discussion

Glucose levels after interaction. The hypothesis was that dis-

cussing racially sensitive material with a member of another race

would require self-control and therefore deplete glucose—mainly

for people who do not habitually stifle prejudicial thoughts and

feelings (i.e., for people low in IMS). A regression analysis that

regressed standardized IMS, condition (same-race vs. interracial

interaction), and their interaction upon postinteraction glucose

levels yielded the predicted significant interaction between IMS

and experimental condition (b ⫽ 4.41), t(1, 33) ⫽ 2.20, p ⬍ .05

(see Figure 1). The postinteraction glucose levels were controlled

for baseline (preinteraction) glucose levels, which did not differ by

condition or IMS (ts ⬍ 1). To interpret the interaction, we assessed

the simple effect of condition among participants who were rela-

tively high versus low in IMS (one standard deviation above and

below the mean on IMS, respectively; Aiken & West, 1991).

Results indicated that the effect of condition was significant for

low-IMS participants (b ⫽⫺3.28), t(1, 33) ⫽⫺2.33, p ⬍ .05, but

not for high-IMS participants (b ⫽ 1.16), t(1, 33) ⫽ 0.92, p ⫽ .36.

Thus, discussing a sensitive topic with a member of a different race

used up a significant amount of glucose among people with low

IMS. Glucose was not depleted in people who discussed the same

topics with members of their own race or among people who are

dispositionally motivated to stifle prejudicial thoughts and feel-

ings. This pattern is consistent with the view that acts of self-

control deplete blood glucose.

Effort. Interracial interactions require self-control because one

often exerts effort to avoid expressing negative attitudes or opin-

ions (Richeson & Trawalter, 2005). In support of this, in the

interracial interaction condition, IMS scores predicted self-

reported effort, r(21) ⫽⫺.48, p ⬍ .05; effort predicted postinter-

action glucose levels (controlling for preinteraction glucose levels)

r(18) ⫽⫺.54, p ⬍ .05; and a Sobel test for mediation pointed

toward the conclusion that effort mediated the effect of IMS on

postinteraction glucose levels, although it fell short of two-tailed

significance (z ⫽ 1.79, p ⫽ .07). Glucose dropped primarily

among low-IMS participants because they found the interracial

interaction more effortful than did high-IMS participants. Effort

did not appear to mediate the effect of IMS in the same-race

condition (z ⫽⫺.20, ns), and the preconditions for mediation were

not met either. These findings suggest that the self-regulatory

effort needed to avoid negative responses during an interracial

interaction depletes an energy source on which self-control relies

(i.e., glucose).

Internal Motivation to Respond Without Prejudice

-4.00

-2.00

0.00

Low

High

Other-Race

Same-Race

Glucose

Figure 1. Glucose levels after an interaction (controlling for glucose

levels before the interaction) as a function of interaction condition and

internal motivation to respond without prejudice (see Study 2).

328

GAILLIOT ET AL.

Study 3

With Study 3, we began testing the hypothesis that low levels of

blood glucose following a self-control task would predict poor

performance on behavioral measures of self-control. In Studies

3– 6, all participants were subjected to an initial depleting task, and

we assumed that these would be more depleting to some than to

others. The prediction for Studies 3– 6 was that the participants

with the lowest levels of blood glucose would perform worst at

self-control. After measuring blood glucose, we used the attention

control task from Study 1 to create depletion. Following a second

glucose measurement, participants completed the Stroop task,

which is one of the most frequently used measures of self-control

(e.g., Richeson & Shelton, 2003; von Hippel & Gonsalkorale,

2005). The Stroop task requires the participant to override an

incipient response (i.e., to read aloud the name of the word) in

order to say instead the color in which the word is printed, and in

that sense it requires self-regulation. Lower blood glucose should

impair Stroop performance in the sense of causing the person to

take longer to get the right answer and in terms of making more

errors along the way.

Method

Participants. Participants were 16 college undergraduates (12

women). We excluded from this sample 1 additional participant

because of equipment malfunction.

Procedure. Participants were told that the study was examin-

ing the relationship between physiological factors and task perfor-

mance. First, baseline blood glucose levels were assessed. Next,

participants completed the attention control task used in Study 1.

All participants were instructed to exert self-control by refraining

from looking at the words in the video. At the end of the task,

glucose levels were assessed a second time.

Last, participants completed the Stroop task. They were shown

words (i.e., red, green, and blue) that appeared in a font color (i.e.,

red, green, or blue) that diverged from the meaning of the word

(e.g., red appeared in blue ink). Participants completed 80 trials for

which they were to state aloud the color ink that each word

appeared in and to refrain from reading the word. The amount of

time participants took to complete the Stroop task (i.e., speed) and

the number of errors (i.e., accuracy) constituted the dependent

measures of Stroop performance. The assumption behind that

measure is that effective self-control enables the person to override

the initial response to say the word so as to be able to state the

color of the ink. When self-control is weak or ineffective, the

person takes longer to override the initial (wrong) response, or the

person makes more errors.

Results and Discussion

Glucose levels at the start of the experiment did not predict

Stroop performance (i.e., neither time to complete the Stroop nor

errors; ⫺.20 ⬍ rs ⱕ ⫺.13, ps ⬎ .48). In contrast, lower glucose

after having watched the video was associated with poorer Stroop

performance (i.e., taking more time to complete the Stroop task),

r(14) ⫽⫺.62, p ⬍ .05, and this relationship remained significant

after controlling for baseline glucose levels, r(12) ⫽⫺.66, p ⬍

.05. Errors on the Stroop task showed a similar though nonsignif-

icant pattern, such that lower glucose was associated with making

more errors, r(14) ⫽⫺.23, ns.

These findings are consistent with the idea that self-control

impairments stemming from a prior act of self-control are attrib-

utable to low glucose. Participants with less glucose after an initial

self-control task performed worse at a subsequent self-control task.

One alternative explanation for these findings is that low glu-

cose simply made participants slow to respond rather than impair-

ing their self-control. In Studies 4 – 6, we addressed this limitation

by using a dependent measure that was not directly related to speed

of responding.

Studies 4 – 6

Studies 4 – 6 provided conceptual replications of Study 3. In

each study, we had participants engage in a self-control task

designed to impair their self-control afterwards. Participants either

controlled their attention (see Study 4), completed the Stroop task

(see Study 5), or regulated their emotions (see Study 6). Regarding

the last of these, emotion regulation is a common, well-recognized

form of self-control and likewise has been shown to impair later

attempts at self-control (e.g., Muraven, Tice, & Baumeister, 1998).

In each study, blood glucose was measured before and after the

initial self-control task.

Last, participants were assigned to perform a figure-tracing task.

Unbeknownst to participants, the task was unsolvable, and we

timed how long they persisted. Persistence at the unsolvable task

requires self-control because the discouraging, frustrating failures

at the task give rise to impulses to quit, which the person must

override in order to continue striving on the task (see Baumeister

et al., 1998). We predicted that low glucose after the initial

self-control task would be associated with less effortful persis-

tence.

Method

Participants. Study 4 had a final sample of 12 participants (10

women). Two additional participants were excluded, 1 because a

second glucose reading could not be obtained and the other be-

cause the participant erroneously claimed to have solved the

figure-tracing task. Study 5 had a final sample of 23 participants

(15 women). Four additional participants were excluded, 1 because

a second glucose reading could not be obtained and 3 others who

either suspected the figure-tracing task was unsolvable or errone-

ously claimed to have solved it. The final sample in Study 6 was

17 participants (7 women). Two additional participants were ex-

cluded from analyses. One of the excluded participants did not

complete the emotion regulation task, and the other was an outlier

(three standard deviations above the mean) on the dependent

measure.

Procedure. Participants were told that the study was investi-

gating the relationship between physiological factors and task

performance. Participants first provided a blood sample to assess

glucose level. Next, they either completed the same attention

control task used in the previous studies (see Study 4), the Stroop

task used in Study 3 for 4 min (see Study 5), or an emotion

regulation task (see Study 6). For the emotion regulation task,

participants watched a 2-min video clip of animals in a slaughter-

house and a 2-min video clip of the comedy show Jay Leno.

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SELF-CONTROL AND GLUCOSE

Participants in Study 6 were instructed to suppress any emotional

feelings or reactions they had while watching the clips. After their

respective task, participants provided a second sample of blood to

assess glucose.

Last, all participants completed the figure-tracing task, adapted

from Glass, Singer, and Friedman (1969) and Feather (1961).

Participants were given a figure and asked to trace the lines of the

figure without lifting their pen. Participants were told that they

would be given five figures in total and were to solve each of them

as quickly as possible. They were to notify the experimenter as

soon as they solved each figure or decided that they could not and

wanted to give up. The first figure given to participants, however,

was unsolvable. Participants were surreptitiously timed. Partici-

pants were given a maximum of 20 min for the task, at which time

any participants still working were asked to stop.

Results and Discussion

Glucose levels at the start of each experiment were not signif-

icantly related to persistence (all rs ⬍ .27, ps ⬎ .24). Low glucose

after the initial self-control task, however, was associated with

persisting less on the figure-tracing task in all three studies; Study

4 r(10) ⫽ .56, p ⬍ .05; Study 5 r(21) ⫽ .45, p ⬍ .05; Study 6

r(15) ⫽ .43, p ⬍ .05. (Because these studies were replications, we

used one-tailed tests to examine our hypothesis.) Furthermore, the

correlation between glucose levels after the initial task and persis-

tence remained significant when controlling for glucose levels at

the start of each experiment ( ps ⬍ .05). The link between posttest

glucose levels and persistence, therefore, did not appear attribut-

able to initial glucose levels.

Although the samples in these three studies were small, the

magnitude of the correlations was consistently quite robust. The

consistency of findings across the three different samples and

methods lends confidence to the conclusion that low levels of

glucose are associated with decrements in self-control.

Moreover, these results help to refute the alternative explanation

for Study 3 that low glucose simply makes people generally slower

at tasks rather than impairing their self-control. If low glucose only

made people slow at various tasks, then one might have expected

that low glucose would have predicted greater persistence (i.e.,

being slower to give up).

Study 7

The results thus far support the hypothesis that impairments to

self-control following an initial self-control task are linked to low

glucose in the bloodstream. Studies 1 and 2 showed that exerting

self-control reduced blood glucose levels. Studies 3– 6 showed that

lower glucose after an initial exertion of self-control predicted

poorer self-control on the second task (e.g., quitting sooner on the

frustrating puzzle). The remaining studies sought to provide evi-

dence of a causal relationship between glucose and self-control by

using direct manipulations of blood glucose, namely, drinking a

glucose beverage. Glucose is absorbed into the bloodstream at a

rate of 30 calories per minute and after about 10 min can be

metabolized to the brain (Donohoe & Benton, 1999). Hence,

glucose drinks are a viable means of increasing the amount of

glucose available for self-regulatory tasks, provided that one al-

lows some time for the glucose to reach the bloodstream. Partic-

ipants in Study 7 first completed either a task that required self-

control (attention control as in the previous studies) or a task that

did not require self-control.

Participants then drank Kool-Aid lemonade that had been sweet-

ened either with sugar (and hence glucose) or Splenda (a good-

tasting sugar substitute that does not increase blood glucose). The

sugar lemonade shake should restore glucose and therefore would

be expected to replenish self-control strength after the initial

self-control task.

Hence, we predicted that participants who controlled their at-

tention during the video task would perform worse on a subsequent

Stroop task than participants who did not control their attention,

but that a glucose drink would attenuate this effect.

Method

Participants. Participants were 62 college undergraduates (49

women and 1 unreported). We excluded from this final sample 1

additional participant who chose not to consume the drink. Partic-

ipants were randomly assigned to depletion and glucose condi-

tions. They were randomly assigned among the conditions. The

two variables (attention control and lemonade content) were ma-

nipulated orthogonally.

Procedure. Participants were told that the study was examin-

ing factors related to different tasks and food. Participants first

completed 20 Stroop trials (as in Study 3) as a baseline measure of

Stroop ability. They then were administered the attention control

manipulation used in Study 1, with half simply watching the video

and the other half being instructed to keep their attention focused

on the woman and not on the words while watching it. Study 7

added a manipulation check. After watching the tape, participants

rated how often they had looked at the woman and the words,

respectively.

Next, participants were given 14 ounces of lemonade sweetened

with either sugar (glucose condition) or a sugar substitute (placebo

condition). The glucose drink contained approximately 140 calo-

ries, whereas the placebo contained 0 calories. Participants and the

experimenter were blind to condition. Participants consumed the

glucose drink and then completed three measures of liking for the

drink (e.g., “How pleasant was it for you while drinking the

beverage?”; ␣⫽.63) that were embedded among other filler

measures intended to bolster the cover story regarding the taste and

appearance of the drink. Participants then completed filler ques-

tionnaires for 12 min to allow the glucose from the drink (if they

had any) to be metabolized (Donohoe & Benton, 1999). Last,

participants completed 80 Stroop trials separated into four blocks.

Speed and errors on the Stroop task constituted the dependent

measures of self-control performance.

Results and Discussion

Manipulation check. Participants appeared to be successful in

following the video task instructions. Participants in the attention

control condition reported having looked at the words (M ⫽ 2.03,

SD ⫽ 0.68) less often and at the woman (M ⫽ 6.72, SD ⫽ 0.45)

more often than participants in the watch normally condition

reported having looked at the words (M ⫽ 4.76, SD ⫽ 1.95) and

woman (M ⫽ 5.12, SD ⫽ 1.44), respectively (ts ⬎ 4.30, ps ⬍

.001).

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GAILLIOT ET AL.

Stroop performance. A 2 (self-control condition) ⫻ 2 (glucose

condition) between-subjects analysis of covariance (ANCOVA) on

the number of errors made on the final Stroop task (controlling for

errors at baseline) indicated that the main effects of self-control

condition and glucose condition were not significant (Fs ⬍ 2.23,

ps ⬎ .14). Their interaction, however, was significant, F(1, 57) ⫽

5.04, p ⬍ .05, and consistent with predictions (see Figure 2 for

adjusted means). In the placebo condition, attention control par-

ticipants made more errors than watch normally participants, F(1,

58) ⫽ 5.14, p ⬍ .05, d ⫽ 0.73. This, however, was not the case in

the glucose condition (F ⫽ 1.12, ns). A glucose drink thus elim-

inated the tendency for an initial self-control task to impair Stroop

performance, consistent with the hypothesis that glucose replen-

ishes what has been depleted.

A 2 (self-control condition) ⫻ 2 (glucose condition) ANOVA

on speed on the Stroop indicated no significant effects (Fs ⬍ 1.76,

ps ⱖ .19). To be sure, there was no speed–accuracy trade-off, such

as if attention control participants in the placebo condition made

more errors because they responded faster. The above interaction

between self-control condition and glucose condition on the num-

ber of errors on the Stroop remained significant even when con-

trolling for speed, F(1, 56) ⫽ 6.18, p ⬍ .05.

This pattern of results diverges from Study 3, in which speed on

the Stroop rather than errors was more sensitive to glucose. This

difference is probably attributable to the different methods used in

Studies 3 and 8. In Study 3, participants completed 80 Stroop trials

without a break, and so performing quickly (rather than making

errors) was probably more onerous and hence more sensitive to

glucose. In Study 7, the Stroop trials were divided into four blocks.

The delay between blocks probably allowed participants to main-

tain their speed, and so avoiding errors was probably more oner-

ous.

Liking for the drinks. The effects of depletion and glucose did

not appear to be the result of liking for the drinks. In particular, the

interaction between self-control condition and glucose condition

on the number of errors on the Stroop approached significance

when controlling for liking (F ⫽ 3.47, p ⫽ .07).

Study 8

Study 8 sought additional evidence that increasing glucose

levels can counteract the detrimental effects of prior efforts at

self-control. More precisely, in Study 8, we examined whether

glucose would improve self-control among people who have coped

with thoughts and fears of dying. The thought of death can be

aversive and threatening, and people therefore seem motivated to

avoid thinking about death (e.g., Aries, 1981; Becker, 1973). Past

research has indicated that avoiding the thought of death is effort-

ful and demanding (Arndt, Greenberg, Solomon, Pyszczynski, &

Simon, 1997; Greenberg, Arndt, Schimel, Pyszczynski, & So-

lomon, 2001) and requires self-control (Gailliot et al., 2006).

Likewise, thinking about death (i.e., mortality salience) has been

found to impair subsequent performance on tasks that require

self-control (e.g., the Stroop task), presumably because the self-

control expended to cope with the thought of death depletes the

energy source on which self-control relies (Gailliot et al., 2006).

To assess whether glucose underlies self-control, we examined

whether a glucose drink would benefit self-control after a mortality

salience induction.

First, participants received either a glucose drink or a placebo.

We administered it first on the assumption that the glucose would

start to reach the bloodstream around the time participants com-

pleted the next task, at which point those who had coped with

thoughts of death would otherwise be suffering from depleted

glucose. They then wrote about either death or a control topic

(dental pain). For the subsequent self-control task, participants

were given a sheet filled with word fragments and were asked to

solve them all. The word fragments were relatively easy, and with

enough persistence (self-control), one could eventually solve each

one by substituting into the word fragment different letters in the

alphabet. Consistent with the idea that coping with thoughts of

death impairs self-control afterwards, Gailliot et al. (2006) found

that mortality salience caused participants to leave more word

fragments unsolved, suggesting that mortality salience activated

thoughts of death, and the person had to use his or her resources in

order to defend against that threat, resulting in less resources

available for making oneself persist on the word fragment task.

The present experiment was designed to extend this finding by

showing that a glucose drink attenuates this effect.

Method

Participants. Participants were 73 undergraduates (51

women). One additional participant was excluded from all analy-

ses because this participant declined the drink. Participants were

randomly assigned to mortality salience versus dental pain and

glucose drink versus placebo conditions.

Procedure. Participants were told that the study was investi-

gating food and personality. First, participants consumed either a

glucose drink or a placebo and completed measures of taste and

liking for the drink. The drinks and measures were the same as

those used in Study 7.

Next, participants in the mortality salience condition were asked

to describe the emotions that the thought of their own death

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Glucose Placebo

Glucose Condition

Errors on Stroop

Watch Normally

Control Attention

Figure 2. Stroop performance as a function of self-control and glucose

conditions (see Study 7).

331

SELF-CONTROL AND GLUCOSE

aroused in them and to write about what would happen to their

bodies as they physically die (see Rosenblatt, Greenberg, So-

lomon, Pyszczynski, & Lyon, 1989). Participants in the dental pain

condition answered parallel questions about dental pain.

Participants next worked on a crossword puzzle and completed

a filler questionnaire. These tasks took approximately 6 min. This

delay was intended to allow participants to cope with any thoughts

of death they might have had and hence consume the energy on

which self-control relies (see Gailliot et al., 2006).

Participants then received a sheet that contained 20 word frag-

ments (e.g., —WER) and were asked to solve all of them. The

number of word fragments left unsolved constituted the dependent

measure of self-control. The word fragments were relatively easy

to solve, and with enough persistence (self-control), participants

could have solved them all. Therefore, we used the number of

unsolved fragments as the measure of poor self-control.

Last, participants completed the 33-item Marlowe-Crowne So-

cial Desirability Scale (Crowne & Marlowe, 1960), which allowed

us to assess whether any differences in the number of unsolved

word fragments may have been attributable to response bias rather

than to poor self-control. Participants then estimated the number of

calories they thought that their drink had contained, as a check of

whether there was any discernible difference in taste between the

glucose and placebo drinks that may have influenced persistence at

solving the word fragments.

Results and Discussion

Unsolved word fragments. We predicted that mortality sa-

lience would cause participants to leave more word fragments

unsolved but that a glucose drink would eliminate this effect, and

hence that mortality salience participants who consumed the pla-

cebo drink would leave more word fragments unsolved than any

other group of participants. A 2 (mortality salience condition) ⫻ 2

(glucose condition) ANOVA on the number of unsolved word

fragments produced no significant effects (Fs ⬍ 2.02, ns). To test

our specific prediction, which constituted a theoretical replication

of Study 7, we conducted a planned comparison, which confirmed

our prediction, F(1, 69) ⫽ 5.45, p ⬍ .05 (see Figure 3).

In the placebo condition, mortality salience participants left

unsolved more word fragments than did dental pain participants,

F(1, 69) ⫽ 3.91, p ⫽ .05, d ⫽ 0.33. Thus, mortality salience

impaired self-control. In the glucose drink condition, mortality

salience participants did not differ from dental pain participants in

the number of word fragments left unsolved (F ⬍ 1). A glucose

drink thus eliminated the detrimental effects of mortality salience

on self-control. These results suggest that the self-regulatory im-

pairments following mortality salience may be at least partly

attributable to low glucose.

Social desirability, calories, and ratings of liking. Analyses

indicated that the obtained pattern of results was likely not attrib-

utable to differential liking for the drinks, social desirability, or

estimations of the caloric content of the drinks. For instance, the

number of word fragments left unsolved was not significantly

related to any of these factors ( ps ⬎ .53; p ⫽ .21 for social

desirability), and the planned comparison reported above remained

significant or approached significance when controlling for each of

these factors ( ps ⱕ .06). Moreover, estimations of the caloric

content of the drinks did not differ between those who consumed

the glucose drink and those who consumed the placebo (F ⬍ 1).

Study 9

In the final study in this investigation, we sought to apply the

link between glucose and self-control to an important form of

interpersonal behavior, namely, helping. Helping is oftentimes

effortful (see, e.g., Dawkins, 1976) and hence requires some

amount of self-control. Indeed, previous studies show that an

initial act of self-control reduces helpfulness later on (Gailliot,

Maner, DeWall, & Baumeister, 2005) as well as other forms of

prosocial behavior (Finkel & Campbell, 2001; Vohs, 2004).

Study 9 was a field experiment. We had participants exert

self-control by taking an actual examination in their psychology

course. Previous work has confirmed that taking exams or engag-

ing in logical reasoning both require self-control (Gailliot et al.,

2006; Schmeichel, Vohs, & Baumeister, 2003), and other work has

shown that taking a difficult exam reduces glucose (Hall & Brown,

1979). Study 9 did not have a no-self-control (control) condition,

insofar as everyone took the exam, but we were able to assess

individual differences in the extent of self-control exertion. Spe-

cifically, the experimenter recorded the sequential order in which

participants finished the exam. Gailliot et al. (2005) found that

taking longer to complete an exam was associated with higher

scores on a questionnaire measure of impaired self-control (see

Twenge, Muraven, & Tice, 2004).

After finishing the exam, participants were randomly assigned

to receive a glass of lemonade that was either high or low in

glucose. Then their willingness to help others was measured with

two items: one involved donating money to charity, and the other

involved helping a fellow student with a housing problem. If

glucose underlies self-control and self-control facilitates helping,

then participants who consume the glucose rather than the placebo

drink should be more willing to help.

0.3

0.1

0.2

0.4

0.5

0.6

0.7

Glucose Placebo

Glucose Drink Condition

Unsolved Word Fragments

Dental Pain

Mortality Salience

Figure 3. Effortful persistence as a function of mortality salience and

glucose conditions (see Study 8).

332

GAILLIOT ET AL.

Method

Participants. Participants were 18 undergraduates (10 men).

We excluded from all analyses 1 additional participant who was an

outlier (nearly three standard deviations above the grand mean) on

the dependent measure. Participants were randomly assigned to a

glucose or placebo drink condition.

Procedure. Participants were told the study was investigating

the relationship between their attitudes and factors related to food.

First, participants completed a 26-item multiple-choice exam that

was an actual examination that counted toward the course grade.

The experimenter recorded the order in which participants finished

(e.g., first, second, third), as a measure of the duration of self-

control exertion.

The actual experiment began after participants finished their

exam. First, when participants turned in their exam, they were

handed either a glucose or placebo drink. Participants and the

experimenter were blind to condition. They then completed mea-

sures of liking for the drink (␣⫽.63) that were embedded among

other filler measures regarding the taste and appearance of the

drink intended to bolster the cover story. Participants then sat

quietly for approximately 14 min to allow the glucose to be

metabolized. (The reason for the delay was not explained to

participants.)

Last, participants responded to two hypothetical scenarios that

assessed their willingness to help strangers. For the first scenario,

participants were to indicate how much money they would donate

to a charity. For the second scenario, participants were told to

imagine that a stranger from their class had been evicted from his

or her apartment. Participants were to indicate, on a 7-point scale,

what they would do to help him or her, with higher points on the

scale representing more labor-intensive forms of helping. Specif-

ically, participants’ options were to: (1) do nothing, (2) give him or

her an apartment guide, (3) help him or her find a new place to live

by driving him or her around for a few hours, (4) offer to have him

or her come to stay with the participant for a couple of days

(provided the participant had space), (5) offer to have him or her

come stay with the participant for a week (provided the participant

had space), (6) offer to have him or her come stay with the

participant until he or she found a new place (provided the partic-

ipant had space), or (7) offer to have him or her come live with the

participant rent free (provided the participant had space). Re-

sponses to the two scenarios correlated moderately strong, r(16) ⫽

.63, p ⬍ .05, and so they were standardized and averaged to form

the final dependent measure of helping that ranged from ⫺.90 to

2.46.

Results and Discussion

Likelihood of helping. Participants in the glucose condition

(M ⫽ 0.48, SD ⫽ 0.95) indicated that they were willing to give

significantly more help than those in the placebo condition (M ⫽

⫺0.61, SD ⫽ 0.27), t(16) ⫽ 3.13, p ⬍ .01, d ⫽ 1.57. Thus, after

exerting self-control, consuming a glucose drink increased the

likelihood and extent of helping.

Exam effort and likelihood of helping. We used the order in

which participants completed the exam as a rough index of self-

regulatory effort expended on the exam. Exam completion order

does not appear to be related to intelligence or exam performance

(e.g., Bridges, 1985), and it therefore seemed reasonable that

taking longer on the exam reflected a greater expenditure of effort.

In other words, independent of ability, the students who spent

longer on the exam would have exerted effort for a longer period

of time and therefore were likely to have used more self-control

than the others.

Among those participants who received no glucose, likelihood

of helping correlated negatively and quite strongly with exam

completion order, r(6) ⫽⫺.83, p ⫽ .01. Those who took longer

on the exam offered less help. This fits the hypothesis that working

long and hard on the exam depletes some resource that could

otherwise be used to increase helpfulness. In the glucose condition,

however, the relationship between helping and completion order

was not significant, r(8) ⫽ .29, ns, and if anything, the relationship

was in the opposite direction. Moreover, the strength of this

relationship was significantly different than in the placebo condi-

tion (z ⫽ 2.52, p ⬍ .05). To the extent that taking longer on the

exam reflected expending more self-control effort, then it appears

that glucose eliminated the effect of self-control exertion (taking

an exam) on helping.

Ratings of liking. Ratings of liking for the drinks did not differ

between the glucose and placebo conditions (t ⬍ 1). Furthermore,

these ratings were not significantly related to helping (r ⬍ .22, ns).

It therefore does not appear that the difference in helping between

the two conditions was attributable to liking for the drinks. The

most parsimonious conclusion seems to be that glucose increased

helping by counteracting the effects of prior self-control exertion.

Because of the small sample size, however, we do present these

results with some caution.

General Discussion

Self-control is one of the most important and central processes

in personality and self-theory. Past work has shown that self-

control operates as if a limited resource is consumed or expended

in the process. The nature of such a resource, however, has

remained a matter of conjecture and metaphor. The goal of this

investigation was to flesh out that model by moving from meta-

phor to at least one plausible physiological process. Specifically,

we investigated the hypothesis that some patterns of poor self-

control are attributable to drops in glucose.

Three main sets of findings supported the hypothesis that self-

control depends on glucose. First, measurements of blood glucose

showed significant drops following acts of self-control (e.g., dur-

ing an interracial interaction), primarily among participants who

worked hardest. Second, low glucose after an initial self-control

task (e.g., emotion regulation) was linked to poor self-control on a

subsequent task. Third, experimental manipulations of glucose

reduced or eliminated self-control decrements stemming from an

initial self-control task.

By testing hypotheses using a variety of methods, the present

findings sought to rule out a variety of alternate explanations. The

improvements and impairments of self-control were not linked to

mood, arousal, or other emotional states. Neither the taste nor the

pleasure of the glucose snacks appeared to be a factor. The effects

were not limited to any particular sort of task or measure. In fact,

the procedures of these nine studies were deliberately set up to

encompass a variety of behaviors, including well-validated and

artificial laboratory measures (e.g., the Stroop task) and complex

social behaviors (e.g., willingness to help a stranger).

333

SELF-CONTROL AND GLUCOSE

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

334

GAILLIOT ET AL.

capacity, was probably one of the last to develop and hence may

be one of the first to suffer impairments when resources are

inadequate. The present findings suggest that relatively small acts

of self-control are sufficient to deplete the available supply of

glucose, thereby impairing the control of thought and behavior, at

least until the body can retrieve more glucose from its stores or

ingest more calories. More generally, the body’s variable ability to

mobilize glucose may be an important determinant of people’s

capacity to live up to their ideals, pursue their goals, and realize

their virtues.

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Received February 2, 2006

Revision received September 12, 2006

Accepted September 18, 2006 䡲

336

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Discussion

The brain’s glucose stores can compensate for only a few minutes of utilization. In effect, for glucose consumption, the brain is dependent on a constant supply from the blood circulation. A task where participants are asked to trace a figure without retracing any lines and without lifting their pen from the paper. ![puzzle](https://i.imgur.com/8ahyse5.png) #### ANOVA Anova stands for *Analysis of Variance*. It is a collection of statistical models developed by statistician and evolutionary biologist Ronald Fisher. The observed variance in a variable is partitioned into components attributable to the different sources of variation. A Mixed 2X2 ANOVA compares the mean differences between two groups that have been split on two factors. One factor is a within-subjects factor and the other is a between-subjects factor. In this case the within subjects factor is the time of measurement and the between subjects-factors is the attention control condition. If you want to learn more about this topic I would suggest [Chapter 11 of "Experimental Design and Analysis" by Howard Seltman](http://www.stat.cmu.edu/~hseltman/309/Book/chapter11.pdf) #### Brief Mood Introspection Scale The *Brief Mood Introspection Scale* (**BMIS**) is an open-source mood scale consisting of 16 mood adjectives to which a person responds (e.g., Are you “ Sad"?). The scale can yield measures of overall pleasant-unpleasant mood, arousal-calm mood, and it also can be scored according to positive-tired and negative-calm mood. #### Positive and Negative Affect Schedule In this context, affect refers to whether you feel emotionally in a positive or negative mood. The *Positive and Negative Affect Schedule* (**PANAS**) is a mood scale. PANAS requires subjects to take a very short survey to determine their mood score. #### UWIST Mood Adjective Checklist Is another mood scale similar to BMIS and PANAS A Stroop effect is a demonstration of interference in the reaction time of a task. Here is an example of a Stroop Task (where you can observe the stroop effect): Name the colors of the words bellow ( do not read the words, instead say the colors the words are written with): ![colors](https://faculty.washington.edu/chudler/colors2.gif) As you probably noticed, naming the color of the word takes longer and is more error prone than if the color of the ink matched the name of the color. The effect is named after John Ridley Stroop, [who first published the effect in English in 1935](http://psychclassics.yorku.ca/Stroop/). The original paper is one of the most cited papers in experimental psychology. ### Glucose and our body **Glucose** ($C_{6}H_{12}O_{6}$) is the main simple sugar that circulates in our blood. Our cells use it as fuel for most of its functions. Glucose is made during photosynthesis from carbon dioxide and water. When we eat carbohydrates, a portion of those are eventually reduced to their simplest form, glucose. Excess glucose can be toxic, therefore it is removed from our blood and stored as *glycogen* in our liver or converted to fat (via *Fatty Acid Synthesis*). Insulin is the hormone responsible for making sure that your blood sugar level doesn’t rise to toxic levels. When blood glucose levels drop, your liver will start converting glycogen stores into glucose. If your glycogen stores dwindle your body can also resort to generating glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids (aka *Gluconeogenesis*). This work has been captivating for those two decades since Baumeister, Bratslavsky, Muraven, & Tice, 1998, but this skepticism about the results is more profound than a fringe belief. From what I understand, ego depletion was one of the [hardest](https://www.wikiwand.com/en/Ego_depletion#/Reproducibility_controversy_and_conflicting_meta_analyses) (or [loudest](http://www.slate.com/articles/health_and_science/cover_story/2016/03/ego_depletion_an_influential_theory_in_psychology_may_have_just_been_debunked.html)) to fall as a part of the ongoing [replication crisis](https://www.wikiwand.com/en/Replication_crisis#/In_psychology). As only an armchair observer of psychology, these opinions are my own and were informed of this controversy through the easy listening of [podcasts](https://youarenotsosmart.com/2017/07/19/yanss-100-the-replication-crisis/) via [You Are Not So Smart](https://youarenotsosmart.com/). However, I would want to engage this conversation here to learn about and challenge the inclusion of such research in Fermat's Library, as this work may effectively be superseded or redacted for some if not all intents and purposes, if not more explicitly in the near future. ### Ego Depletion This research is part of the broader topic of *Ego Depletion*. Ego depletion is the idea that self control and willpower draw upon a limited resource in order to be exerted. Obviously, both self control and willpower are incredibly important in an individuals life and are closely related to susceptibility to criminality, eating disorders, drug abuse and poor academic performance. One of the first experiments to study ego depletion was performed by [*Baumeister et al*](http://psycnet.apa.org/doiLanding?doi=10.1037%2F0022-3514.74.5.1252) in 1998. In the experiment, researchers observed that people who initially resisted the temptation of eating chocolates had a harder time solving a difficult puzzle. Since then, many studies have been published about the topic, tho many researchers are still skeptical about the results.

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