The Relation Between Sluggish Cognitive Tempo and

C 2004)
Journal of Abnormal Child Psychology, Vol. 32, No. 5, October 2004, pp. 491–503 (
The Relation Between Sluggish Cognitive
Tempo and DSM-IV ADHD1
Christie A. Hartman,2,4 Erik G. Willcutt,2 Soo Hyun Rhee,2 and Bruce F. Pennington3
Received August 28, 2003; revision received March 12, 2004; accepted March 21, 2004
To test the relation between sluggish cognitive tempo (SCT) and DSM-IV ADHD symptoms, parent
and teacher ratings of the 18 DSM-IV ADHD items and five potential SCT items were obtained in a
community sample of 8–18 year-old twins that was overselected for ADHD and learning disabilities
(n = 296). Confirmatory factor analyses revealed that a three-factor model provided the best fit
to the data for both parent and teacher ratings. DSM-IV inattention and hyperactivity–impulsivity
symptoms loaded on two factors consistent with the DSM-IV model, and five SCT symptoms loaded
primarily on a third factor. The SCT and inattention factors were highly correlated, whereas SCT and
hyperactivity–impulsivity were weakly related. Both raters indicated that children meeting symptom
criteria for the combined and inattentive subtypes exhibited significantly more SCT symptoms than
those meeting symptom criteria for hyperactive–impulsive type and the comparison group without
ADHD. Children meeting symptom criteria for the inattentive type exhibited significantly more SCT
symptoms than those meeting criteria for the combined type, based on teacher ratings. These results
suggest that SCT is an internally consistent construct that is significantly associated with DSM-IV
inattention.
KEY WORDS: attention-deficit/hyperactivity disorder; DSM-IV; subtypes; factor analysis; sluggish cognitive
tempo.
INTRODUCTION
subtype describes children who exhibit significant symptoms of hyperactivity and impulsivity in the absence of significant symptoms of inattention. The combined subtype
includes individuals with significant elevations on both
symptom dimensions. The overall goal of this study was
to examine the relation between the DSM-IV subtypes and
the putative construct of sluggish cognitive tempo (SCT)
as one test of the validity of the subtypes.
The optimal nosology of ADHD would discriminate
among meaningful subtypes to reduce the heterogeneity that has characterized previous studies (e.g., Barkley,
1998; Lahey & Willcutt, 2002; Milich, Balentine, &
Lynam, 2001). If subtypes of ADHD differ consistently in
their etiology, severity, or treatment response, studies that
include multiple subtypes in the same group may mask
effects associated primarily with one of the subtypes. In
contrast, the most parsimonious taxonomy would not include subtypes that lack empirical support, as the inclusion
of such subtypes tacitly reifies the validity of these subclassifications, and may therefore mislead both clinical
diagnosis and research design.
The diagnostic criteria for attention-deficit/hyperactivity disorder (ADHD) in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSMIV; American Psychiatric Association, 1994) define three
diagnostic subtypes based on differential elevations of
inattention and hyperactivity–impulsivity symptoms. The
predominantly inattentive subtype includes individuals
who exhibit significant symptoms of inattention and disorganization without significant hyperactivity or impulsivity, whereas the predominantly hyperactive–impulsive
1 Portions of these results were presented at the 2003 meeting of the Inter-
national Society for Research in Child and Adolescent Psychopathology, Sydney, Australia.
2 Institute for Behavioral Genetics and Department of Psychology,
University of Colorado at Boulder, Boulder, Colorado.
3 Department of Psychology, University of Denver, Denver, Colorado.
4 Address all correspondence to Christie Hartman, Institute for Behavioral Genetics, UCB 447, University of Colorado, Boulder, Colorado
80309-0447; e-mail: christie.hartman@colorado.edu.
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0091-0627/04/1000-0491/0 492
Some recent reviews concluded that the inattentive
type is a valid subtype within the overarching DSM-IV
ADHD diagnosis (e.g., Carlson, Shin, & Booth, 1999;
Lahey & Willcutt, 2002; McBurnett, 1997), whereas others have suggested that the inattentive type may be a distinct attentional disorder that is not a subtype of ADHD
at all (e.g., Barkley, 2001; Milich et al., 2001). Studies
that compared the inattentive and combined types suggest
that the combined type is associated with greater overall
impairment (e.g., Faraone, Biederman, Weber, & Russell,
1998), a higher frequency of accidental injuries requiring care from a physician (e.g., Lahey et al., 1998), and
greater social impairment in many domains (e.g., Gaub
& Carlson, 1997), whereas individuals with the inattentive type are more likely to be socially passive or isolated (e.g., Hinshaw, 2002; Maegden & Carlson, 2000).
On the other hand, available data reveal few consistent
differences between the inattentive and combined types
on neurocognitive tasks or measures of academic functioning (e.g., Chhabildas, Pennington, & Willcutt, 2001;
Hinshaw, Carte, Sami, Treuting, & Zupan, 2002), and family and twin studies suggest that these subtypes may be attributable largely to common etiological influences (e.g.,
Todd et al., 2001; Willcutt, Chhabildas, & Pennington,
2001).
Taken together, previous studies provide some support for the discriminant validity of the DSM-IV inattentive and combined types, but these results are inconsistent.
One possible explanation for these mixed results may be
heterogeneity within the group identified by the DSM-IV
criteria for the inattentive type. Recent studies suggest that
a subset of individuals who meet criteria for the DSM-IV
inattentive type exhibit elevations of hyperactivityimpulsivity that fall only slightly below the diagnostic
threshold on the hyperactivity-impulsivity symptom dimension (e.g., Barkley, 2001), whereas a second subset
of individuals with the inattentive type exhibits few or no
hyperactive-impulsive symptoms (e.g., Chhabildas et al.,
2001; Milich et al., 2001). This heterogeneity may be exacerbated by the fact that the inattentive type is defined by the
presence of a subset of the same symptoms used to define
the combined type (i.e., inattention) in the absence of clinically significant elevations of hyperactivity-impulsivity.
To address this concern, several authors have suggested
that rather than defining a primarily inattentive subtype
based on a negative diagnostic criterion such as the absence of hyperactivity, the validity of this subtype may be
improved by developing positive diagnostic criteria based
on additional attentional difficulties that specifically characterize this group (e.g., Carlson & Mann, 2002; Hinshaw,
2001; McBurnett, Pfiffner, & Frick, 2001; Milich et al.,
2001; Willcutt et al., 2001).
Hartman, Willcutt, Rhee, and Pennington
Sluggish Cognitive Tempo
Studies of ADHD subtypes as defined in the third
edition of the DSM (DSM-III; American Psychiatric Association, 1980) revealed some differences in the specific
attentional dysfunction exhibited by each of the groups.
Specifically, children with DSM-III attention deficit disorder with hyperactivity exhibited inattention symptoms
characterized by sloppy work and distractibility, whereas
children with DSM-III attention deficit disorder without
hyperactivity were rated higher on inattention items measuring daydreaming, tendency to become confused, lack of
mental alertness, and physical hypoactivity (e.g., Barkley
et al., 1990; Hynd et al., 1989; Lahey & Carlson, 1991;
Stanford & Hynd, 1994), a cluster of symptoms that has
subsequently been labeled sluggish cognitive tempo
(SCT). On the basis of these findings, the DSM-IV field trials (Lahey et al., 1994) tested the utility of SCT symptoms
for the diagnosis of the inattentive type. Consistent with
studies of DSM-III ADD, the field trials found that SCT
symptoms were associated most strongly with the DSM-IV
predominantly inattentive type (Frick et al., 1994). However, further analyses indicated that the majority of individuals with the DSM-IV inattentive type did not exhibit
SCT symptoms, so these symptoms were not included in
the diagnostic criteria for DSM-IV ADHD.
The finding that few individuals with the inattentive type exhibited SCT symptoms in the field trials suggests that these symptoms are relatively weak measures
of the construct assessed by DSM-IV inattention symptoms. However, it is still possible that SCT symptoms
may identify an important subgroup of individuals with
DSM-IV ADHD or may even be a marker for a specific
subset of individuals who meet symptom criteria for the
DSM-IV inattentive type but are in fact better conceptualized as having a distinct primarily inattentive disorder.
Therefore, several subsequent studies have examined the
relation between ADHD and SCT symptoms and reevaluated the potential utility of SCT symptoms as part of the
diagnostic criteria for ADHD (Carlson & Mann, 2002;
Hinshaw et al., 2002; McBurnett et al., 2001; Skansgaard
& Burns, 1998).
Factor Analyses of SCT Symptoms
McBurnett et al. (2001) factor analyzed parent and
teacher ratings of DSM-IV ADHD symptoms plus the
two SCT symptoms daydreams and sluggish/drowsy in a
large clinic-referred sample. When all DSM-IV ADHD
symptoms and the two SCT symptoms were included in
the analysis, a two-factor solution was obtained in
Sluggish Tempo and ADHD
which SCT items loaded on a factor with DSM-IV inattention symptoms and DSM-IV hyperactivity-impulsivity
symptoms loaded on a second factor. In contrast, when
hyperactivity-impulsivity items were excluded from the
analysis the inattention and SCT items separated into two
factors: the first factor included all DSM-IV inattention
items with the exception of often forgetful in daily activities, which loaded on a second factor along with the two
SCT items.
The authors then examined the potential implications
of these results for the DSM-IV model of ADHD by analyzing the relation between SCT and the DSM-IV ADHD
subtypes. Results of these analyses revealed that the mean
severity of the SCT items was significantly higher in the
group with the DSM-IV inattentive type than the combined
type, hyperactive-impulsive type, or a clinic-referred comparison group without ADHD, and similar results were obtained in two studies of teacher ratings (Carlson & Mann,
2002; Skansgaard & Burns, 1998). Moreover, whereas
McBurnett et al. (2001) found that the positive predictive
power of the SCT symptoms was similar for the combined and inattentive types, the negative predictive power
of these symptoms was substantially higher for the inattentive type. Thus, although the initial two-factor result
suggests that SCT symptoms are related closely to DSMIV inattention, analyses of the inattention symptoms alone
suggest that SCT symptoms may represent a separable
symptom dimension that relates differentially to the inattentive and combined types.
External Validity of SCT Symptoms
To test the external validity of SCT symptoms,
Carlson and Mann (2002) reanalyzed a large sample of
nonreferred children with DSM-IV ADHD (Gaub &
Carlson, 1997). They identified a subgroup (28%) of children with the inattentive type who also exhibited extreme
scores on a summary measure comprised of several possible SCT items from the Child Behavior Checklist
(Achenbach, 1991b). The subgroup with elevations on the
SCT items was rated by teachers as showing less externalizing behavior and higher levels of unhappiness, anxiety/
depression, and withdrawn behavior than the remaining
children with the DSM-IV inattentive type or combined
type, as well as more social dysfunction than children
with the DSM-IV inattentive type without significant SCT.
In contrast to the results reported by Carlson and Mann
(2002), however, another recent study found few group
differences on neurocognitive measures when females
with the inattentive type were subdivided into groups with
and without elevations of SCT (Hinshaw et al., 2002).
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Discriminant Validity of SCT
Lahey (2001) pointed out that in addition to examining the association between ADHD and SCT, it is also essential to understand the relation between SCT symptoms
and other related disorders. For example, many aspects of
SCT are similar to symptoms of internalizing disorders
such as major depressive disorder (e.g., drowsiness and
psychomotor retardation) and generalized anxiety disorder (e.g., mind going blank), and slow processing of cognitive stimuli is also a correlate of learning disabilities and
mental retardation (e.g., Stanford & Hynd, 1994; Willcutt,
Pennington, et al., 2001). Because a diagnosis of an attentional disorder may be more palatable to parents than a
diagnosis of major depression, it is essential that the discriminant validity of SCT symptoms be carefully demonstrated to avoid precipitating inappropriate treatment for
individuals with these other disorders (e.g., Lahey, 2001).
Carlson and Mann (2002) found that the inattentive
groups with low and high SCT did not differ on measures
of learning problems, suggesting that the relation between
SCT symptoms and the inattentive type is at least partially
independent of comorbid learning difficulties. However,
no previous studies have examined the relation between
SCT symptoms and intelligence or internalizing and externalizing psychopathology, suggesting that additional
research is needed.
This Study
In summary, previous studies of the relation between
SCT and ADHD have yielded promising but mixed results, suggesting that additional research is needed to clarify these findings. In this study, parent and teacher ratings
of the 18 DSM-IV ADHD symptoms and five putative
SCT items were obtained for a sample of children with
and without DSM-IV ADHD. Three sets of analyses were
conducted to test the relation between SCT symptoms
and DSM-IV ADHD symptom dimensions and diagnostic
subtypes.
(1) Confirmatory factor analyses were conducted to
test the fit of several competing models of the
relation between DSM-IV ADHD and SCT
symptoms.
(2) Scores on the obtained factors were compared
among groups with the DSM-IV ADHD subtypes
and a comparison sample without ADHD to test
if SCT symptoms are associated differentially
with the DSM-IV inattentive type.
(3) Finally, to address the concern that SCT symptoms may be confounded with lower intelligence,
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Hartman, Willcutt, Rhee, and Pennington
learning difficulties, or another disorder such as
depression or anxiety (e.g., Lahey, 2001), we
examined the phenotypic relation between parent and teacher ratings of SCT symptoms and
measures of general cognitive ability, academic
achievement, and internalizing and externalizing
behaviors, and tested whether these correlated
variables influenced the association between
ADHD and SCT.
METHOD
Participants
Recruitment
Participants completed the measures described in this
paper as part of their participation in the Colorado Learning Disabilities Research Center (CLDRC) twin study,
an ongoing study of the etiology of learning disabilities,
ADHD, and other related disorders (e.g., DeFries et al.,
1997; Willcutt et al., 2003). In collaboration with 27 local
school districts, parents of all twins between the ages of
8 and 18 were contacted by letter and invited to participate in the study. Approximately 35% of the families who
were contacted agreed to participate in the initial screening
procedure.
To preserve family confidentiality until they provided
informed consent, most school districts mailed the recruitment letter directly to the family so that the twins’
names would remain confidential until the family replied
to our letter to indicate their interest in participating in
the study. Therefore, it is not possible to compare directly
the families who participated in the study to the specific
families who did not respond to the initial recruitment
letter. Instead, the representativeness of the sample that
agreed to participate was indirectly examined by comparing the characteristics of the families who participated in
the study to the demographic information for each school
that is available in the public record. These comparisons
indicated that the families who agreed to participate in the
screening were drawn proportionally from the schools in
each district, and were representative of the overall population of each district in terms of gender ratio and ethnicity.
After obtaining parental consent, parents and teachers were asked to complete the Disruptive Behavior Rating
Scale (Barkley & Murphy, 1998) to assess symptoms of
DSM-IV ADHD. If one of the twins met symptom criteria for any DSM-IV ADHD subtype based on parent or
teacher ratings, the twin pair was recruited for the larger
study. In addition, twin pairs in which one twin had a
history of academic difficulties were recruited independent of the procedure to ascertain the sample of twins
with ADHD, and a third group of twins without ADHD or
academic difficulties was recruited from the same school
districts as a comparison sample. These procedures yield a
community-based sample that is enriched for ADHD and
learning difficulties. Approximately 95% of the families
in the screening sample agreed to participate in the larger
study if invited.
Exclusionary Criteria
After the initial screening but prior to the inception
of any testing, CLDRC staff conducted an additional telephone screening interview. Because the focus of the overall project is on familial ADHD and its correlates, potential
participants with a documented brain injury or other rare
genetic or environmental etiology that might produce a
phenotypic manifestation of ADHD were excluded from
the sample. In addition, three participants were excluded
from analyses because their Full Scale IQ scores were
below 75.
The Sample Used for This Paper
The use of twins for phenotypic analyses in which
each twin is considered as an individual data point presents
a methodological difficulty, as the scores of the twins in
each pair do not represent fully independent observations.
To address this issue most conservatively, one twin was
randomly selected from each twin pair in which both twins
met inclusion criteria for the study. Results were virtually identical when analyses were repeated in a sample in
which the selected twin was replaced by the cotwin that
was excluded from the first set of analyses, suggesting
that the random selection of one twin from each of these
pairs did not inadvertently bias the results. A dummy code
for zygosity was included in all initial models to control
for any differences between participants from MZ and DZ
pairs, but this code was dropped from all final models
because it had no significant impact on any result.
The final sample for this analyses includes all participants for whom complete data were available for parent or
teacher ratings of ADHD and SCT. The SCT items were
added to the battery in January, 1999, whereas the overall project has been ongoing since 1984. Therefore, the
sample described in this paper is a subset of the overall
CLDRC sample. After randomly selecting one twin from
each pair, parent ratings were available for 286 individuals (141 males and 145 females) and teacher ratings were
available for 229 individuals (119 males and 110 females).
Sluggish Tempo and ADHD
Although the overall sample ranged in age from 8
to 18 years, recruitment was weighted toward individuals
between 8 and 13 years of age (85% of the overall sample)
to facilitate follow-up analyses in a separate longitudinal
component of the study. Therefore, the mean age of the
present sample was 10.6 years (SD = 2.5). The pattern of
results was virtually identical when each set of analyses
was conducted after regressing variance associated with
age out of each variable; therefore, to simplify interpretation we present results of analyses without controlling for
age.
The ethnicity of the participants included in these
analyses (82% Caucasian, 9% Hispanic, 4% African
American, 5% other ethnicity) is consistent with the ethnic breakdown of the overall CLDRC sample and the total
population of students in the school districts from which
the twins were recruited. Ratings on the Hollingshead
2-factor inventory (1975) indicate that the socioeconomic
status (SES) of the overall sample falls near the middle of
the range on this scale (M = 3.02, SD = 1.13)
Measures
495
as predominantly hyperactive/impulsive type, and individuals with six or more symptoms on both dimensions
were coded as combined type. Because the primary goal
of this paper is to better understand the relation among
the DSM-IV ADHD symptom dimensions and SCT symptoms, the DSM-IV age-of-onset and cross-situation impairment criteria were not used to assign individuals to groups
for the categorical analyses.
A total of 76 participants met symptom criteria for
DSM-IV ADHD based on parent ratings (51 males and
25 females), and 57 individuals met symptom criteria
based on teacher ratings (38 males and 19 females). Consistent with other community samples (e.g., DuPaul et al.,
1998; Gaub & Carlson, 1997), the majority of participants
who met symptom criteria for DSM-IV ADHD met criteria
for the inattentive type (30 males and 15 females based on
parent ratings, 24 males and 12 females based on teacher
ratings). Most of the remaining participants met symptom criteria for the combined type (14 males and seven
females based on parent ratings; seven males and five females based on teacher ratings), and a small subset met
criteria for the hyperactive-impulsive type (seven males
and three females based on parent ratings; seven males
and two females based on teacher ratings).
Symptoms of ADHD
The Disruptive Behavior Rating Scale (DBRS;
Barkley & Murphy, 1998) was used to obtain parent and
teacher ratings of the 18 symptoms of DSM-IV ADHD.
Because maternal report was available for more participants than paternal report (95% vs. 73% of the sample),
maternal report was used for the analyses described in
this report. Each symptom on the DBRS is rated on a
four-point scale (never or rarely, sometimes, often, and
very often). Previous results from this sample and others
indicate that parent and teacher ratings on the DBRS or
other similar scales are internally consistent (α = .92–.96)
and have adequate to high test–retest reliability (r = .59–
.89; e.g., DuPaul, Power, & Anastopoulos, 1998; Willcutt,
Chhabildas, et al., 2001).
To facilitate the assignment of participants to groups
for comparisons among the DSM-IV ADHD subtypes,
items rated as often or very often were scored as positive
symptoms and items rated as never or rarely or sometimes
were scored as negative symptoms, consistent with the
procedure used in previous studies of similar rating scales
(e.g., Pelham, Gnagy, Greenslade, & Milich, 1992). Individuals with six or more symptoms of inattention but fewer
than six symptoms of hyperactivity-impulsivity were identified as predominantly inattentive type, participants with
six or more symptoms of hyperactivity-impulsivity but
fewer than six symptoms of inattention were categorized
SCT Items
Although several published measures include at least
a few items that may reflect SCT (e.g., Achenbach, 1991a,
1991b), there is no clear consensus regarding the behaviors that best capture the construct of SCT (e.g., McBurnett
et al., 2001). Therefore, an item pool of five potential SCT
symptoms was developed based on items used in other
studies (e.g., Achenbach, 1991; Frick et al., 1994) or created for this study, including sluggish/slow to respond,
seems to be “in a fog,” drowsy or sleepy, easily confused,
and daydreams/stares into space. The items daydreams
and sluggish/drowsy were used in the study by McBurnett
et al. (2001) and the DSM-IV field trials (e.g., Frick et al.,
1994). The five SCT items were then added to the parent
and teacher rating forms and administered on the same
four-point scale as the DBRS.
Measures of Other Psychopathology
The DSM-III-R parent-report version of the Diagnostic Interview for Children and Adolescents (DICA-P;
Reich & Welner, 1988) was administered to parents to
assess symptoms of oppositional defiant disorder (ODD),
conduct disorder (CD), generalized anxiety disorder
(GAD), and major depressive disorder. The DSM-III-R
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Hartman, Willcutt, Rhee, and Pennington
version of the DICA has been maintained to allow analyses of the entire sample in the overall study. To facilitate
the assessment of DSM-IV disorders, DICA modules were
modified as necessary to include all DSM-IV symptom
criteria.
In addition to the DICA, the parent and primary classroom teacher of each participant completed the appropriate version of the Child Behavior Checklist (CBCL;
Achenbach, 1991a, 1991b), a widely used measure of
child and adolescent psychopathology. Broad-band internalizing and externalizing summary scores were used for
this report. The externalizing score is based on the Delinquent Behavior and Aggressive Behavior scales, whereas
the internalizing score includes the Somatic Complaints,
Withdrawn, Anxious/Depressed scales. Neither the CBCL
Attention Problems scale nor any putative SCT items used
in previous studies are included in the internalizing or
externalizing summary scores. Finally, each youth also
completed the GAD module from the self-report version
of the DICA and the Children’s Depression Inventory
(CDI; Kovacs, 1988), a self-report checklist that is widely
used to screen for symptoms of depression.
Intelligence and Academic Achievement
The revised version of the Wechsler Intelligence
Scale for Children (WISC-R; Wechsler, 1974) was used
to assess the Full Scale IQ (FSIQ) of participants 16 years
of age or younger, and the Wechsler Adult Intelligence
Scale (WAIS; Wechsler, 1981) was administered to participants who were 17 or 18 years of age. Academic achievement in reading and mathematics was assessed with the
Peabody Individual Achievement Test (PIAT; Dunn &
Markwardt, 1970). To simplify interpretation a reading
composite score was created based on a previous discriminant function analysis of the PIAT Reading Recognition,
Reading Comprehension, and Spelling subtests (DeFries,
1985).
Procedures
Each participant completed a detailed psychoeducational battery that included the WISC-R, PIAT, and
self-report measures of internalizing disorders. While the
child completed these measures their parent completed the
DICA and CBCL. All interviews and tests were administered by graduate students in psychology who had experience working with children and families. Interviewers
and testers were unaware of the child’s diagnostic status or
the results of the other measures in the battery. Parents of
participants who were taking psychostimulant medication
were asked to withhold medication for 24 hr prior to each
Fig. 1. Confirmatory factor analysis models.
session of study to minimize the influence of medication
on the results.
Data Analysis
Confirmatory Factor Analyses
CFA was conducted to compare the fit of three nested
models (Fig. 1). In all specified models each symptom
loaded only on its respective latent factor, and the latent
factors were allowed to correlate with one another. The
first model was specified such that all DSM-IV ADHD
and SCT symptoms loaded on a single factor. This model
is most parsimonious, and the fit of this one-factor model
can then be compared to the fit of the more complex models. A two-factor model was then fitted based on the hypothesis that SCT items are additional manifest indicators
of the same latent trait measured by DSM-IV inattention
symptoms. Specifically, one factor included all DSM-IV
hyperactivity-impulsivity items, and the second factor included both DSM-IV inattention and SCT items. The final
three-factor model retained the hyperactivity-impulsivity
factor, but DSM-IV inattention symptoms and SCT symptoms were specified to load on separate factors.
Sluggish Tempo and ADHD
Fit Indices
A first appraisal of the fit of each CFA model was obtained by examining the overall chi-square for the model,
and the relative fit of each pair of competing models was
compared using the chi-square difference test. This
method compares the fit of two nested models by testing
if the more complex model yields a significant improvement in fit (as indicated by reduction in chi-square), with
degrees of freedom equal to the difference in degrees of
freedom between the two models (Kline, 1998).
Because chi-square values are extremely sensitive to
sample size, three additional fit indices were also used to
compare the relative fit of the competing models. Bentler’s
Comparative Fit Index (CFI; Bentler, 1990) describes the
overall proportion of variance explained by the model,
with a value of .90–.95 indicating acceptable fit (Bollen
and Long, 1993). Akaike’s Information Criterion (AIC;
Akaike, 1987) utilizes chi-square while adjusting for
model complexity; a decreasing value is desired when
comparing models (Kline, 1998). Root mean square error of approximation (RMSEA; Steiger and Lind, 1980)
compares the observed and expected covariances for a particular model; a value close to zero, and less than .10, is
desired.
The Relation Between SCT and DSM-IV ADHD Subtypes
Correlations among the latent factors yielded by the
CFA provided initial information regarding the relation
between SCT scores and DSM-IV inattention and
hyperactivity-impulsivity symptoms. To examine the relation between SCT and the DSM-IV ADHD subtypes,
the mean score of the items that loaded on each factor
was compared among the groups. As in other studies, the
symptom ratings were positively skewed and highly kurtotic (i.e., most children scored at or near zero), and were
best described by a random Poisson distribution. Therefore, planned comparisons between each pair of groups
were conducted by fitting a Poisson regression model (e.g.,
Gardner, 1995; McCullagh & Nelder, 1983).
The Relation Between SCT and Intelligence
and Other Psychopathology
To address the concern that SCT symptoms may simply be proxies for lower intelligence, comorbid internalizing disorders, or other variables that are typically correlated with ADHD (e.g., Lahey, 2001), we first examined
the zero-order correlations between SCT symptoms and
internalizing and externalizing psychopathology, reading
497
and mathematics achievement, and intelligence. Two sets
of analyses were then conducted to test if parent or teacher
ratings of DSM-IV ADHD symptom dimensions and subtypes were associated with elevations of SCT after controlling for the influence of these variables. The first analysis
examined the partial correlation between SCT scores and
DSM-IV inattention and hyperactivity-impulsivity symptoms, controlling for intelligence, academic achievement,
and internalizing and externalizing symptoms. Similarly,
these covariates were added to the initial Poisson regression models to test if any of the DSM-IV ADHD subtypes exhibited significant elevations of SCT symptoms
when scores on the measures of psychopathology, academic achievement, and intelligence were controlled.
RESULTS
Confirmatory Factor Analyses
Overall, confirmatory factor analyses showed that the
three-factor model represented the best fit when compared
to the two-factor or one-factor models (Table I). Chisquare values decreased significantly between the onefactor and two-factor models and between the two-factor
and three-factor models, indicating that the three-factor
model provided a significant improvement in fit over the
other models. Similarly, decreasing AIC values indicated
that the fit of the three-factor model was better than the fit
of the two-factor or one-factor models even after accounting for increased model complexity, and the increase in
CFI and decrease in RMSEA values across models provide additional evidence that the three-factor model best
represents the data. These results were consistent for both
parent and teacher ratings. Please see Fig. 2 for 3-factor
model loadings.
The Relation Between SCT and DSM-IV ADHD
Symptom Dimensions and Subtypes
To evaluate the implications of the present results
for the DSM-IV model of ADHD, correlations among
the three factors obtained from the CFA were examined.
Consistent with previous studies (e.g., Chhabildas et al.,
2001; DuPaul et al., 1998), the correlation between the
Inattention factor score and the Hyperactivity-impulsivity
factor score was high for both parent ratings (r = .73)
and teacher ratings (r = .63). The correlation between the
Inattention and SCT factors was similarly high (r = .75
for teacher ratings, r = .76 for parent ratings). In contrast,
the correlation between the Hyperactivity-impulsivity and
SCT factors was moderate for parent ratings (r = .51), and
was small and nonsignificant for teacher ratings (r = .18).
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Hartman, Willcutt, Rhee, and Pennington
Table I. Confirmatory Factor Analysis of Parent and Teacher Ratings of ADHD and SCT Symptoms
Fit indices
Model
Parent ratings
1. Single-factor model
2. Two-factor model
Difference between model 1 and model 2
3. Three-factor model
Difference between model 2 and model 3
Teacher ratings
1. Single-factor model
2. Two-factor model
Difference between model 1 and model 2
3. Three-factor model
Difference between model 2 and model 3
χ 2 (df)
CFI
AIC
RMSEA
1296(230)∗
854(229)∗
.75
.85
836
397
.130
.100
652(227)∗
.90
198
.083
1478(230)∗
831(229)∗
.64
.83
1018
373
.174
.121
667(227)∗
.87
213
.104
2 (df)
χdiff
442(1)∗
202(2)∗
647(1)∗
164(2)∗
Note. CFI = Comparative Fit Index; AIC = Akaike’s Information Criterion; RMSEA = root mean square
error of approximation.
∗ p < .0001.
The CFA results also provided further support for
the hypothesis that SCT symptoms are more strongly associated with symptoms of inattention than symptoms of
hyperactivity-impulsivity: For both parent and teacher ratings, when the correlation between the SCT and Inattention factors was dropped from the 3-factor model, the result was a large and significant decrease in model fit (data
not shown). In contrast, when the correlation between SCT
and hyperactivity-impulsivity factors was dropped from
the 3-factor model, the resulting decrease in fit was much
smaller.
Comparisons among groups with and without ADHD
revealed significant differences for both parent ratings
and teacher ratings of SCT symptoms (Table II). To facilitate comparison to results from other studies (e.g.,
McBurnett et al., 2001), the scores presented in Table II
are the means of the 0–3 scale of the DBRS (not at all–
very often) for the five SCT items, the nine DSM-IV inattention symptoms, and the nine DSM-IV hyperactivityimpulsivity symptoms. The pattern of results was identical
when factor scores from the CFA were compared
across groups. Pairwise comparisons between the groups
Fig. 2. Three-factor model with loadings. Note. Top loadings and correlations are based on parent ratings, bottom loadings
and correlations are based on teacher ratings. S1–S5 = Sluggish cognitive tempo items in the order listed on pg. 495,
I1–I9 and H1–H9 = Inattention and Hyperactivity/Impulsivity items in the order listed in DSM-IV.
Sluggish Tempo and ADHD
499
Table II. Mean Scores of Groups With and Without DSM-IV ADHD on Sluggish Cognitive Tempo and
DSM-IV ADHD Symptoms
Parent ratings
Sluggish cognitive tempo
DSM-IV Inattention
DSM-IV Hyp–imp
Teacher ratings
Sluggish cognitive tempo
DSM-IV Inattention
DSM-IV Hyp-imp
Comparison group
without ADHD
Hyperactive–
impulsive type
Combined
type
Inattentive
type
N = 210
0.19 (0.33)a
0.46 (0.34)a
0.38 (0.39)a
N = 172
0.27 (0.43)a
0.38 (0.47)a
0.21 (0.35)a
N = 10
0.30 (0.19)a
1.04 (0.33)b
1.93 (0.34)b
N =9
0.34 (0.36)a
1.23 (0.37)b
1.89 (0.27)b
N = 21
1.10 (0.75)b
2.36 (0.39)c
2.14 (0.36)b
N = 12
0.80 (0.49)b
2.34 (0.29)c
2.16 (0.41)b
N = 45
1.26 (0.65)b
2.11 (0.36)c
0.95 (0.51)c
N = 36
1.35 (0.71)c
2.13 (0.32)c
0.74 (0.54)c
Note. Means with different superscripts are significantly different, p < .05.
indicated that both parents and teachers rated the combined and inattentive groups significantly higher on the
SCT symptoms than the hyperactive-impulsive and comparison groups. The SCT mean of the inattentive group
was also significantly higher than the mean of the combined type based on teacher ratings, but the inattentive
and combined types were not significantly different when
ratings were completed by parents.
The Relation Between SCT
and Other Psychopathology
Many of the correlations between SCT ratings and
measures of externalizing psychopathology, academic
functioning, and FSIQ were significant, but the correlations were generally small in magnitude and similar
to the correlations between these measures and symptom scores on the DSM-IV inattention and hyperactivity-impulsivity items (Table III). In contrast, the relation between SCT and internalizing symptoms was more
variable across measures. Although correlations were
small if different raters completed the measures of SCT
and internalizing symptoms, correlations between SCT
and internalizing ratings completed by the same individual were substantially larger. For example, the correlation between SCT and CBCL internalizing behaviors
was significant for both parents (r = .52) and teachers
(r = .51).
Table III. Zero-Order Correlations Between Parent and Teacher Ratings of Sluggish Cognitive Tempo and DSM-IV ADHD
and Measures of Psychopathology, Intelligence, and Academic Achievement
Intelligence/academic achievement
WISC-R full scale IQ
PIAT reading
PIAT math
Externalizing symptoms
Parent CBCL
Teacher CBCL
DICA-P ODD symptoms
DICA-P CD symptoms
Internalizing symptoms
Parent CBCL
Teacher CBCL
DICA-P MDD symptoms
DICA-P GAD symptoms
DICA-C GAD symptoms
CDI total score
Sluggish cognitive tempo
DSM-IV inattention
DSM-IV hyp/imp
Teacher
Parent
Teacher
Parent
Teacher
Parent
−.33∗
−.33∗
−.23∗
−.19∗
−.16∗
−.11
−.39∗
−.39∗
−.32∗
−.22∗
−.25
−.18∗
−.22∗
−.25∗
−.13∗
−.12
−.15∗
−.10
.25∗
.25∗
.17∗
.12
.38∗
.27∗
.30∗
.23∗
.30∗
.53∗
.22∗
.15∗
.51∗
.32∗
.35
.24∗
.33∗
.68∗
.24∗
.19∗
.64∗
.36∗
.43∗
.28∗
.21∗
.51∗
.05
−.03
.07
.10
.52∗
.24∗
.33∗
.19∗
.20∗
.33∗
.17∗
.46∗
.12
.00
.17∗
.18∗
.38∗
.29∗
.34∗
.14∗
.15∗
.24∗
.13∗
.33∗
.10
.04
.15∗
.13∗
.39∗
.26∗
.22∗
.13∗
.17∗
.12
Note. WISC-R = Wechsler Intelligence Scale for Children—Revised; PIAT = Peabody Individual Achievement Test; CBCL = Child
Behavior Checklist; DICA-P = Diagnostic Interview for Children and Adolescents—Parent Report version; ODD = Oppositional
Defiant Disorder; CD = Conduct Disorder; DICA-C = Diagnostic Interview for Children and Adolescents—Self-report version;
MDD = Major Depressive Disorder; GAD = Generalized Anxiety Disorder; CDI = Children’s Depression Inventory.
∗ p < .05 (1-tailed).
500
Because SCT symptoms were significantly associated with these other constructs that are related to ADHD,
a final set of analyses was conducted to test if the relation
between ADHD and SCT remained significant after covarying scores on each of these other measures. For each
analysis all measures that were significantly correlated
with SCT ratings were included as covariates in the model.
Partial correlations between SCT and ADHD symptoms
remained significant for both parent ratings and teacher
ratings of inattention (pr = .53 for parent ratings; pr =
.58 for teacher ratings) and parent ratings of hyperactivityimpulsivity (pr = .26). Similarly, parent ratings of SCT
symptoms remained significantly higher in the inattentive and combined types than the hyperactive-impulsive
type and comparison group. In contrast, whereas the
inattentive type exhibited more SCT symptoms than the
other three groups based on teacher ratings, the combined
type no longer differed significantly from the comparison
group when the covariates were included in the model.
Hartman, Willcutt, Rhee, and Pennington
toms (daydreams and sluggish/drowsy) and one DSM-IV
inattention symptom (forgetful) loaded on a factor separate from the other DSM-IV inattention symptoms when
the inattention items were analyzed alone. In contrast,
the DSM-IV inattention and SCT symptoms loaded on
the same factor when DSM-IV hyperactivity-impulsivity
symptoms were also included in the model. Thus, the
present results provide even stronger support for the internal validity of the SCT construct, as the third SCT factor
emerged even when symptoms of both DSM-IV inattention and DSM-IV hyperactivity-impulsivity were included
in the analysis. Moreover, similar results were obtained for
both parent and teacher ratings. Therefore, these results
suggest that when a sufficient number of SCT items are
included in the analysis, SCT symptoms load on a factor
separate from both DSM-IV inattention and hyperactivityimpulsivity.
The Relation Between SCT and DSM-IV ADHD
DISCUSSION
This study examined the relation between symptoms
of DSM-IV ADHD and five potential symptoms of SCT
in a sample of twins overselected for ADHD and learning
difficulties. Confirmatory factor analyses showed that the
three-factor model resulted in an improvement in fit over
both a two-factor structure in which SCT items loaded on
the same factor as symptoms of DSM-IV Inattention and a
single-factor model in which all symptoms loaded on the
same factor.
The five items that loaded on the SCT factor (sluggish/slow to respond, in a fog, drowsy or sleepy, easily
confused, daydreams) are strongly characterized by slow
processing and hypoactivity, and appear to capture both
cognitive and physiological sluggishness. Moreover, although items from the Child Behavior Checklist were not
included in the final factor analyses because the items are
assessed on a different scale of measurement and
are somewhat redundant with the items described here,
initial exploratory analyses indicated that several CBCL
items (confused/in a fog, daydreams, stares blankly, overtired, and underactive/lacks energy) also generally
loaded on the same factor as the five SCT symptoms described in this report. Taken together, these findings support the internal validity of the SCT construct, and suggest
that the five items described here and several additional
items from the CBCL may be useful manifest indicators
of SCT.
These results replicate and extend the results of
McBurnett et al. (2001), who found that two SCT symp-
SCT factor scores were highly correlated with ratings of DSM-IV inattention by both parents and teachers,
and mean SCT scores were significantly higher in both
DSM-IV ADHD subtypes that are characterized by significant elevations of inattention (i.e., the inattentive and
combined types) than in the hyperactive-impulsive type or
comparison sample without ADHD. In contrast, the correlation between SCT symptoms and DSM-IV hyperactivityimpulsivity was substantially lower for both parent and
teacher ratings, and the mean SCT score of the hyperactiveimpulsive type was not significantly different from the
mean of the comparison sample. These results suggest
that SCT is related to DSM-IV ADHD, but primarily to
the inattention symptom dimension.
As noted previously, several authors have suggested
that SCT symptoms might better distinguish the inattentive and combined types than the inattention symptoms included in DSM-IV (e.g., Carlson & Mann,
2002; Hinshaw, 2001; McBurnett et al., 2001; Willcutt,
Chhabildas, et al., 2001). Interpretation of the present results is complicated by differences between parent and
teacher ratings. Teacher ratings indicated that the inattentive type exhibited significantly more SCT symptoms than
any other group, and the combined type did not differ from
the comparison group when symptoms of comorbid
disorders and other related variables were controlled. In
contrast, parent ratings indicated that both the inattentive type and combined type exhibited significant elevations of SCT in comparison to the group without ADHD,
and the means of the two subtypes were not significantly
different.
Sluggish Tempo and ADHD
In summary, teacher ratings provide fairly strong support for the hypothesis that SCT symptoms are associated
primarily with the inattentive type, whereas parent ratings suggest that SCT is associated more generally with
both the inattentive and combined types. This rater difference limits the conclusions that can be drawn at this
time regarding the potential utility of SCT symptoms as
part of the diagnostic criteria for ADHD in DSM-V. Future
studies of larger samples are needed to test further the difference between parent and teacher ratings, and to directly
compare the functional and neurocognitive correlates of
groups with each DSM-IV ADHD subtype with and without SCT. Until such studies are able to demonstrate that
the presence of significant SCT identifies a meaningful
subgroup of individuals with ADHD, it is premature to
suggest that SCT symptoms be included in future diagnostic criteria for ADHD.
The Relation Between SCT
and Other Psychopathology
In addition to those who suggest that SCT symptoms may distinguish between the inattentive and combined types, some authors have hypothesized that these
symptoms might even be a marker for a new attentional
disorder that is distinct from ADHD (e.g., Barkley, 2001;
Carlson & Mann, 2002; Milich et al., 2001). As future
research begins to evaluate this possibility, it will be essential to test not only the internal and external validity
of diagnostic criteria based on elevations of SCT, but also
the discriminant validity of the new disorder and other
psychopathology (Lahey, 2001).
Although this study provides no information regarding the diagnostic validity of a group defined based on elevations of SCT, our findings do have implications for the
relation between SCT and several other disorders. These
results suggest that SCT is significantly correlated with
internalizing and externalizing symptoms, academic difficulties, and lower intelligence. Most of these correlations
are small in magnitude, however, and elevations of SCT in
individuals with the DSM-IV inattentive type remain significant when all of these variables are controlled. Therefore, although future research is clearly needed to answer
this question more definitively, these results suggest that
SCT is associated with internalizing symptoms and some
specific neurocognitive difficulties, but that a substantial
proportion of the variance in SCT is independent of the
variance associated with these other variables. Nonetheless, if a new attentional disorder is defined based on SCT
in the future, it will be essential to ensure that this new
diagnosis does not inadvertently preclude children with
501
other disorders from receiving an appropriate diagnosis
and treatment, especially among individuals with severe
internalizing disorders that may be experiencing suicidal
ideation.
Limitations and Future Directions
Sampling Issues
Because this sample was overselected for both
ADHD and learning difficulties (e.g., DeFries et al., 1997;
Willcutt et al., 2003), these results warrant replication
in samples that are representative of the population and
samples ascertained from clinics. Similarly, the average
age of the participants in this study (M = 10.6 years)
is somewhat older than the age at which many children
with ADHD are first referred for a clinical evaluation. Future studies should examine the relation between SCT and
ADHD in samples of both younger children and older adolescents and adults. Finally, although the overall sample
size is adequate for the procedures described in this report,
our current sample is too small to conduct factor analyses separately in males and females. Preliminary results
from a larger sample in another laboratory suggest that
the three-factor model with separate SCT, inattention, and
hyperactivity-impulsivity factors may hold more strongly
for males than females (R. Todd, personal communication,
July 10, 2003) suggesting that additional studies of potential gender differences in the relation between ADHD and
SCT are needed.
Measurement
Although the availability of both parent and teacher
ratings is a strength of this study, SCT symptoms were
measured exclusively on a four-point Likert rating scale.
Therefore, future studies should examine other methods
of assessing SCT, such as diagnostic interviews, ratings by
peers or the children themselves, or direct observation in
the classroom or testing setting. Due to the relatively unobtrusive nature of SCT behaviors, structured behavioral
observations may be especially useful to complement parent and teacher ratings.
Further Development of Potential SCT Symptoms
As noted previously, the internal and external validity
of an exclusively inattentive disorder may be improved by
developing positive diagnostic criteria rather than defining
the disorder based on a negative diagnostic criterion such
502
as the absence of hyperactivity. By using the largest initial
pool of potential SCT items to date, this study provides an
important step in this direction. However, our final list of
five items is still smaller than the number that is typically
recommended to reliably assess a latent construct (e.g.,
DeVellis, 1991). Therefore, additional research is needed
to identify additional SCT items to facilitate the development of a reliable SCT scale. These studies should use
an initial pool of potential SCT items of sufficient size to
ensure that an adequate number of items remain for the
final symptom list even after some items are inevitably
discarded due to weak psychometric characteristics.
Functional Impairment and Other External
Correlates of ADHD and SCT
Data from this study and others increasingly support the hypothesis that SCT is an internally consistent
construct that relates to ADHD and other disorders in important ways. Therefore, it is essential that future studies
test further the external validity of the SCT construct by
examining the relation between SCT and important external correlates such as functional impairment and neuropsychological functioning. Such analyses will provide
an important step toward determining whether SCT symptoms mediate or moderate the relation between ADHD and
these external correlates, or if SCT symptoms identify a
unique disorder that is independent of DSM-IV ADHD.
Conclusions
Confirmatory factor analyses revealed that five putative SCT symptoms loaded on a third factor separate from
DSM-IV inattention and hyperactivity-impulsivity symptoms. Teacher ratings of SCT were most strongly associated with the inattentive type, whereas parent ratings
indicated that both the combined and inattentive subtypes
exhibited significant SCT. In summary, these results suggest that SCT is an internally consistent construct that is
significantly associated with DSM-IV inattention.
ACKNOWLEDGMENTS
Funding for the present research was provided by
NICHD grant HD 27802 (Center Director: J. C. DeFries).
The authors were also supported in part during the preparation of this manuscript by NIH grants T31 MH 16880
(J. Wehner), F32 MH 12100, R01 MH 62120, and R01 MH
63941 (E. Willcutt), K01 DA 13956 (S. H. Rhee), and MH
38820 and MH 04024 (B. Pennington). The authors thank
Hartman, Willcutt, Rhee, and Pennington
Ben Lahey, Caryn Carlson, and Keith McBurnett for their
helpful suggestions regarding the analyses described in
this report.
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