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
Journal of Personality and Social Psychology Copyright 2007 by the American Psychological Association
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-
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GAILLIOT ET AL.
ticipants already perform poorly on self-control tasks,
1
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 &
1
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
