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Distinguishing Sluggish Cognitive Tempo From Attention-Deficit/Hyperactivity Disorder in Adults

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This study sought to do so using a general population sample in which those having high levels of SCT symptoms were identified (95th percentile) and compared to adults having high levels of ADHD symptoms and adults having both SCT and ADHD symptoms.
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Content Preview
Journal of Abnormal Psychology
(c) 2011 American Psychological Association
2012, Vol. 121, No. 4, 978 -990
0021-843X/11/$12.00
DOI: 10.1037/a0023961
Distinguishing Sluggish Cognitive Tempo From
Attention-Deficit/Hyperactivity Disorder in Adults
Russell A. Barkley
Medical University of South Carolina
Researchers who study subtypes of attention-deficit/hyperactivity disorder (ADHD) in children have identified
a subset having a sluggish cognitive tempo (SCT) typified by symptoms of daydreaming, mental confusion,
sluggish-lethargic behavior, and hypoactivity, among others who differ in many respects from ADHD. No
studies have examined the nature and correlates of SCT in adults. This study sought to do so using a general
population sample in which those having high levels of SCT symptoms were identified ( 95th percentile) and
compared to adults having high levels of ADHD symptoms and adults having both SCT and ADHD
symptoms. From a representative sample of 1,249 U.S. adults 18 -96 years four groups were created: (a) high
levels of SCT but not ADHD (N
33), (b) high levels of ADHD but not SCT (N
46), (c) high levels of
both SCT and ADHD (N
39), and (d) the remaining adults as a control group (N
1,131). As in children,
SCT formed a distinct dimension from ADHD symptoms that was unrelated to age, sex, or ethnicity. Adults
in both ADHD groups were younger than those with SCT only or control adults. The SCT-only group had less
education than the control group, whereas both SCT groups earned less annual income than the control or
ADHD-only group. More individuals in the combined group were out of work on disability. In their EF, both
SCT groups reported greater difficulties with self-organization and problem solving than controls or the
ADHD-only group. Otherwise, the SCT
ADHD group reported significantly greater problems with all other
domains of EF than the other groups. But both the SCT-only and ADHD-only groups had significantly more
EF difficulties than controls though not differing from each other. A similar pattern was evident on most
ratings of psychosocial impairment, except in work and education where SCT was more impairing than
ADHD alone and in driving where ADHD was more impairing. SCT contributed unique variance to EF
deficits and psychosocial impairment apart from ADHD inattention and hyperactive-impulsive symptoms.
Results further suggested that a symptom threshold of 5 or more out of 9 along with a requirement of
impairment would result in 5.1% of the population as having SCT. It is concluded that SCT may be a separate
disorder from ADHD yet with comorbidity occurring in approximately half of all cases of each.
Keywords: sluggish cognitive tempo, SCT, attention-deficit/hyperactivity disorder, ADHD, adults,
executive functioning, psychosocial impairment
Attention-deficit/hyperactivity disorder (ADHD) is currently
not an individual surpasses the diagnostic threshold of six or more
identified by diagnostic criteria that include two related symptom
symptoms on either or both dimensions: the Predominantly Inat-
dimensions consisting of difficulties with inattention and with
tentive (I) Type, the Predominantly Hyperactive-Impulsive Type
hyperactive-impulsive behavior (Diagnostic and Statistical Man-
(HI), and the Combined (C) Type, respectively. Some research
ual for Mental Disorders- 4th edition; DSM-IV; American Psy-
suggests that the HI-Type is a milder or earlier stage of the C-Type
chiatric Association, 2000). These criteria use the two symptom
leading to little further research on the validity of this subtype and
dimensions to create three subtypes of ADHD based on whether or
to it being largely ignored or collapsed in with cases of C-Type in
current research.
Early research suggested some validity to the I-Type relative to
the C-Type (Carlson, Lahey, & Neeper, 1986; Carlson & Mann,
This article was published Online First May 23, 2011.
2002; Milich, Balentine, & Lynam, 2001) as did some studies
This research was supported in part by a grant from Guilford Press
using neuropsychological measures (Solanto et al., 2007). More
Publications, New York, NY. The author is grateful for the assistance of
recent reviews that compare these types have typically concluded
Wendy Mansfield and Sergei Rodkin of Knowledge Networks (Menlo
Park, CA) for their assistance with the collection of the measures on the
that this approach does not so much identify distinct subtypes of a
representative general population sample of adults used in this study. Dr.
disorder but variations in disorder severity (Baeyens, Roeyers, &
Barkley receives royalties for books, videos, and rating scales from Guil-
Walle, 2006; Lahey & Willcutt, 2010; Nigg, Tannock, & Rohde,
ford Press and is the author of the Adult ADHD Rating Scale, Deficits in
2010). The subtypes are also unreliable, being highly related to the
Executive Functioning Scale, and Functional Impairment Scale published
methods and sources of information used to assess ADHD (Valo &
by Guilford Press and used in this article.
Tannock, 2010) and are not especially stable over development
Correspondence concerning this article should be addressed to Russell
(Lahey & Willcutt, 2010).
A. Barkley, Department of Psychiatry, Medical University of South Car-
olina, 1752 Greenspoint Court, Mt. Pleasant, SC 29466. E-mail:
Another approach to subtyping that may have some merit was
drbarkley@russellbarkley.org
suggested in early studies comparing the C- and I-Types of ADHD
978

SCT AND ADHD IN ADULTS
979
(Carlson et al., 1986; Carlson & Mann, 2000) where a distinct set
general population samples (Penny et al., 2009; Wahlstedt &
of inattention symptoms not represented in DSM criteria charac-
Bohlin, 2010) or general clinical samples (Garner et al., 2010) and
terized a sizable minority of cases diagnosed with I-Type. Those
not requiring the presence of significant ADHD inattention has
I-Type cases had very low levels of HI symptoms and were likely
shown that SCT may be distinct from but also associated with
to manifest problems with daydreaming, staring, mental fogginess,
ADHD inattention.
and confusion, hypoactivity, sluggishness or slow movement, leth-
The major aim of this study was to begin to fill in this void of
argy, apathy, and sleepiness (Barkley, DuPaul, & McMurray,
information on the utility of identifying SCT in adults relative to
1990; Carlson & Mann, 2002; Diamond, 2005; McBurnett,
ADHD. This was achieved initially by adding symptoms of SCT
Pfiffner, & Frick, 2001; Milich et al., 2001). This constellation
into a project that was collecting normative information on an
came to be labeled "sluggish cognitive tempo" (SCT; McBurnett et
adult ADHD rating scale. Using a general population sample
al., 2001) or more recently simply as attention-deficit disorder
permitted the examination of the independence of SCT from
(ADD; Adams, Milich, & Fillmore, 2010; Diamond, 2005).
ADHD, which may have been confounded in prior studies due to
Factor analytic studies support the distinction of SCT from
recruitment procedures as previously noted. The first objective of
ADHD symptoms in children when parent and teacher ratings are
this article therefore focused on the potential distinctiveness of
used (Garner, Marceaux, Mrug, Patterson, & Hodgens, 2010;
adults with high levels of SCT from those with high levels
Hartman, Willcutt, Rhee, & Pennington, 2004; Milich et al., 2001;
of ADHD, drawing both from this large representative sample of
Penny, Waschbusch, Corkum, Klein, & Eskes, 2009), or when
U.S. adults whose identification would not be contaminated by
direct observations of child behavior in school (McConaughy,
referral biases. Specifically, adults with high levels of SCT only
Ivanova, Antshel, Eiraldi, & Dumenci, 2009) or in a clinic (Mc-
were compared to those with high levels of ADHD only and to
Conaughy, Ivanova, Antshel, & Eiraldi, 2009) have included SCT
those with elevations on both sets of symptoms along with a
items. SCT symptoms also show a stronger association with degree
control group in their demographic correlates, EF ratings, and
of internalizing symptoms and social withdrawal (Garner et al.,
ratings of psychosocial impairment. A second objective was to
2010; Milich et al., 2001; Penny et al., 2009) and a weaker
evaluate the extent to which SCT symptoms contributed unique
association with measures of executive functioning (EF) and state
variance to the EF deficits and functional impairment besides that
regulation (Wahlstedt & Bohlin, 2010). It may be the daydreamy-
contributed by the two ADHD dimensions. Finally, a third objec-
sleepy features of SCT symptoms that are the most distinctive
tive of this article was the determination of the most appropriate
(rather than simply slow movement) from ADHD symptoms
SCT symptom threshold that might be useful for its diagnosis.
(Penny et al., 2009). These findings imply that the nature of the
inattention seen in cases of SCT may be of a distinctly different
Method
form than that found in ADHD-C, representing a separate disorder
from ADHD (Carlson & Mann, 2002; Diamond, 2005; Milich et
Participants
al., 2001). Nevertheless, SCT symptoms may correlate to a
moderate-to-high degree with traditional ADHD symptoms of
To obtain a nationally representative sample of adults in the
inattention while having a much lower relationship to HI symp-
United States ages 18 to 70
with equal representation of men
toms (Garner et al., 2010; Hartman et al., 2004; Penny et al., 2009;
and women in each of nine regions representing the United States,
Wahlstedt & Bohlin, 2010). Indeed, the relationship between SCT
we hired a national survey company, Knowledge Networks of
and HI may even become negative once the overlap of SCT with
Menlo Park, California (see website knowledgenetworks.com for
ADHD Inattention has been removed (Penny et al., 2009).
more information on the company). Knowledge Networks con-
The literature to date on the value of SCT symptoms for iden-
ducted the survey by using the web-enabled Knowledge Panel, a
tifying a separate disorder of inattention or separate type of ADHD
probability-based panel designed to be representative of the U.S.
is limited in several important respects. First, it has been conducted
population. Initially, participants were chosen scientifically by a
entirely with children. It is unclear to what degree such symptoms
random selection of telephone numbers and residential addresses.
even exist in the adult population. Nor do any longitudinal studies
Persons in selected households were then invited by telephone or
appear to exist that have followed children with SCT to determine
by mail to participate in the web-enabled Knowledge Panel (http:
not only the persistence of these symptoms into adulthood but also
www.knowledgenetworks.com/ganp/reviewer-info.html). For
the risks SCT may convey in adulthood for impairment in major
those who agree to participate but do not already have access,
domains of life activities, such as in education and occupational
Knowledge Networks provides at no cost a laptop and ISP con-
functioning. It is also unclear how adult SCT as a disorder may
nection. People who already have computers and internet service
differ from adult ADHD in its demographic correlates and in these
are permitted to participate by using their own equipment. Panel-
patterns of deficiencies and impairments.
ists then receive unique log-in information for accessing surveys
Second, the vast majority of research on SCT in children re-
online and then are sent e-mails throughout each month, inviting
cruited them from cases that were initially referred for evaluation
them to participate in research. Panelists are paid for their com-
of possible ADHD. The SCT cases were then selected from among
pletion of the survey.
those children who met criteria for the I-Type of ADHD. This
A total of 1,249 adults completed the survey (see Barkley,
creates an automatic confound of the SCT symptom set with the
2011a, for a detailed description of the sample). Of these adults,
ADHD inattention symptom set making it difficult to identify
623 were male, ages 18 -93 years (49.9%) and 626 female, ages
distinctive features that may be associated with SCT. It would also
18 -96 years. Approximately 8% of the sample fell into each sex
make the disorder of SCT appear as if it were a subtype of ADHD
within each age group. The education levels for the sample were
cases simply as an artifact of the recruitment process. Using
11.4% less than high school education, 29.5% completed high

980
BARKLEY
school, 29.2% completed some college, and 29.9% completed a
DSM-IV is invalid, having no scientific basis and resulting in more
bachelor's degree or higher. The racial- ethnic breakdown of the
than 35% of children and 50% of adults with ADHD going
sample was 77.3% White (non-Hispanic), 6.8% Black (non-
undiagnosed though they meet all other criteria for the disorder
Hispanic), 8.9% Hispanic, 3.8% other (non-Hispanic, chiefly
(Applegate et al., 1997; Barkley & Biederman, 1997; Barkley et
Asian), and 3.2% self-identified as belonging to two or more
al., 2008). Consequently, the DSM-5 committee is proposing to
ethnic groups. The marital status of the sample was 52.8% married,
abandon this criterion in favor of a higher age as yet to be
6.8% widowed, 9.4% divorced, 2.2% separated, 21.1% never
determined (most likely 12 to 16 years). No age of onset was
married, and 7.6% living with a partner. The employment status
specified for SCT symptoms as well, given that there are no
was 45.9% working as a paid employee, 6.5% self-employed,
diagnostic criteria for this condition. The DSM-IV criterion of
1.6% on temporary layoff, 7.2% seeking employment but not
functional impairment in home, school, or work was not used for
currently working, 22.6% retired, 7.9% disabled, and 8.3% not
placement in the ADHD group because impairment served as a
working for other reasons. All of these findings were very similar
dependent measure in the group comparisons in this study. This
to those for the U.S. adult population (2000 U.S. Census, www
ADHD group therefore represents a general population sample
.census.gov).
with high levels of ADHD symptoms. It may not be representative
For this article, a symptom of ADHD or SCT was identified as
of adults with a clinical diagnosis of ADHD.
such if it was rated as occurring often or very often on the Adult
ADHD Rating Scale-IV or SCT Rating Scale (see below), con-
Procedures
sistent with the use of this symptom frequency in DSM-IV. The
requirement in the DSM-IV that at least six such symptoms be
The self-report version of the Adult ADHD Rating Scale-IV, the
present on both lists to make the diagnosis of ADHD was not used
Deficits in Executive Functioning scale, and the Functional Im-
because it identified just 1.6% of adults as having ADHD in this
pairment scale were provided to Knowledge Networks to be up-
sample. The threshold has also been criticized as being inapplica-
loaded to an internet site such that members of the Knowledge
ble to adults given that it was based entirely on children and has
Panel could be invited to complete the scales. The survey was
been found to be an excessively high threshold for diagnosing
conducted in April 2010 and was completed within 3 weeks of the
adults with ADHD (Barkley, Murphy, & Fischer, 2008; McGough
initial invitation. Knowledge Networks requires that participants
& Barkley, 2004; Murphy & Barkley, 1996a). Moreover, longitu-
be given the option of choosing not to answer a question. For most
dinal research has shown that at least a third of all cases of ADHD
questions on the scales, this did not occur. But for some questions,
in childhood whose symptoms persist to adulthood and who place
a rare participant did refuse to answer the question resulting in a
at the 98th percentile for adults no longer meet DSM diagnostic
refusal rate of less than 2% for any item on any scale and typically
criteria for ADHD (Barkley, Fischer, Smallish, & Fletcher, 2002;
averaging less than 0.3% refusal rate. In these rare cases, the
Barkley et al., 2008). Prior studies have shown that a threshold of
missing answer was replaced with a 1, the lowest rating, so that
four symptoms corresponds to the 93rd to 95th percentile in
the data for this participant could be used in the project. This
various general population samples, identifies individuals more
practice was based on the argument that this would create a
impaired than those with a lesser number of symptoms and so may
conservative bias in the responses to that item.
be more appropriate for diagnosing ADHD in adults (Barkley et
al., 2008; Kooij et al., 2005; Murphy & Barkley, 1996a). In the
Measures
present sample, having four or more symptoms puts the person at
or above the 95th percentile for the inattention symptoms and at or
Demographic Questionnaire.
The survey company rou-
above the same percentile for the hyperactive-impulsive symptom
tinely obtains information from each participant at the start of the
list. On the basis of other studies recommending this threshold for
survey completion about the participants' age, education, income,
identifying adult ADHD, a threshold of four or more symptoms on
marital status, employment status, and region of the United States
either list was used here to identify the ADHD group. So as to
in which they currently reside.
equate the threshold for identifying high levels of SCT with this
Adult ADHD Rating Scale-IV (Barkley, 2011a).
This scale
same threshold, a symptom cutpoint corresponding to the 95th
contains the 18 items from the DSM-IV criteria for ADHD with
percentile in the sample for the SCT symptom count was 5 or
each item answered on a 4-point scale (from 1- 4; not at all,
greater. Therefore, this was the threshold used to identify partici-
sometimes, often, and very often, respectively). If any symptom of
pants as having high levels of SCT.
ADHD was rated often or more frequently, raters indicated the age
Using these symptom count thresholds, four groups were
of onset of those symptoms. This scale has been used extensively
formed from this sample: (a) high levels of SCT but not ADHD
in prior studies of adults with ADHD (see Barkley 2011a). Inter-
(N
33), (b) high levels on either ADHD symptom list but not
observer agreement between adult respondents and someone who
SCT (N
46), (c) high levels of both SCT and ADHD (N
39),
knows them well for the ADHD symptom rating scores has been
and (d) the remaining adults as a control group (N
1,131). Of all
found to range from .59 -.76 (Adler et al., 2008; Barkley et al.,
85 participants that qualified for ADHD, 32% were inattentive
2008; Magnusson et al., 2006; Murphy & Barkley, 1996b; Mur-
only (N
28), 36% were hyperactive-impulsive only (N
31),
phy, Barkley, & Bush, 2002; P. Murphy & Schachar, 2000;
and 30% were combined type (N
26). Because of the small
Zucker, Morris, Ingram, Morris, & Baker, 2002). Internal consis-
samples for each subtype, especially when examining the overlap
tency (Cronbach's alpha) of the subscales in this sample was quite
with SCT, subtypes were combined into a single ADHD sample.
acceptable (ADHD Inattention
.902; ADHD Hyperactive-
No age of onset criterion was specified for placement in this
Impulsive
.798) and consistent with other population-based
ADHD group as the current age of onset criterion of 7 years in
studies (alphas ranging from .72-.83; Kooij et al., 2005). Test-

SCT AND ADHD IN ADULTS
981
retest reliability based on the 62 adults from this sample who
score reflects the number of domains with ratings placing at or
completed the scale a second time after 2-3 weeks was also
above the 93rd percentile for the entire normative sample. Test-
acceptable: ADHD Inattention
.66, ADHD Hyperactive-
retest reliability over a 2-3 week interval was .72 for the mean
impulsive
.74. Comparisons of the prepost test scores revealed
impairment score. Interjudge agreement on the ratings from an
no significant changes between the first and second administration
earlier 10 domain version of this scale was .68 between adult
of the scale (Barkley, 2011a). Validity of the scale has been
respondents and others who knew them well (Barkley, 2011b). The
demonstrated through studies of the prototype version of this scale
validity of the current version of the scale has been evident in its
where, as noted above, ratings were obtained from collaterals who
association with degree of education, level of income, employment
know the patient well. The scale is also significantly associated
status, and marital status, whereas the earlier prototype version of
with self-reported, other-reported, and archival measures of im-
the scale was found to be associated with adverse outcomes in
pairment in education, occupational functioning, marital satisfac-
employment, education, marital satisfaction, driving, criminal his-
tion, driving, money management, parenting stress, offspring risk
tory, financial difficulties, health risks, parenting stress, and im-
for psychopathology, and health concerns, among others (Barkley,
pairment in other domains of major life activities as indexed
2011a).
through self-reports, other reports, employer ratings, and archival
Adult SCT ratings (Barkley, 2011a).
For this project, nine
records (Barkley, 2011b).
symptoms reflecting SCT were also collected and rated identically
The Deficits in Executive Functioning Scale (Barkley,
to those for ADHD. These symptoms were chosen from the symp-
2011c).
This 89-item rating scale requires participants to rate the
tom sets used in prior studies of SCT in children and included the
frequency with which they experience difficulties in five domains
following items: "Prone to daydreaming when I should be con-
of deficits in executive functioning on a 1- 4 Likert scale identical
centrating"; "have trouble staying alert or awake in boring situa-
to the ADHD scale above. The DSM-IV ADHD symptoms were
tions"; "easily confused"; "easily bored"; "spacey or in a fog";
excluded from this scale intentionally so as to permit the evalua-
"lethargic, more tired than others"; "underactive or have less
tion of the relationship of EF ratings to ADHD symptom ratings
energy than others"; "slow moving"; "I don't seem to process
(Barkley, 2011c; Barkley & Murphy, 2011). EF items were chosen
information as quickly or as accurately as others." These symp-
to represent the most commonly cited constructs believed to reside
toms have been found in earlier studies of children (Garner et al.,
under the umbrella term of EF: inhibition, nonverbal working
2010; Penny et al., 2009) and in the present sample to form a
memory, verbal working memory, organization, problem solving,
separate factor or dimension from the DSM symptoms of ADHD
time management, self-motivation, and self-regulation of emotion
(Barkley, 2011a). The score used here was the number of SCT
(see Barkley, 2011c, for a brief review of EF constructs and
symptoms answered often or very often. Internal consistency
models reviewed in generating the item pool). The five-factor
(Cronbach's alpha) of the nine items for this sample was .898.
based scales are self-management to time, self-organization and
Test-retest reliability for the scale based on the 62 adults that
problem-solving, self-restraint (inhibition), self-motivation, and
completed it a second time after 2-3 weeks was r
.88. A
self-regulation of emotion. Internal consistency of the five scales is
comparison of the prepost test scores revealed no significant
high with the Cronbach's alphas, ranging from .914 to .958.
change between the first and second administration of the scale
Test-retest reliability of the scale over a 2-3 week period has been
(Barkley, 2011a).
reported to range across the five scales from .62 (self-motivation)
Functional Impairment Rating Scale (Barkley, 2011b).
to .90 (self-organization) and scores were not found to change
The Functional Impairment Rating Scale has participants rate their
significantly between the two administrations (Barkley, 2011c).
degree of perceived current impairment in 15 different major life
Interobserver agreement was collected on an earlier 91-item pro-
activities using a 10-point Likert scale, rating from 0 (not im-
totype of the scale and was found to be satisfactory for all scales,
paired) to 9 (severely impaired) without reference to any medical
ranging from .66 to .79. The scale is significantly associated with
or psychiatric disorder. The domains were home life with your
self-ratings of impairment and with various measures of occupa-
immediate family; getting chores completed at home and manag-
tional functioning (Barkley, 2011c). The prototype of the scale was
ing your household; work or occupation; social interactions with
also found to be significantly predictive of impairment in various
strangers and acquaintances; relationships with friends; activities
domains of major life activities including educational, occupa-
in the community (church, clubs, social groups, organizations);
tional, marital, psychiatric, criminal, parenting, driving, financial,
any educational activities (college, night classes, technical train-
and health domains as indicated in self-reports, other-reports,
ing, occupational training); marital, coliving, or dating relation-
employer reports, and archival records (Barkley, 2011c; Barkley &
ships; management of your money, your bills, and your debts;
Fischer, 2011; Barkley & Murphy, 2010a; Barkley & Murphy,
driving a motor vehicle and in your history of citations & acci-
2011).
dents; sexual activities and sex relations with others; organization
and management of your daily responsibilities; caring for yourself
Results
daily (dressing, bathing & hygiene, eating, sleeping, etc.); main-
taining your health (exercise, nutrition, preventive medical and
Some information pertinent to the distinctiveness of the SCT
dental care, etc.); and taking care of and raising your children. The
symptoms from the ADHD symptoms was previously published in
scale comprises a single factor that reflects impairment and has
the manual for the ADHD rating scale (Barkley, 2011a). Results of
high internal consistency (reliability) as reflected in a Cronbach's
the factor analysis of the ADHD and SCT item ratings were
alpha of .969 (Barkley, 2011b). Scores can be obtained for each of
consistent with earlier studies on children that identified the SCT
the 15 domains along with two summary scores: the mean impair-
symptoms as comprising a separate factor from those representing
ment rating and the percentage of domains impaired. The latter
ADHD. Men and women did not differ in their ratings of either of

982
BARKLEY
the two ADHD symptom dimensions or the SCT dimension. Age
bined Type of ADHD. Considering the reverse relationship,
did not correlate significantly with the SCT score (r
.043, p
among those individuals who qualified for the Inattentive only
ns) but did to a small but significant degree with the ADHD scores:
subset of ADHD, 68% also qualified for SCT. Of those placing
ADHD Inattention
.075 ( p
.008), Current ADHD HI
in the Hyperactive-Impulsive only subset of ADHD, just 10%
.18 ( p
.001). Comparisons of six age groups, spanning 18 - 89
had SCT. And of those falling in the Combined subset of
years of age on their ADHD symptom ratings found that inatten-
ADHD, 65% also had SCT. Overall, 46% of those having SCT
tion symptoms showed a marginally significant decline with age
did not have any form of ADHD while 54% of those having any
( p
.066), whereas HI symptoms showed a significant decline
type of ADHD did not have SCT.
with age ( p
.001). In contrast, there was no significant change
in SCT symptoms with age. There were no ethnic differences for
Demographic Correlates of SCT Versus ADHD
ADHD inattention or SCT scores, but Black and Hispanic partic-
ipants reported significantly higher levels of HI symptoms than did
The comparisons of the groups on their SCT and ADHD
White participants in the sample even after covarying age given
symptom counts as well as in their age, education, and income
that the Hispanic sample was significantly younger than the other
are shown in Table 1. As expected from the selection criteria,
ethnic groups. Thus, SCT symptoms formed a distinct dimension
the two SCT groups (SCT only, SCT
ADHD) had signifi-
apart from ADHD symptoms in adults; are not affected by age or
cantly more symptoms of SCT than did the other two groups.
sex, as may be the case for some symptoms of ADHD; and are not
Also expected were the findings that the two ADHD groups had
significantly associated with ethnic group, as may be some symp-
significantly more ADHD inattention and ADHD HI symptoms
toms of ADHD. The highest loading items on the SCT scale from
than did the SCT or control groups, although the SCT group
the factor analysis (Promax rotation) were: "Lethargic, more tired
also had more ADHD symptoms on both dimensions than did
than others" (factor loading
.861), "underactive or have less
the control group. The two ADHD groups did not differ in their
energy than others" (.866), "slow moving" (.798), "I don't seem to
levels of HI symptoms, but the SCT
ADHD group had
process information as quickly or as accurately as others" (.697),
significantly higher levels of ADHD inattention than did the
and "spacey or `in a fog'" (.696).
ADHD-only group.
The groups differed significantly in their current age. Subse-
quent pairwise contrasts indicated that the two ADHD groups were
The Overlap of SCT With ADHD Subtypes
significantly (and comparably) younger compared to the SCT or
As noted above, 1,131 participants did not qualify as having
control groups. The latter two groups did not differ significantly
SCT or ADHD and served as the control group for all subse-
from each other.
quent group comparisons. There were 72 participants that qual-
The education of the participants was coded based on their
ified for SCT of which 39 (or 54%) also qualified for ADHD.
highest educational level attained ranging from 1 (no formal edu-
Twenty-six percent of those having SCT placed in the Inatten-
cation) to 14 ( professional or doctorate degree). Given that the
tive only subset of the ADHD group, just 4% placed in the
groups differed in their age, it served as a covariate in this and all
Hyperactive-Impulsive only subset, and 24% fell in the Com-
other parametric analyses. The groups differed significantly in
Table 1
Comparisons of the Groups on Number of Sluggish Cognitive Tempo (SCT) and Attention-Deficit/Hyperactivity Disorder (ADHD)
Symptoms and Dimensional Demographic Features

Group
Control
SCT
ADHD
SCT
ADHD
Measure
M
SD
M
SD
M
SD
M
SD
F
Cont
ADHD Inattention symptoms
0.2
0.5
1.2
1.2
2.4
2.5
5.7
2.3
656.57
1
2
3
4
ADHD Hyp-Imp symptoms
0.3
0.7
0.9
0.9
4.3
2.2
4.0
2.5
488.42
1
2
3,4
SCT symptoms
0.4
0.9
6.2
1.2
2.2
1.5
6.9
1.6
876.37
1
3
2
4
Age (in years)
50.5
18.0
50.3
16.0
40.2
15.5
38.9
16.6
9.83
1,2
3,4
Education rankinga
2.8
1.0
2.4
0.9
2.6
1.0
2.5
1.0
2.77
1
2
Income rankinga
11.5
4.2
8.3
3.6
11.9
3.7
9.1
5.0
11.09
1,3
2,4
Note.
Control
general population control group that does not have either SCT or ADHD group; SCT
sluggish cognitive tempo only; ADHD
ADHD only; SCT
ADHD
has both SCT and ADHD; F
Cont
results of the omnibus F-test. Where the F test was significant, the results of the
pairwise group contrasts are shown as well. ADHD Inattention
no. of ADHD Inattention symptoms; ADHD Hyp-Imp
no. of ADHD Hyperactive-
Impulsive symptoms. aAge was used as a covariate in this analysis.
p
.05.
p
.001.

SCT AND ADHD IN ADULTS
983
their education. Pairwise contrasts revealed that the SCT-only
more likely to be associated with a younger age, as noted
group had significantly less education than the control group. The
earlier.
two ADHD groups fell between these two groups and did not
Comparisons of the groups in their marital status revealed a
differ from either of them. Household annual income was coded in
significant omnibus test. Subsequent comparisons showed that
the survey as ranging from 1 ( $5,000) to 19 ($175,000 or more).
the SCT
ADHD group had a significantly lower percentage
The omnibus test for this measure was significant. Pairwise com-
of individuals who were married (25.6%) than the control group
parisons of the groups indicated that the two SCT groups (SCT
(54.5%) and thus a significantly higher percentage who had
only, SCT
ADHD) did not differ from each other yet both had
never been married (SCT
ADHD
38.5%, control
significantly less household income than did the control group or
20.0%). The SCT-only and ADHD-only groups fell between
than the ADHD-only group. The latter two groups did not differ
these two groups in their percentages that were married or never
from each other in this respect.
married and did not differ significantly from either of them.
The comparisons of the groups on the categorical demo-
Again, the difference between the SCT
ADHD and control
graphic variables are shown in Table 2. The groups were
group could be a function of the significantly younger age of the
compared on their proportion of men and women, using Pearson
ADHD groups and hence their decreased opportunity for mar-
chi-square. The omnibus test was not significant. The ethnic
riage at the time of the survey.
representation of the groups differed significantly. Comparisons
The groups were then compared in their employment status
among the groups indicated that the SCT
ADHD group had
and the omnibus chi-square was significant. Subsequent com-
a significant underrepresentation of White individuals (59%)
parisons revealed that the SCT-only and control groups did not
than did the control group (78%), most likely owing to an
differ in their work-status categorizations. But the two ADHD
overrepresentation of Hispanic members. The other three
groups had significantly fewer participants categorized as re-
groups did not differ from each other, nor did the SCT
ADHD group differ significantly from the SCT-only or ADHD-
tired than the control group. Once again, this may be a function
only groups. It seems likely that the overrepresentation of
of the younger age of the ADHD groups noted above. However,
Hispanic persons in the ADHD groups may be due to the
the SCT
ADHD group also had a significantly greater per-
Hispanic group being significantly younger than the other eth-
centage of cases that were not working currently due to dis-
nic groups in this sample (Barkley, 2011a) and that ADHD was
ability (28.2%) than the control group (7.1%). This difference is
Table 2
Comparisons of the Sluggish Cognitive Tempo (SCT), Attention-Deficit/Hyperactivity Disorder (ADHD), and Control Groups on the
Categorical Demographic Measures

Group
Measure
Control
SCT
ADHD
SCT
ADHD
2
p
Sex
2.31
ns
Males
50.1
45.5
41.3
56.4
Females
49.9
54.5
58.7
43.6
Ethnic group
21.57
.043
White
78.3
75.8
69.9
59.0
1
4
Black
6.8
3.0
6.5
10.3
Other
3.8
6.1
2.2
2.6
Hispanic
7.9
12.1
19.6
23.1
2
Races
3.2
3.0
2.2
5.1
Marital status
35.82
.002
Married
54.6
39.4
41.3
25.6
1
4
Widowed
6.9
15.2
2.2
2.6
Divorced
9.3
15.2
8.7
10.3
Separated
2.0
0.0
4.3
7.7
Never married
20.0
24.2
32.6
38.5
Living with partner
7.3
6.1
10.9
15.4
Employment status
54.41
.001
Working: employee
45.9
36.4
54.3
43.6
Working: self-employed
6.5
9.1
4.3
5.1
Temporarily laid off
1.4
3.0
6.5
0.0
Looking for work
7.1
6.1
4.3
15.4
Retired
24.0
15.2
8.7
5.1
1
3,4
Disabled
7.1
12.1
8.7
28.2
1
4
Not working: other
8.0
18.2
13.0
2.6
Note.
Control
general population control group having no SCT or ADHD, SCT
SCT only; ADHD
ADHD group only; SCT
ADHD
has both
SCT and ADHD; 2
results for the omnibus Pearson chi-square test; p
probability value for the omnibus chi-square test; Contrasts
results of pairwise
comparisons of the groups provided the omnibus test was significant.

984
BARKLEY
probably not due to the fact that the ADHD groups were
Relative Contribution of SCT Versus ADHD
younger at the time of the survey.
Symptoms to EF Deficits and Functional Impairment
To more fully evaluate the relative contributions of symptoms of
Executive Function Deficits in SCT Versus ADHD
SCT to EF deficits and psychosocial impairment relative to the two
ADHD symptom dimensions, multiple linear regression analyses
Table 3 also shows the group comparisons on the ratings of EF
were computed by using the entire sample of 1,249 adults. Step-
deficits (DEFS) and in impairment in psychosocial functioning for
wise entry was used here because the focus of this article is on the
the 15 domains rated on the FIS. The multivariate analysis of
distinct variance that the SCT dimension may explain apart from
covariance (MANCOVA; age as a covariate) applied to the five
any significant contribution of the two ADHD symptom dimen-
subscales of the DEFS was significant: Wilks's , F
48.17, df
sions. Stepwise entry permits the computation of the increment in
15/3423.49, p
.001. All of the subsequent univariate analyses of
variance explained by each significant variable that enters the
covariance (ANCOVAs) for each subscale were also significant.
equation and not just that explained by all variables, significant or
Pairwise contrasts indicated that both the two SCT groups had
not. The results appear in Table 4. The results indicated that both
significantly more deficits in self-organization and problem solv-
the ADHD inattention and SCT symptom dimensions contributed
ing than did the control group and the ADHD-only group. But the
59.4% of the variance to the degree of deficits in the DEFS
SCT
ADHD group had even more such problems than did the
Self-Management to Time scale with the former accounting for the
SCT-only group. Although having fewer such problems than the
vast majority of variance explained (54.5% vs. 5%). In the pre-
two SCT groups, the ADHD-only group nevertheless reported
diction of the DEFS Self-Organization and Problem-Solving scale,
the reverse was the case. Both inattention dimensions explained
significantly more problems in this domain than did the control
nearly 70% of the variance in this scale but the vast majority was
group. In contrast, the SCT
ADHD group reported significantly
contributed by the SCT symptom dimension (65.9% vs. 3.8%).
more deficits in the remaining four DEFS subscales than did either
The ADHD HI dimension did not contribute significantly to either
the SCT-only or the ADHD-only groups, who did not differ from
dimension of EF deficits. For the DEFS Self-Discipline scale, all
each other. But both of those groups reported more EF deficits
three symptom dimensions contributed significantly explaining
across these four domains than did the control group.
56.1% of the variance in this scale. The vast majority was con-
The ANCOVAs for the two summary scores from the psycho-
tributed by the SCT symptoms (45.8%) relative to the other two
social impairment scale (FIS) were both significant. Pairwise com-
ADHD symptom dimensions (HI
8.5%, Inattention
1.8%).
parisons showed that the SCT
ADHD group had a higher mean
Similarly, all three symptom dimensions contributed significantly
impairment score than either the SCT-only or ADHD-only groups,
to the DEFS Self-Regulation of Emotion scale and, again, the SCT
who did not differ from each other. Both of those groups, however,
symptom dimension explained the vast majority of variance in this
reported more impairment than did the control group. For the
domain of EF deficits (44.5%) than did the ADHD-HI dimension
percentage of domains impaired score, the SCT
ADHD group
(7.7%) or the ADHD Inattention dimension (1.4%). The DEFS
had significantly more pervasive impairment than did the other
Self-Motivation scale was significantly predicted by all three
three groups. The ADHD-only group was more pervasively im-
symptom dimensions explaining 72.5% of the variance in this EF
paired than either the SCT-only or control group, whereas the
scale. This time, ADHD Inattention explained the most variance
SCT-only group was more impaired in this respect than the control
(48.8%) followed by SCT symptoms (3.1%) and ADHD HI symp-
group. This table also shows the results for the 15 individual FIS
toms (0.5%).
domain scores. For most domains, the SCT, ADHD, and SCT
The same analyses were applied to the two summary scores
ADHD groups all were significantly more impaired than the con-
from the FIS impairment ratings. Again, the two ADHD symptom
trol group. The SCT and ADHD groups did not differ from each
dimensions and the SCT dimension were entered in stepwise
other yet both were significantly less impaired than the SCT
fashion in predicting each of these two summary scores. For both
the FIS Mean Impairment score and the FIS Percentage of Do-
ADHD group. However, in the domains of occupational function-
mains Impaired score, ADHD Inattention explained the most vari-
ing, the results indicated that although all three disordered groups
ance (39.9% & 32.2%, respectively) followed by the SCT Symp-
again were more impaired than controls, the ADHD-only group
toms dimension (3.2% & 2.3%, respectively). The ADHD HI
was less impaired than the SCT, which was less impaired than the
symptoms contributed the least variance to these impairment
SCT
ADHD group. In short, SCT is more impairing in this
scores (0.7% & 0.4%).
domain than is ADHD only. In the educational domain, all three
disordered groups were again more impaired than controls. The
SCT
ADHD group was the most impaired yet it did not differ
Identifying an SCT Symptom Threshold for Diagnosis
from the SCT-only group in this regard. The comorbid group was
No formal diagnostic criteria for SCT currently exists that might
more impaired than the ADHD-only group. This same pattern was
be comparable to the DSM-IV criteria for ADHD in which both a
evident for the domain of sexual activities where once again the
symptom threshold and impairment in one or more domains of
SCT-only group did not differ from the comorbid group. In the
major life activity are required to be diagnosed as ADHD. To
driving domain, the SCT group did not differ from the control
determine what symptom threshold might be most useful for
group, but both ADHD groups were more impaired than controls.
identifying adults as having a diagnosis of SCT, information is
Finally, in the child-rearing domain, only the comorbid SCT
reported in Table 5 that is pertinent to making such a decision. This
ADHD group was more impaired than the control group.
table reports the percentage of the sample that met criteria for an

SCT AND ADHD IN ADULTS
985
Table 3
Comparisons of the Sluggish Cognitive Tempo (SCT), Attention-Deficit/Hyperactivity Disorder (ADHD), and Control Groups on
Executive Function Deficits and Psychosocial Impairment

Group
Control
SCT
ADHD
SCT
ADHD
Measure
M
SD
M
SD
M
SD
M
SD
F
Cont
DEFS Ratings
Self-organizea
34.2
9.1
56.3
13.9
44.6
11.3
66.2
17.1
206.85
1
3
2
4
Time mgmta
32.7
9.1
45.9
10.6
43.6
11.8
58.9
12.8
125.60
1
2,3
4
Self-restrainta
27.1
6.8
38.7
9.5
38.7
10.0
47.1
12.4
148.13
2,3
4
Emotiona
18.3
5.6
28.1
8.9
28.1
9.9
33.4
10.9
128.72
1
2,3
4
Self-motivationa
14.6
3.6
20.4
5.9
20.4
7.2
27.6
9.1
157.68
1
2,3
4
FIS Ratings
Mean imp scorea
1.8
1.6
3.4
1.5
3.8
1.8
5.6
2.0
90.78
1
2,3
4
Domains imp %a
7.9
16.7
21.5
19.5
30.1
23.9
59.4
31.0
123.27
1
2
3
4
Home life with family
1.7
1.9
3.3
2.6
3.5
2.5
5.3
2.8
44.97
1
2,3
4
Completing chores at home and
2.5
2.3
4.7
2.2
4.8
2.8
6.4
2.3
53.75
managing a household
1
2,3
4
Work-Occupation
1.8
2.0
4.2
2.2
3.1
2.6
6.4
2.4
61.40
1
3
2
4
Social: strangers
2.0
2.1
3.4
2.2
4.0
2.8
6.1
2.7
53.14
1
2,3
4
Social: friends
1.6
1.9
3.2
2.1
3.6
2.4
5.1
2.9
48.79
1
2,3
4
Community Activities
1.8
2.0
3.4
2.4
3.3
2.0
5.5
3.3
34.26
1
2,3
4
Educational activities
1.9
2.2
5.6
3.6
4.4
3.1
6.7
2.9
40.47
1
2,3,4
3
4
Marital or cohabiting relationships
2.0
2.2
4.3
2.9
4.7
2.7
5.9
2.8
42.24
1
2,3
4
Managing finances
1.8
2.2
3.8
2.9
4.4
3.0
5.6
3.4
46.11
1
2,3
4
Driving a vehicle
1.0
1.6
1.6
2.2
2.0
2.5
2.9
3.0
14.05
1,2
3
4
Sexual activities
2.2
2.5
4.5
3.0
3.8
3.0
5.7
3.2
26.67
1
2,3,4
3
4
Managing daily responsibilities
1.8
2.0
3.9
2.2
4.3
2.7
6.6
2.4
88.99
1 178 2,3
4
Daily self-care
1.2
1.9
2.3
2.7
2.9
3.0
4.8
2.8
47.39
1
2,3
4
Health maintenance
2.0
2.2
4.3
2.6
4.3
3.1
5.8
2.8
51.14
1
2,3
4
Child rearing
1.5
2.1
1.7
1.7
3.0
2.6
5.2
2.5
13.94
1,2,3
4
Note.
Control
general population control group that does not have either SCT or ADHD group; SCT
ADHD
has both SCT and ADHD; F
Cont
results of the omnibus F test. Where significant, the results of the pairwise group contrasts are shown as well; DEFS
Deficits in Executive
Functioning scale. Organization
Self-Organization and Problem-Solving scale; Time mgmt
Self-Management to Time scale; Emotion
Emotional
Self-Regulation scale; Motivation
Self-Motivation scale. FIS
Functional Impairment scale; Mean impairment
mean impairment rating across all
domains answered by the participant; Domains imp %
the percentage of domains answered by the participant which fell in the impaired range
(approximately 93rd percentile) for the general population sample.
a Indicates that age was used as a covariate in the omnibus F test.
p
.05.
p
.001.

986
BARKLEY
Table 4
Prediction of the Deficits in Executive Functioning Scale Ratings and Functional Impairment Scale Ratings From Attention-Deficit/
Hyperactivity Disorder (ADHD) Inattention, ADHD Hyperactive-Impulsive, and Sluggish Cognitive Tempo (SCT)
Symptom Dimensions

Rating scale and symptom predictors
Beta
R
R2
R2
F
DEFS Self-Management to Time
ADHD Inattention symptoms
.472
.738
.545
.545
1491.02
SCT symptoms
.347
.771
.594
.050
152.75
DEFS Self-Organization & Problem Solving
SCT symptoms
.578
.812
.659
.659
2410.20
ADHD Inattention symptoms
.306
.835
.698
.038
158.58
DEFS Self-Restraint
SCT symptoms
.346
.677
.458
.458
1054.16
ADHD Hyperactive-Impulsive Symptoms
.282
.737
.543
.085
231.72
ADHD Inattention symptoms
.227
.749
.561
.018
51.42
DEFS Self-Regulation of Emotion
SCT symptoms
.362
.667
.445
.445
998.53
ADHD Hyperactive-Impulsive symptoms
.272
.722
.522
.077
200.33
ADHD Inattention symptoms
.200
.732
.536
.014
37.71
DEFS Self-Motivation
ADHD Inattention symptoms
.441
.699
.488
.488
1190.61
SCT symptoms
.261
.721
.520
.031
81.67
ADHD Hyperactive-Impulsive symptoms
.092
.725
.525
.005
13.13
FIS Mean Impairment score
ADHD Inattention symptoms
.366
.632
.399
.399
816.17
SCT symptoms
.259
.656
.431
.032
68.64
ADHD Hyperactive-Impulsive symptoms
.107
.661
.437
.007
14.86
FIS Percentage of Domains Impaired
ADHD Inattention symptoms
.344
.568
.322
.322
584.48
SCT symptoms
.222
.588
.345
.023
43.57
ADHD Hyperactive-Impulsive symptoms
.085
.591
.350
.004
8.13
Note.
DEFS
Deficits in Executive Functioning scale; FIS
Functional Impairment scale. Analyses are for linear multiple regression with stepwise
entry; Beta
standardized beta coefficient from the final model; R
regression coefficient; R2
percentage of explained variance accounted for by all
variables at this step; R2
(change)
percentage of explained variance accounted for by this variable added at this step; F
F to change results, p
probability value for the F test.
p
.001.
increasing number of SCT symptoms from none, to 1 or more, 2 or
symptom threshold who had a mean impairment score at the 93rd
more, and so forth. For each symptom threshold, the mean impair-
percentile or higher ( 1.5 SDs above the sample mean, or a score
ment score for the Functional Impairment scale is provided. Also
of 5.2). Next to this is indicated the percentage of people who meet
provided is the percentage of people who meet or exceed this
this diagnostic threshold who rated at least one or more domains
Table 5
Percentage of Sample Placing at or Above Each of 9 Sluggish Cognitive Tempo (SCT) Symptom Thresholds and the Corresponding
Impairment Scores From the Functional Impairment Scale

Mean impairment
Percent rated 1
Percentage of domains
SCT symptom threshold
Sample percent
score (SD)
Percentage impaireda
Domains impairedb
impaired (SD)
No symptoms
71.7
1.5 (1.5)
3.1
24.6
5.7 (14.4)
1 or more
28.3
3.3 (1.9)
16.1
66.1
23.1 (26.5)
2 or more
19.1
3.8 (1.9)
21.4
80.3
29.3 (27.6)
3 or more
13.4
4.0 (1.9)
25.1
85.0
32.9 (28.5)
4 or more
9.4
4.3 (1.9)
30.8
88.0
36.4 (29.3)
5 or more
5.8
4.6 (2.1)
38.9
87.5
42.0 (32.4)
6 or more
3.9
5.0 (2.1)
46.9
87.8
47.2 (32.7)
7 or more
2.8
5.1 (2.2)
48.6
88.6
49.2 (34.0)
8 or more
1.6
6.0 (2.2)
70.0
95.0
65.7 (32.9)
9 symptoms
0.9
5.8 (2.5)
63.6
90.9
65.3 (36.3)
a The percentage of this group that had a mean impairment score that fell at least 1.5
SD above the mean for the entire sample (score of 5.2 or higher;
M
2.0, SD
1.8).
b The percentage of this group that rated at least one or more domains as placing in the impaired range (had an impairment rating
of 1.5
SD above the mean score for that domain).
p
.001, for comparison of this group to group having fewer symptoms using either analysis of variance or chi-square, as appropriate.

SCT AND ADHD IN ADULTS
987
on the FIS as placing in the impaired range, which again is a rating
This further supported the view that unusually elevated levels of
for that domain at or above the 93rd percentile for the sample
SCT symptoms may comprise a separate disorder of attention from
( 1.5 SDs above the sample mean rating). Finally, the last column
that associated with ADHD. It is the first study, however, to extend
of this table shows the mean score for the percentage of domains
this conclusion to adults. Despite being distinct from ADHD, SCT
that the individual rated as placing in the impaired range ( 1.5
symptoms shared up to 50% of their variance with ADHD Inat-
SDs above the mean rating for that domain). Recall that this score
tention while sharing less than 25% with ADHD HI symptoms.
is computed on the FIS as being the number of domains they rated
The pattern is similar to that found for children (Garner et al.,
as being applicable to their life on which they gave a rating that fell
2010; Penny et al., 2009). Also, whereas ADHD symptoms were
at or above the 93rd percentile for that domain. As in the devel-
found to decline with age, especially HI symptoms, and to vary
opment of the DSM-IV, it is important to select a symptom
somewhat with ethnic grouping (HI symptoms only), this was not
threshold for SCT that does not identify too many or too few
the case for SCT symptoms.
members of the population as being inappropriate in their symp-
In this study, 6.8% of the normative sample had high levels of
tom levels.
ADHD (Combined, Inattentive, or Hyperactive-Impulsive; ap-
For instance a score of four or more symptoms would identify
proximately 95th percentile or more). A total of 5.8% of the
9.4% of this sample as being SCT, which is slightly less than 1 in
sample met criteria for high SCT symptoms (approximately 95th
10 people. Important to note is that 88% of the people who met this
percentile or more). Approximately, half (54%) of those partici-
threshold were rated as impaired in at least one or more domains
pants qualifying for SCT had ADHD, yet nearly half did not.
on the FIS. On average, they reported being impaired in at least
Where overlap existed, it was principally with those subtypes of
36% of the domains they rated as being applicable to them on this
ADHD having significant inattention. Similarly, approximately
scale. Because impairment is an essential criterion for virtually all
half of individuals qualifying for ADHD of any type (46%) also
Axis I disorders in the DSM-IV, it seems necessary that individuals
qualified for SCT. The overlap with SCT mainly involved indi-
who meet a symptom threshold should also be required to be
viduals having high symptoms of the ADHD Inattention. These
impaired to be called disordered. Imposing both criteria (four or
findings agree with those for children (Garner et al., 2010; Hart-
more symptoms and one or more domains of impairment) would
man et al., 2004) that suggested that SCT is not a subtype of
result in 8.3% of the sample being considered as SCT or disor-
ADHD but a relatively distinct disorder from it. Yet it is one in
dered, or about 1 of every 12 people (9.4%
88%
8.3%). This
which about half of cases of one disorder may also have signifi-
may be acceptable but seems to be a bit excessive.
cantly elevated symptoms of the other, in particular, those sub-
In contrast, choosing a threshold of five or more symptoms as
types having high inattention. The pattern here is one similar to
was done in this study to select the SCT group identified 5.8% of
comorbidity between two disorders, such as between anxiety and
the sample as meeting this threshold. This seems to be a more
depression, than one of subtyping within a single disorder or in
reasonable percentage of the population to identify as possibly
which subgroups share the same disorder of attention. When
having a disorder, or about 1 in every 17 people. As with the
sample recruitment is not biased toward using ADHD referrals as
threshold of four or more symptoms, the vast majority (87.5%) of
a starting point, as was done here and in some studies of children
individuals who met this threshold were rated as being impaired in
(Garner et al., 2010; Hartman et al., 2004), SCT is not found to be
at least one or more domains on the FIS. Again, requiring both
a subtype of ADHD but a statistically valid disorder from it.
criteria (five or more symptoms and one or more impaired do-
Several demographic and other features distinguished the SCT
mains) would result in 5.1% of the population (5.8%
87.5%
and ADHD groups. The ADHD groups were considerably younger
5.1%) having this disorder, or 1 in 20. Also noteworthy is that
than the SCT group by nearly a decade. The SCT-only group
people who met this symptom threshold reported an average of
reported significantly less education and earned significantly less
42% of the domains on the FIS applicable to them as being
income than control participants, whereas the ADHD groups did
impaired (falling at the 93rd percentile for that domain). In either
not differ from either of these groups, falling between them in
case, having four or more or five or more symptoms is not a trivial
these features. There was no difference in sex representation
matter. Most people who met these thresholds were impaired in at
among any of these groups. The ADHD groups, especially the
least one domain and were rated as impaired in a substantial
SCT
ADHD group, were significantly less likely to have mar-
percentage of those domains on the FIS. Either of these thresholds
ried than the control group. But this may have been the result of the
may be acceptable for selecting a sample as having a disorder of
ADHD groups being significantly younger and therefore having
adult SCT. But the symptom threshold of five or more along with
had less opportunity to marry at the time of the survey. It may also
one or more impaired domains appears preferable to avoid overi-
have accounted for significantly fewer ADHD group members
dentification of adults as being disordered.
being retired than was the case in the control group. But age
differences among the groups could not explain the overrepresen-
Discussion
tation of participants out of work due to disability in the SCT
ADHD group than was the case in the other groups. This implies
This study evaluated the utility of identifying sluggish cognitive
that where both disorders are comorbid, occupational impairment
tempo, or SCT, as a distinct disorder of attention from that char-
is likely to be higher than for either disorder alone. In general,
acterizing ADHD in adults by using a representative sample of
there is a suggestion here that SCT is associated with a different
U.S. adults. As noted earlier, the initial stage of this project
demographic pattern than is ADHD.
(Barkley, 2011a) found that SCT symptoms comprised a separate
Comparison of these groups on more clinically relevant mea-
factor from the traditional ADHD symptom dimensions consistent
sures indicated that the groups with high SCT
ADHD symptoms
with studies of children (Garner et al., 2

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