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ADHD Combined Type and ADHD Predominantly
Inattentive Type Are Distinct and Unrelated Disorders
Richard Milich, Amy C. Balentine, and Donald R. Lynam
University of Kentucky
We comprehensively reviewed research assessing differences
(ADHD) subtypes to examine the possibility that ADHD/
combined type (ADHD/C) and ADHD/predominantly inattentive type (ADHD/I) are distinct and unrelated disorders. Differences among subtypes were examined
along dimensions identified as being important in documenting the distinctiveness of two disorders. These include essential and associated features, demographics,
measures of cognitive and neuropsychological functioning, family history, treatment response, and prognosis.
Important differences among subtypes were found in
several areas of study, supporting the conclusion that
ADHD/C and ADHD/I may best be characterized as distinct disorders. We identify major limitations of the
available research and present future directions for research.
Key words: attention defict hyperactivity disorder,
ADHD combined, ADHD inattentive, ADHD subtypes,
ADHD classification. [Clin Psychol Sci Prac 8:463–488,
The history of the classification of attention-deficit hyperactivity disorder (ADHD) is rife with controversies. Early
concerns were raised over whether ADHD and conduct
disorders (CD) were distinct disorders or whether the
classification of these disorders could be handled more
parsimoniously by combining them into a broader, externalizing category. Although this controversy has been
resolved in favor of distinct externalizing categories (Hinshaw, 1987), other diagnostic debates have come to the
Address correspondence to Richard Milich, Ph.D., Department
of Psychology, 115 Kastle Hall, Lexington, KY 40506-0044.
E-mail: [email protected]
fore. For example, the changing DSM classification schemas have emphasized different primary symptoms underlying the disorder, starting with hyperactivity in DSM-II,
moving to attention problems in DSM-III, and currently
emphasizing both attention problems and hyperactive/
impulsive behaviors in DSM-IV. The identification of the
primary symptom of ADHD is a debate that continues
to the present, with some proposing deficient inhibitory
processes (e.g., Barkley, 1998) and others adhering to the
earlier emphasis on attention problems as the core difficulty (e.g., Brown, 1999).
Although these classification controversies have come
and gone as the zeitgeist and research findings have
changed, one diagnostic debate has persisted for the past
20 years. This concerns the validity of the inattentive subtype. Before the introduction of this disorder with the
publication of DSM-III in 1980, the literature had not
raised the possibility that children may have significant
attentional problems without the concomitant overactivity and impulsivity. Since 1980, when the inattentive
subtype was introduced, a large number of studies have
appeared examining the validity of this subtype of
ADHD, attempting to identify commonalities and differences between the two major subtypes of ADHD:
those with concomitant hyperactivity and impulsivity and
those with attention problems alone. As the literature
addressing this issue grew, a number of reviews appeared
arguing for the validity of the inattentive subtype (Carlson, 1986; Carlson & Mann, 2000; Carlson, Shin, &
Booth, 1999; Goodyear & Hynd, 1992; Lahey, Carlson, &
Frick, 1997; Lahey & Willcutt, 2000; Wheeler & Carlson,
1994). This conclusion appears to hold regardless of
whether the review involves studies of the DSM-III
subtypes, attention deficit disorder with hyperactivity
(ADD/H) and attention deficit disorder without hyperactivity (ADD/WO) (Edelbrock, Costello, & Kessler, 1984;
Lahey et al., 1997) or studies of DSM-IV subtypes,
ADHD/C (combined type) and ADHD/I (predominantly inattentive type) (Gaub & Carlson, 1997; Morgan,
Hynd, Riccio, & Hall, 1996).
Despite all the current research on the validity of the
inattentive subtype, few investigators seem to be addressing the most important, albeit controversial, issue
concerning this disorder: Is the inattentive subtype actually a subtype of ADHD or a completely distinct diagnostic disorder? This is the question we address in this review.
The answer to this question has important implications for
future investigations in this area, especially in regard to
the inclusion and exclusion criteria for studies of ADHD.
If the inattentive subtype is not a subtype of ADHD, as
some investigators have suggested (see, e.g., Barkley,
DuPaul, & McMurray, 1990), then studies that indiscriminately combine these samples are misguided at best, and
the conclusions that are drawn from these studies may be
seriously flawed. The answer to this question can also have
important implications for our understanding of ADHD,
especially in terms of what constitutes the core features of
the disorder. Finally, if it is the case that the inattentive
subtype is a separate disorder and not part of the ADHD
family, then studies focusing on the similarities of and
differences between the inattentive and combined subtypes are unnecessarily narrow in scope, and attention
could better be directed toward identifying the nature of
the inattentive disorder.
To address the question of whether the combined and
inattentive subtypes are distinct and unrelated disorders,
we first review the history of ADHD, including the
changes in theory and diagnostic criteria. Next, we review
the literature examining the correspondence of the
different DSM diagnostic categories to examine whether
and how to collapse across studies using different diagnostic criteria. Third, we discuss several taxonomic perspectives for determining whether two disorders belong to
common or distinct diagnostic categories and use these
taxonomic perspectives as guidelines to review the available literature on ADHD/C and ADHD/I. Finally, we
identify significant gaps in the literature that preclude
making definitive conclusions concerning this issue, and
we make recommendations for future research.
Attention-deficit hyperactivity disorder has been reconceptualized several times since it first appeared as a diagnostic category in the DSM-II (American Psychiatric
Association [APA], 1968), at which time it was classified
as hyperkinetic reaction of childhood. The term indicates
that the disorder was primarily conceptualized as having a
core dysfunction of excess motor activity. However, with
the publication of DSM-III (APA, 1980) came a new diagnostic label, attention deficit disorder (ADD), and a new
conceptualization of the disorder as one of a dysfunction
in attentional processes. Of greater historical significance,
for the first time the disorder was subdivided into subtypes
to distinguish individuals with hyperactivity, labeled as
having attention deficit disorder with hyperactivity, from
those without hyperactivity, labeled as having attention
deficit disorder without hyperactivity. The absence of
hyperactivity in the latter category further demonstrated
that the role of hyperactivity in ADD had shifted from being a core component to being a nonessential, concomitant symptom.
It is interesting to note that little research existed at the
time DSM-III was published to support the validity of the
ADD/WO diagnostic category. In fact, one of the primary reasons this subtype was included in the DSM-III
appeared to be to stimulate investigations to determine
whether this subtype was a valid diagnostic disorder. The
clinical literature had suggested that children with ADD/
WO were being seen in clinics, and the originators of this
category hoped that formalizing the disorder in the DSM
would encourage researchers to undertake the necessary
validation studies.
Such validity studies began appearing almost immediately thereafter (e.g., Maurer & Stewart, 1980), and an
early review of this literature (Carlson, 1986) suggested
that ADD/WO was a valid behavioral category. Thus, it
was surprising when the DSM-III-R was released in 1987
with ADD once again reconceptualized as a unidimensional category (APA, 1987) and given the term attentiondeficit hyperactivity disorder (ADHD). DSM-III-R did
not provide for the subdivision of ADHD into subtypes
to distinguish between individuals with or without hyperactivity, and it did not even require that a significant number of attention symptoms be present. A diagnosis of
ADHD was made if an individual met any 8 or more of
the possible 14 symptoms of hyperactivity, impulsivity,
and inattention. A separate category, undifferentiated
attention-deficit disorder (UADD), was also included in
the DSM-III-R. However, no explicit diagnostic criteria
were given for it, and the category was listed in a separate
section of the manual. The result was that the DSM was
perceived to have abandoned the inattentive subtype.
V8 N4, WINTER 2001
While the UADD category provided a way to diagnose
individuals who displayed no hyperactivity or impulsivity,
it was also possible to meet criteria for ADHD without
endorsing any symptoms of hyperactivity. Thus, the unidimensional approach to ADHD resulted in a diagnosed
population that was more heterogeneous than before
(Lahey et al., 1997).
After publication of the DSM-III-R, evidence supporting the validity of a multidimensional approach to ADHD
began accumulating. Several factor analytic studies found
that symptoms of ADHD could be grouped into two factors: inattention and hyperactivity-impulsivity (Lahey et
al., 1997). As a result of these and other studies, ADHD
began to be reconceptualized once again, this time as a
disorder having two distinct areas of dysfunction. Thus,
the criteria for ADHD presented in the more researchdriven DSM-IV (APA, 1994) reflect this multidimensional
conceptualization of the disorder. ADHD is now subdivided into three categories, reflecting the possible
combinations of the two dimensions: ADHD/predominantly hyperactive-impulsive type (ADHD/HI), which
is marked by maladaptive symptoms of hyperactivityimpulsivity; ADHD/predominantly inattentive type
(ADHD/I), which is marked by maladaptive symptoms of
inattention; and ADHD/C, which is marked by maladaptive symptoms of hyperactivity, impulsivity, and inattention.
DSM-IV presents ADHD in a manner similar to that of
DSM-III, returning to a subtyping system of classification.
Conceptually, ADHD/C corresponds well to the DSMIII diagnosis ADD/H, as both are distinguished by symptoms of inattention and hyperactivity/impulsivity.
ADHD/I corresponds well to the DSM-III diagnosis of
ADD/WO, as both are distinguished by symptoms of
inattention and a lack of hyperactivity. ADHD/HI shows
the least correspondence to a prior ADHD diagnostic category and has the least empirical support at this time
(Lahey et al., 1997). In fact, similar to the motivation
behind the original identification of the ADD/WO subtype, the identification of the ADHD/HI subtype seems
partially driven by the desire to encourage research examining the validity of this subtype.1
To properly review the literature on the ADHD subtypes,
it is necessary to determine which studies should be
included in the review. Some articles have examined
ADHD as it is classified in the DSM-III, while others have
examined the DSM-III-R or DSM-IV version of ADHD.
Several studies have explored the correspondence among
the different DSM versions of ADHD. These results have
helped researchers to determine if findings from studies
examining one particular DSM version of ADHD can be
assumed to generalize to other DSM versions of ADHD.
A study by Biederman et al. (1997) examined the correspondence between DSM-III-R and DSM-IV ADHD.
Out of 405 consecutive referrals to a pediatric psychopharmacology program, 302 were given a DSM-III-R
diagnosis and 302 were given a DSM-IV diagnosis.
Ninety-three percent of children with a DSM-III-R
ADHD diagnosis also received a DSM-IV ADHD diagnosis. Of these children, 64% received a DSM-IV diagnosis of ADHD/C, 25% received a DSM-IV diagnosis of
ADHD/I, and 11% received a DSM-IV diagnosis of
ADHD/HI. Of the 21 children who met criteria for only
DSM-IV ADHD, 90% were the inattentive type, 5% were
the hyperactive-impulsive type, and 5% were the combined type.
A study by Morgan et al. (1996) examined 62 children
who had a previous DSM-III ADD diagnosis or a DSMIII-R ADHD diagnosis. The children were given a retrospective DSM-IV ADHD diagnosis; 30 were diagnosed as
predominantly inattentive type and 26 were diagnosed as
combined type. It was found that children with a DSMIII ADD/WO diagnosis received the predominantly inattentive type diagnosis more often than they received the
combined type diagnosis. Conversely, the children with a
DSM-III ADD/H diagnosis received the combined type
diagnosis more often than they received the predominantly inattentive type diagnosis. No differences in
receiving DSM-IV diagnoses were found for children
with a DSM-III-R ADHD diagnosis, although more did
receive a combined type diagnosis than a predominantly
inattentive type diagnosis.
In a third study by Baumgaertel, Woraich, and Dietrich
(1995), teachers rated 1077 children in a school sample on
DSM-III, DSM-III-R, and DSM-IV criteria for ADHD.
The prevalence rates for the various ADHD diagnoses
were examined and the congruence among these disorders compared. They found that 95% of children meeting
criteria for DSM-III and 95% meeting criteria for DSMIII-R also met criteria for a DSM-IV ADHD diagnosis.
One-hundred percent of children with ADHD/C, 70%
with ADHD/HI, and 30% with ADHD/I met criteria for
DSM-III-R ADHD. Ninety percent of children with
ADHD/C also met criteria for DSM-III ADD/H diagnosis. Only 32% of children with ADHD/I and 30% of children with ADHD/HI met DSM-III ADD criteria.
Finally, in a DSM-IV field trial study, structured interviews were given to multiple informants to assess the presence of DSM-III, DSM-III-R, and DSM-IV ADHD in
380 clinic-referred children (Lahey et al., 1994). Ninetyseven percent of children meeting DSM-III ADHD
criteria also met DSM-IV criteria. Of children who met
criteria for DSM-III ADD/H, 88% also met criteria for
DSM-IV ADHD/C or ADHD/HI. Of children who met
criteria for DSM-III ADD/WO, 91.7% met DSM-IV
ADHD/I criteria. Ninety-seven percent of children who
met criteria for DSM-III-R ADHD also met criteria for
one of the DSM-IV ADHD subtypes. For children meeting criteria for a DSM-IV subtypes, 100% of the combined type, 74% of the predominantly inattentive type,
and 61% of the predominantly hyperactive-impulsive type
also met DSM-III-R ADHD criteria. The DSM-IV
ADHD criteria resulted in a net 15% increase in number
of ADHD cases over the DSM-III-R criteria. The new
cases of DSM-IV ADHD were more than twice as likely
to be female, with most of these cases being in the predominantly inattentive type.
These four studies yield consistent findings regarding
the correspondence of the DSM-III, DSM-III-R, and
DSM-IV ADHD diagnoses. There is a high correspondence among them, with 93–97% of DSM-III-R cases
meeting DSM-IV criteria, and 95–97% of DSM-III cases
meeting DSM-IV criteria. The DSM-III subtypes ADD/
H and ADD/WO correspond well to the DSM-IV subtypes ADHD/C and ADHD/I, respectively. The DSMIII-R ADHD diagnosis yields the most heterogeneous
diagnostic group. The DSM-IV ADHD/C subtype corresponds more closely to ADHD than do the ADHD/I and
ADHD/HI subtypes. However, 30–74% of ADHD/I
cases meet criteria for DSM-III-R ADHD as do 61–70%
of ADHD/HI cases. From these findings it can be concluded that results from studies based on DSM-III ADHD
subtypes can be generalized with a good degree of accuracy to ADHD/C and ADHD/I subtypes. Results from
studies based on DSM-III-R ADHD should only be generalized to ADHD/C and ADHD/I subtypes with a great
deal of caution. The majority of studies we reviewed
examined DSM-III and DSM-IV subtypes. Before proceeding to these studies to assess the validity of ADHD/
C and ADHD/I as distinct disorders, we discuss the most
appropriate approach to use in establishing the validity of
a mental disorder.
There is debate over the most appropriate approach to use
in establishing the validity of a psychiatric disorder. The
most dominant and long-standing approach to classification is the neo-Kraepelinian approach (Widiger & Clark,
2000). This approach is based on the classification system
originated by Emil Kraepelin, who believed that mental
disorders represented disease states that could be identified
through the careful grouping of patients with disorders of
similar course and appearance. While he believed in the
organic etiology of most mental disorders, he asserted that
careful observation of patient behaviors was initially the
most fruitful step toward classification. During the past
few decades, this approach has been supported and developed by those within the neo-Kraeplinian movement
who believe in the need to classify disorders and develop
explicit diagnostic criteria for them. They view psychiatry
as a branch of medicine whose task is to investigate the
causes, diagnoses, and treatment of discrete mental disorders (Blashfield, 1984).
This movement became more influential with the publication of a journal article titled “Diagnostic Criteria for
Use in Psychiatric Research” (Feighner et al., 1972). This
article set forth explicit diagnostic criteria for 16 diagnostic categories and led to the formulation of explicit diagnostic criteria in the DSM-III (APA, 1980). The authors
also described criteria for establishing the validity of a psychiatric illness that are divided into five phases. In line
with its Kraepelinian origins, the first phase involves
obtaining a clinical description of the disorder. This
includes not only describing associated symptoms, but
also the distribution across race, sex, age at onset, and precipitating factors, among others. The second phase
includes conducting reliable laboratory studies to help
develop a more precise clinical description. Laboratory
studies include the use of certain psychological tests as
well as chemical, physiological, radiological, and anatomical examinations. In the third phase, exclusionary criteria
are developed so that boundaries can be drawn between
the disorder under investigation and other disorders that
bear a resemblance to it. The fourth phase is to carry out
follow-up studies of patients. This step helps illuminate
whether the patients being examined form a homogenous
group that is experiencing the same illness. Differential
V8 N4, WINTER 2001
outcomes for patients suggest that they may not all have
the same disorder. Although a given disorder may not
have a definitive outcome or prognosis, differential outcomes at least call into question the validity of the diagnosis under study. The final phase involves investigating the
family history of patients with the given disorder. The
validity of the diagnosis is strengthened if an increased
prevalence of the disorder is found among close relatives
of the patients (Feighner et al., 1972).
These five phases were expanded by Cantwell and
Baker (1988; see also Cantwell, 1995) into a six-stage
model to specifically assess the validity of childhood psychiatric disorders. Their stages include (a) studies of essential and associated features of the disorder and the
exclusionary criteria, (b) physical and neurological studies, (c) laboratory studies, (d) family psychopathology
studies and family interaction studies, (e) follow-up studies, and (f) treatment studies.
The traditional psychiatric approach as described above
is, of course, not the only way to classify mental disorders.
Quantitative approaches have become increasingly popular as more sophisticated statistical techniques have been
developed and as advanced computer technology has
made the routine use of these techniques feasible. This
approach involves the analysis of large data sets with
advanced statistical tools such as factor analysis and cluster
analysis to develop psychiatric groupings and symptom
The research assessing the validity of many childhood
disorders, including ADHD subtypes, has been based on
the traditional psychiatric model of classification. The
majority of studies examining the ADHD subtypes have
focused on clinical descriptions and differentiation from
other disorders, laboratory findings, and family history. As
a result, in this review we mainly focus on this body of
literature and examine ADHD subtypes from the perspective of the traditional psychiatric approach. However,
attention is also given to the small number of studies that
have taken a quantitative perspective and employed factor
analytic and cluster analytic techniques in an effort to
identify and categorize the ADHD subtypes. The statistical and categorical approaches can be viewed as offering
different and complementary perspectives on the question
of whether ADHD/I and ADHD/C are subtypes of the
same disorder. The statistical approach is appropriate for
determining whether meaningful subtypes exist, but this
approach does not offer specific guidelines for ascertaining
whether the subtypes are distinct or represent a common
underlying disorder. Thus, empirically derived factors or
clusters do not necessarily answer the question of interest.
The psychiatric approach, as noted earlier (see Cantwell &
Baker, 1988; Feighner et al., 1972) has offered specific criteria for determining whether distinct disorders exist.
Thus, first we review the few empirically derived classification studies to look for the existence of separate factors
and clusters, and then we review the categorical studies in
an attempt to determine the distinctiveness of the subtypes.
ADHD has been viewed at times as a multidimensional
disorder (e.g., DSM-III, DSM-IV), and at other times as a
unidimensional disorder (e.g., DSM-III-R). Several factor
analytic studies of DSM-III, DSM-III-R, and DSM-IV
ADHD criteria have been carried out to help address this
issue. Lahey et al. (1997) reviewed the initial studies in
this area (see also Lahey & Willcut, 2000), and subsequent
studies (e.g., Baumgaertel et al., 1995; DuPaul et al.,
1997) have come to similar conclusions. Because the
findings are generally consistent across all of the published
studies, we will not systematically summarize each study.
Instead, we review the Lahey et al. (1988) study in detail
as a representative example and then mention additional
studies that may have obtained different findings.
Lahey et al. (1988) examined two separate samples, the
first of which was composed of 677 nonreferred, schoolaged children and the second of which was composed of
663 school-aged children who were referred to a clinic.
For children in both samples, teachers completed a checklist composed of DSM-III symptoms for ADD. For the
clinic-referred sample, clinicians rated children on an
experimental scale consisting of ADD/H and DSM-III-R
ADHD items as well as five items frequently associated
with ADD/WO (e.g., sluggish, forgetful). The teacher ratings for both samples and the clinician ratings for the clinic
sample were separately subjected to principal components
analysis with varimax rotation. The analyses of teacher ratings for both samples yielded nearly identical two-factor
solutions: one factor was composed of items assessing inattention-disorganization, whereas the other factor was
composed of items assessing hyperactivity-impulsivity.
The inattention-disorganization factor consisted of items
relating to concentration problems, distractibility, difficulty finishing tasks, and organizational difficulties. Three
of the purported impulsivity items from DSM-III actually
loaded on this factor. The hyperactivity-impulsivity factor
consisted of the remaining three impulsivity items from
DSM-III (e.g., acts before thinking), as well as items
reflecting motor overactivity. In addition, a three-factor
solution resulted from the clinician ratings: a hyperactivity-impulsivity factor, an inattention-disorganization factor, and a sluggish tempo factor.
A review of the other studies that report factor analyses
of DSM-III, DSM-III-R, and DSM-IV ADHD criteria
consistently yield a two-factor solution that differentiates
symptoms of attention from symptoms of hyperactivityimpulsivity (Lahey et al., 1997). The primary exceptions
to this conclusion arise from studies that added other
internalizing or externalizing disorder symptoms to the
analyses (e.g., Pelham, Gnagy, Greenslade, & Milich,
1992; Wolraich, Hanah, Pinnock, Baumgaertel, &
Brown, 1996). In such cases, additional factors (e.g.,
oppositional defiant disorder [ODD], anxiety) are usually
produced that reflect these additional symptoms. When
additional symptoms of inattention are added (e.g., sluggishness, forgetfulness) that supposedly reflect the problems of the inattentive subgroup, inconsistent findings are
obtained. In Lahey et al.’s (1988) study, a separate sluggish
tempo factor was extracted in addition to an inattentiondisorganization factor for the clinician ratings but not for
the teacher ratings. However, when Bauermeister, Alegra,
Bird, Rubio-Stipec, and Canino (1992) performed a
similar procedure, they failed to obtain a separate sluggish tempo factor. Instead, the associated symptoms of
ADHD/I loaded onto the inattention factor.
Despite these occasional inconsistencies, the results are
quite reliable in documenting that, when the DSM
ADHD criteria are factor analyzed, two distinct factors
emerge reflecting inattention problems and hyperactivity/
impulsivity. It was from data such as these that the DSMIV proposed a two-factor conceptualization of ADHD
(Lahey et al., 1997). However, as noted earlier, such factor
analytic evidence is necessary but not sufficient in
determining whether two distinct syndromes of ADHD
exist. That is, distinct factors are implied by the conception of these problems as subtypes of the same disorder
(necessity), but such results are also consistent with the
conception of these problems as distinct disorders.
Some authors (e.g., Lahey et al., 1997) have argued that
the question of whether ADHD is better conceptualized
as two distinct syndromes is more adequately addressed
through the use of cluster analytic techniques rather than
through factor analysis. The former approaches group
individuals based on their similar symptom picture,
whereas the latter approach groups symptoms according
to their coherence. Unfortunately, there are only two
published studies that have used cluster analysis to better
understand the nature of ADHD. In the study by Lahey
et al. (1988) described above, three factors derived from
clinician’s ratings (inattention-disorganization, hyperactivity-impulsivity, and sluggish tempo) were subjected
to cluster analysis. Three clusters emerged: the first cluster
was low on all three factors, the second cluster was high
on inattention-disorganization and sluggish tempo and
low on hyperactivity-impulsivity, and the third cluster was
high on inattention-disorganization and hyperactivityimpulsivity and low on sluggish tempo. Seventy-five percent of children diagnosed as ADD/H fell into the third
cluster, and 95% of children diagnosed as ADD/WO fell
in the second cluster. Of the remaining ADD/H children,
20% fell in the second cluster and 5% fell in the first cluster. Of the remaining 5% of ADD/WO children, all fell in
the third cluster. These results indicate that children with
ADD/H are best characterized by inattentiondisorganization and hyperactivity-impulsivity, whereas
the children with ADD/WO are best characterized by
inattention-disorganization and sluggish tempo.
Bauermeister et al. (1992) subjected two dimensions
of ADHD (inattention and hyperactivity-impulsivity) derived from factor analysis to cluster analysis. A five-cluster
solution yielded three ADHD profiles and two profiles
reflecting a lack of clinical symptoms. The first cluster was
characterized by high hyperactivity-impulsivity and
moderately high inattention (H), the second cluster was
characterized by very high inattention and very low
hyperactivity-impulsivity (I), and the third cluster was
characterized by both high inattention and high
hyperactivity-impulsivity (HI). Seventy-three percent of
children in the H cluster and 60% of children in the IH
cluster received an ADD/H diagnosis. Children in the I
cluster were equally likely to receive ADD/H, ADD/
WO, or no diagnosis. Children in these three clusters
were compared on several other variables. Children in the
IH cluster were found to be more active and impulsive,
have a higher rate of mother-reported pregnancy complications, and were rated by teachers as more unpopular,
anxious, aggressive, and self-destructive than children in
the I cluster. The I cluster children were rated by teachers
V8 N4, WINTER 2001
as more socially withdrawn than the other children and as
having more reading difficulties than IH children. The
types of inattentive problems appeared to differ for I and
IH children. I cluster children were rated higher on moving slowly, blank staring, and concentration problems but
lower on confusion, getting tired too much, and lack of
persistence. H cluster children were more likely to be
male and from lower socioeconomic status than IH children. They were rated by teachers as less socially withdrawn than I and IH children.
The results of these studies provide support for a distinction between children exhibiting problems with inattention and hyperactivity-impulsivity (ADHD/C) and
those exhibiting predominantly problems with inattention (ADHD/I). Although it is not possible to draw
definitive conclusions based on cluster-analytic work, the
results do provide support for the distinctiveness of the
syndromes of ADHD/C and ADHD/I. Again, however,
these results can be interpreted as supporting the conceptualization of these two disorders being distinct and unrelated or supporting the belief that they are subtypes of the
same disorders. To further explore this issue, it is necessary
to examine the evidence from a categorical perspective,
in which guidelines for identifying the distinctiveness of
disorders have been offered.
R E V I E W O F S T U D I E S C O M PA R I N G A D H D / C A N D
As noted earlier, there are no universally agreed-upon criteria for determining whether two disorders are distinct
and unrelated or instead represent subtypes of a common
disorder. However, Feighner et al. (1972) and Cantwell
(1995; Cantwell & Baker, 1988) offered guidelines for
determining the validity of a disorder, and these criteria
have been used by framers of the DSM and others to identify distinct disorders. Extrapolating from these guidelines,
the more of these criteria that differentiate the two disorders, the more likely they are to represent distinct, unrelated disorders. To facilitate understanding of this issue,
we review the available studies comparing ADHD/C and
ADHD/I samples according to criteria similar to those
proposed by Cantwell and Baker (1988). Thus, in the
review that follows, we have sections devoted to essential
features; associated features; demographics and epidemiology; laboratory studies; family history and family psychopathology studies; treatment response; and prognosis
and follow-up studies.
In selecting studies appropriate for this review, care was
taken to include only articles that identified ADHD
groups using DSM criteria. Although several studies used
strict DSM criteria, others included in this review used
experimental criteria that were based on the DSM. Additionally, some studies using explicit DSM criteria adhered
to only criteria for symptoms, ignoring criteria for age of
onset and impairment in multiple domains. Studies of all
of the above types were included in the review. In addition, some of the studies were carried out using a clinical
population, while the remainder were carried out using a
community or school population. Studies examining both
types of samples are important for gaining a better understanding of ADHD. Specifically, studying community
samples may provide more accurate estimates of demographic characteristics, such as prevalence, age of onset,
and comorbidity, among others. Alternatively, clinic
samples may provide more accurate information about
expected treatment outcomes. Thus, although both types
of samples are profitable to study, their findings may be
discrepant with each another. This factor should be considered when evaluating their results.
Essential Features
There are three essential features in the diagnosis of
ADHD: level of impairment on the inattention symptoms, level of impairment on the hyperactive/impulsive
symptoms, and age of onset. We reviewed the evidence
concerning subtype differences for these three essential
features. It seems reasonable to expect that if ADHD/C
and ADHD/I are simply subtypes of the same general disorder, we would find both groups to exhibit the same patterns of inattention problems, but the ADHD/C group
would show symptoms of hyperactivity and impulsivity as
well. A review of the literature, however, suggests that the
reality is not so straightforward.
Attention Problems. Although there are few studies specifically examining the nature of the attention problems
for the two ADHD subtypes, the available studies suggest
that the two subtypes may experience dramatically
different types of attention problems. This possibility was
first raised in studies examining the DSM-III subtypes, in
which the performance of the ADD/H group was characterized by distractibility and impulsivity, whereas the
behavior of the ADD/WO group was characterized by
what has come to be called a “sluggish cognitive tempo”
(SCT), consisting of behaviors such as drowsiness, leth-
argy, and hypoactivity (Carlson & Mann, in press). Although these latter items sound as though they have to
do with activity level, they tend to load on the inattention factor in factor analytic studies (see Lahey et al.,
1997). Achenbach has reported two similar inattention
dimensions in his factor analyses of both his rating scales
(Achenbach, 1996) and his Direct Observation Form
(Achenbach, 1991; McConaughy, Achenbach, & Gent,
1988). Items relating to this sluggish cognitive tempo were
included in the DSM-IV field trials for ADHD (Frick et
al., 1994), but they were not included in the final symptom lists for ADHD, primarily because the DSM-IV work
group wanted to keep the inattention symptoms identical
for all of the subtypes (Lahey et al., 1994).
This decision by the work group may have artificially
attenuated differences between the combined and inattentive subtypes because it eliminated those attention
symptoms that may have shown the clearest differentiation between the two groups. Following up on work
begun by McBurnett and Pfiffner (1999), Carlson and
Mann (in press) tested this possibility by subdividing a
sample of diagnosed ADHD/I children into two groups
depending on whether they showed elevated scores on the
two SCT items of “daydreams or gets lost in his/her
thoughts” and “underactive, slow moving, or lacks energy.” This dichotomization resulted in 28% of the inattentive group falling in the high SCT group and 72% in
the low group. Comparing the two groups on teacher ratings revealed that the high SCT group was rated higher
on internalizing problems and lower on externalizing
problems than the low SCT group. On measures relating
to internalizing problems, the ratings of the low SCT
group were more similar to those of the ADHD/C group
than they were to the high SCT group.
Similar results have been reported previously in three
studies comparing the inattentive and combined subtypes
on teacher ratings of inattentive symptoms. Lahey,
Schaughency, Frame, and Strauss (1985) examined a
school sample of ADD/H, ADD/WO, and normal control children using teacher ratings on the Revised Behavior Problem Checklist (RBPC). Groups were compared
on teacher responses to items on the Attention ProblemImmaturity (API) factor of the RBPC. Both ADD groups
were rated as exhibiting greater attention problems than
controls on 12 of the 16 API items. However, the two
ADD groups differed from each other on seven of the
items. ADD/H children were rated higher on the irre-
sponsibility, distractibility, impulsivity, sloppiness, and
answering without thinking items. The ADD/WO children were rated higher on the sluggishness and slowness
items. Lahey et al. (1985) suggested that the patterns of
behavior for the two groups may differ so markedly that
ADD/WO could be characterized as an internalizing disorder.
In another study examining differences in cognitive
tempo among ADHD groups, Lahey, Schaughency,
Hynd, Carlson, and Niever (1987) compared ADD/H
and ADD/WO children using teacher ratings. On the
Conners Behavior Rating Scale, teachers rated ADD/
WO children as having a more sluggish tempo than
ADD/H children. Finally, Barkley et al. (1990) also used
teacher rating scales to examine differences in patterns of
attention among ADD/H, ADD/WO, learning disabled,
and normal control groups. On the Teacher Rating Form,
ADD/WO children were rated higher than ADD/H children on items “lost in a fog,” “daydreaming or getting lost
in thought,” and “apathetic or unmotivated.” Thus, consistent with the results of Carlson and Mann (in press), the
results of these three studies all suggest that items dealing
with a sluggish cognitive tempo may well be the most sensitive items for differentiating the attention problems of
the combined and inattentive groups.
The results of these studies raise some interesting possibilities concerning the inattentive subtype. First, as Carlson and Mann (in press) note, this subtype as identified by
the DSM-IV criteria appears to be a somewhat heterogeneous group. Thus, within this subtype there may be at
least two different manifestations of the problem; a group
high on items relating to the sluggish cognitive tempo and
high on internalizing problems, and a group that may be
more similar to the combined type but having fewer
hyperactive/impulsive symptoms, resulting in these children incorrectly receiving an inattentive subtype diagnosis.
Although drawing definitive conclusions from these
few studies is premature, if our speculation is correct,
there are several important implications for our understanding of the ADHD subtypes. First, it appears that the
DSM-IV work group may have erred by excluding items
that tap into the sluggish cognitive tempo. These items
may be necessary to identify a pure inattentive group. Second, the diagnostic system needs to put a limit on how
many hyperactive/impulsive symptoms a child can demonstrate and still receive an ADHD/I diagnosis. For
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example, a child who meets criteria for the inattention
symptoms but evinces three to five H/I symptoms might
be considered subthreshold for the combined type rather
than receiving an ADHD/I diagnosis. As it stands now, a
child can have five H/I symptoms and still receive an
ADHD/I diagnosis. This obviously produces a heterogeneous group of children, some of whom may be considered purely inattentive and some of whom probably could
be classified as subclinical combined type (see Carlson &
Mann, in press). Thus, the studies examining the inattentive subtype are probably examining a heterogeneous
group of children, depending on how carefully the investigators have tried to obtain pure inattentives and exclude
subclinical combined children. This heterogeneity may
explain why there is considerable inconsistency in this literature, as will become apparent later on in our review.
A surprising omission in the literature examining the
validity of the inattentive subtype is the lack of studies
using laboratory or observational procedures to identify
the nature of the attention problems for the different subtypes. In fact, we found only a few studies addressing this
issue. Paternite, Loney, and Roberts (1996) failed to find
differences between ADHD/C and ADHD/I groups on
omission errors from a continuous performance test, a
vigilance task that purportedly measures both sustained
attention (omission errors) and impulsivity (commission
errors). Similarly, Carlson, Lahey, and Neeper (1986)
compared a sample of ADD/H, ADD/WO, and normal
control children obtained from a school population.
These groups were compared on a variety of cognitive
measures designed to assess aspects of attention problems,
including the Detroit Visual Attention Span for Objects
and the Stroop Color Distraction test. The results revealed
no differences between the two ADD groups on these
measures, although both groups were slower than controls
on time to complete the Stroop task. Finally, Barkley et
al. (1990) compared groups of ADD/H, ADD/WO,
learning disabled, and normal control children using the
Continuous Performance Task. No significant differences
were found between the ADD groups, although the combined group did exhibit more ADHD-consistent behaviors (e.g., off-task, fidgeting, out-of-seat) during the task.
In addition, the ADD/H group made more omission and
commission errors than learning disabled and control
groups, while the ADD/WO group also made more
commission errors than the learning disabled group.
The dearth of laboratory or observational studies
examining possible attention differences between the two
subtypes is especially surprising given the many laboratory
studies devoted to identifying the nature of the attention
problems among ADHD children in general. Barkley
(1997) and others have speculated that the combined and
inattentive subtypes may exhibit different types of attention problems, with the former exhibiting more distractibility and the latter more passive-inattentive behavior.
Although this is a plausible and even intriguing hypothesis, to date the only data supporting such a differentiation
come from ratings made by parents and teachers. Such
information does not offer the specificity necessary to
directly test this hypothesis.
Hyperactivity/Impulsivity. Similar to the area of attention
problems, there is a long history of using laboratory and
observational measures to examine the overactivity and
impulsivity problems associated with ADHD (see Tannock, 1998). Given this history, it is therefore surprising
to note how few studies have specifically examined these
symptom constructs as they relate to the combined and
inattentive subtypes of ADHD. Paternite et al. (1996)
failed to find group differences on commission errors on
the Continuous Performance Task, a purported measure
of impulsivity. Paternite et al. (1996) also compared the
validity of DSM-IV subtypes on the Kagan Matching
Familiar Figure Test (MFFT). Although few group
differences were found, the ADHD/C group displayed
significantly shorter response latencies than the clinical
control group. Conte, Kinsbourne, Swanson, Zirk, and
Samuels (1986) compared performances of ADD/H,
ADD/WO, and control groups on the MFFT, a purported measure of impulse control. Somewhat surprisingly, it was found that the ADD/WO group had shorter
latencies than the ADD/H and control groups. Additionally, the ADD/WO group made more errors than the
control group, suggesting that the ADD/WO group was
the most impulsive of the groups examined. Carlson et al.
(1986) found their ADD/WO group to be less accurate
than controls on a visual matching task analogous to the
MFFT. Barkley et al. (1990) also examined differences
among ADD/H, ADD/WO, LD, and normal control
groups using the Kagan MFFT. No differences were
found among groups on this measure. Finally, Trommer,
Hoeppner, Lorber, and Armstrong (1988) administered
the go/no-go paradigm to ADD/H, ADD/WO, and
normal control children. In this paradigm, children were
required to raise and lower their index finger in response
to hearing a single tap (go signal) and refrain from
responding to a double tap (no-go signal). All children
demonstrated the ability to respond properly to both signals. ADD/WO children made more commission errors
(which is suggestive of impulsivity) as well as more overall
errors on the first trial than did ADD/H children and
normal control children. However, ADD/WO children
improved more on the second trial than did ADD/H children, who committed an equal number of commission
errors on both trials.
Taken together, these studies offer few reliable differences between the combined and inattentive subtypes on
laboratory measures of impulsivity, with results from the
MFFT being contradictory. However, the construct validity of the MFFT has been questioned, so it is not clear
exactly what this test is measuring (Milich & Kramer,
Perhaps the most carefully done and compelling study
examining this issue is a recent investigation by Nigg,
Blaskey, Huang, and Rappley (2000). The authors compared combined and inattentive subtypes along with a
normal control group on a stop-signal task, a wellvalidated measure of behavioral inhibition (Oosterlaan,
Logan, & Sergeant, 1998). Nigg et al. found a deficit in
behavioral inhibition for the combined group, even after
controlling for comorbid ODD/CD symptoms. No such
deficit was found for the inattentive group.
The Nigg et al. (2000) study appears to be the only
investigation specifically documenting that the combined
group may have problems in disinhibition that are not
present in the inattentive group, despite the fact that such
problems are the major defining feature in differentiating
the two groups. Clearly, a great deal more research using
well-validated laboratory measures is needed to document
that the two groups do differ in this defining feature, as
well as to elucidate the underlying factors that differentiate
these two subtypes. As noted earlier, such specificity is not
possible when group differences are limited to behavioral
ratings made by parents and teachers.
Age of Onset. Identifying the specific age of onset of
ADHD can be difficult for several reasons (Campbell,
1990). First, very young children are often not placed in
situations that require sustained attention, so that it is
almost impossible to identify attention problems among
preschoolers. Second, parental ratings of ADHD among
young children tend to be quite high, with up to half of
the parents of preschoolers identifying their children as
having problems in the areas of overactivity, distractibility,
and impulsivity (Campbell, 1990). Nevertheless, it is
widely agreed upon that the problems with ADHD
should be evident once the child is placed in a structured
school setting. For this reason, the DSM requires an onset
of the disorder by age seven, or approximately by the time
the child has finished first grade.
Although only a few studies have examined this issue,
evidence is beginning to appear that suggests the combined subtype has an earlier onset and age of referral than
the inattentive subtype. Thus, the DSM-IV field trials for
ADHD found that only 57% of youths meeting symptom
criteria for ADHD/I also met the criterion for age of
onset before seven years, whereas 82% of youths meeting
symptom criteria for ADHD/C met the age of onset criterion (Applegate et al., 1997). Similarly, the field trials
found the ADHD/C group on average to be more than
one year younger than the inattentive type at age of referral (Lahey et al., 1994). Similarly, Paternite et al. (1996)
reported age of onset for ADHD/C children as 3.22 and
for ADHD/I children as 3.61; age at referral was reported
as 8.25 for ADHD/C children and 9.04 for ADHD/I
children. Finally, Faraone, Biederman, Weber, and Russell
(1998) found that the combined group had significantly
earlier age of onset (2.9 vs. 4.0 years) and referral age (6.4
vs. 9.2) than did the inattentive group. In contrast to these
significant differences, Barkley et al. (1990) failed to find
a difference in age at time of study between their ADD/H
group (8.3 years) and their ADD/WO group (9.0 years).
Further, one community-sample study found no differences in age of onset (Gaub & Carlson, 1997).
Thus, although only a few studies have examined subtype differences in age of onset, the evidence suggests that
the combined type is identified and referred at an earlier
age. It is possible that the earlier onset ages for the combined type represent a referral bias, in that the problems of
the combined type may be more evident or at least more
disturbing at an earlier age (Gaub & Carlson, 1997). For
this and related reasons, Barkley and Biederman (1997)
have critiqued the DSM-IV criterion of requiring an age
of onset before age 7. However, the field trial data make
it clear that this criterion works fine for the combined
type and hyperactive/impulsive types; it is only the inattentive subtype that fails to meet this criterion at a significantly high rate.
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Associated Features
In addition to the core or essential features identified with
ADHD, this disorder is noted to co-occur with a number
of associated problems. In fact, a number of these associated features (e.g., conduct disorders, learning disabilities,
social relations problems) show such strong relations with
ADHD that authors have speculated whether these associated features should be considered part of the disorder
itself (see Greene et al., 1996; Jensen, Martin, & Cantwell,
1997; Lynam, 1996). In the following section of the
review we examine associated features of ADHD in three
broad categories: comorbid disorders, social problems,
and academic difficulties.
Comorbidity. Different patterns of comorbidity among
ADHD/C and ADHD/I subtypes have been indicated in
the literature. In terms of externalizing problems, the literature is quite consistent in documenting that the
ADHD/C group is associated with higher rates of conduct disorder and oppositional defiant disorder compared
with the ADHD/I group (see Lahey & Willcutt, 2000).
Several studies have found that children with ADHD/C
are generally rated higher than children with ADHD/I
by parents and teachers on scales measuring aggression,
delinquent behavior, and conduct problems (Barkley et
al., 1990; Eiraldi et al., 1997; Gaub & Carlson, 1997;
King & Young, 1982; Lahey et al., 1987; Maedgen &
Carlson, 2000; Morgan et al., 1996; Nigg et al., 2000;
Paternite et al., 1996). Similarly, many studies have found
higher rates of CD and ODD among ADD/H children
compared with ADD/WO children (Barkley et al., 1990,
Eiraldi, Power, & Nezu, 1997; Faraone et al., 1998; Hynd
et al., 1991; Morgan et al., 1996; Wolraich et al., 1996).
Thus, for example, Faraone et al. found that the combined
group showed significantly higher rates of comorbid conduct disorder (24.3%) and oppositional defiant disorder
(68.1%) than did the inattentive group (8.7% and 44.0%,
Although the evidence concerning group differences
in comorbid externalizing disorders is quite consistent, no
clear distinction in patterns of comorbidity of internalizing problems between groups has emerged from the literature. In fact, elevated rates of internalizing disorders
often have been found in both ADHD/I and ADHD/C
groups (Eiraldi et al., 1997). Although Eiraldi et al. (1997)
found that parent and teacher ratings of internalizing
problems were higher for ADHD/C children than for
ADHD/I children, no difference between groups was
found in rates of co-diagnoses of mood and anxiety disorders. Faraone et al. (1998), Morgan et al. (1996), and Wolraich et al. (1996) also found that children with ADHD/
C and ADHD/I did not differ in rates of anxiety and
depression disorders.
In contrast, there have been at least three studies that
have found differences between ADHD/C and ADHD/I
in rates of comorbid internalizing disorders. Lahey et al.
(1987) found that children with ADD/WO (43%) were
more likely than children with ADD/H (10%) to have a
comorbid internalizing disorder. Similarly, Barkley et al.
(1990) found that the ADD/WO group was more likely
to receive a diagnosis of major depressive disorder than
the ADD/H group, whereas the latter had a higher rate
of separation anxiety disorder than the former. However,
it should be noted that in all cases the occurrence of these
comorbid disorders was less than 2% for each group. In
this study, the two groups did not differ on any parent or
teacher ratings of internalizing problems. Finally, Faraone
et al. (1998) report a somewhat controversial finding (see
Stein, Roizen, & Leventhal, 1999): the combined group
had a higher rate of comorbid bipolar disorder (26.5%)
than the inattentive group (8.7%).
In summary, the literature regarding the presence of
comorbid disorders in the combined and inattentive
groups is clear in documenting important differences in
terms of the externalizing problems of conduct disorder
and oppositional defiant disorder. The results concerning
internalizing problems are less clear cut, with some suggestion that these problems may be more evident in the
inattentive group, although the majority of studies fail to
find such differences. In the next section we compare the
groups in terms of academic difficulties, including comorbid learning disabilities, to further document important
group differences in the rates of associated features.
Academic Achievement. Academic underachievement is a
problem historically related to ADHD (Barkley, 1998).
On tests of academic achievement, both ADHD/C and
ADHD/I groups have been found to perform more
poorly than normal control groups (Barkley et al., 1990;
Faraone et al., 1998; Carlson et al., 1986). Studies examining differences between ADHD groups offer mixed
results. Several studies have found no differences between
groups on tests of academic achievement (Barkley et al.,
1990; Faraone et al., 1998; Frick et al., 1991; Morgan et
al., 1996; Paternite et al., 1996). Barkley et al. found that
equal percentages of ADD/H (31.7%) and ADD/WO
(31.9%) children were held back at least once in school,
but significantly more of the ADD/WO children had a
school placement of learning disabled. In contrast, 15% of
the ADD/H and none of the ADD/WO were suspended
from school at least once, and the former (12%) were significantly more likely than the latter (0%) to receive a
behavior disorders school placement. One study using
teacher ratings to assess academic performance obtained a
similar lack of differences (Baumgaertel et al., 1995).
However, Lahey, Schaughency, Frame, and Strauss (1984)
used teacher ratings to assess academic achievement and
found that normal control children were rated higher in
academic achievement than ADD/H children, who in
turn were rated higher than ADD/WO children. Carlson
et al. (1986) found that although both ADD/WO and
ADD/H groups performed worse than controls on measures of spelling and reading achievement, only the ADD/
WO group performed worse than controls on a measure
of math achievement. Similarly, Hynd et al. (1991) and
Marshall, Hynd, Handwerk, and Hall (1997) found that
children with ADD/H scored higher than children with
ADD/WO on a test of mathematics achievement.
Additionally, several studies have addressed the issue of
associated academic problems by comparing the combined and inattentive groups in terms of their comorbid
diagnoses for specific learning disabilities. Both ADHD/
C and ADHD/I subtypes have been associated with elevated rates of learning disorders. Faraone et al. (1998)
found that ADHD/C children were more likely than
ADHD/I children to have a co-diagnosis of language/
stuttering disorders, but no differences in the rates of
identified reading or math disabilities. Nigg et al. (2000)
also found the combined group to have a higher rate of
comorbid learning disorders (17%) than the inattentive
(7%) and control (0%) groups, which did not differ significantly. Morgan et al. (1996) found that ADHD/I children were more likely than ADHD/C children to have a
math learning disability. However, no differences between
groups emerged when both reading and math learning
disabilities were analyzed together. In contrast, studies by
Hynd et al. (1991) and Marshall et al. (1997) found that
ADD/WO children were more likely than ADD/H children to have some type of developmental learning disorder.
In summary, both children with ADHD/I and
ADHD/C have been found to perform more poorly on
measures of academic achievement than children without
ADHD, and both groups are also consistently more likely
to receive some form of a learning disabilities diagnosis.
Furthermore, when subtype differences are found, children with ADHD/I have been found to perform more
poorly on achievement measures than children with
ADHD/C, particularly on measures of math achievement
(Carlson et al., 1986; Hynd et al., 1991; Marshall et al.,
1997). This latter finding suggests that the processing
deficits in the combined and inattentive groups may be
qualitatively different, with the inattentive group perhaps
showing more deficits in right hemispheric functioning.
Social Functioning. Poor peer relationships have long
been identified among children with ADHD (Milich &
Landau, 1982). King and Young (1982) examined classroom peer perceptions of ADD/H, ADD/WO, and normal control children. Both ADD groups received fewer
“most liked” peer nominations and more “least liked”
peer nominations than the control group. No differences
were found between the ADD groups. Lahey et al. (1994)
found additional evidence indicating that ADHD groups
exhibit social impairment. In a study comparing DSM-IV
ADHD subtype groups and a clinic control group,
ADHD/C and ADHD/I groups were rated more as
“liked least” and less as “liked most” compared with clinic
controls. No differences were found between the ADHD
groups. Lahey et al. (1984) found similar results when
examining a school sample of ADD/H, ADD/WO, and
normal control children. However, it was also found that
ADD/H children received even more “liked least” nominations than ADD/WO children. Additionally, ADD/H
and ADD/WO children were more frequently nominated than control children for “frequently fight,” “poor
leaders,” and “not cute,” although ADD/H children were
even more likely than ADD/WO children to be nominated for “frequently fight.”
Carlson, Lahey, Frame, Walker, and Hynd (1987)
examined the sociometric status of ADD/H, ADD/WO,
and normal control children using peer nominations.
When children without co-diagnoses were compared,
ADD/H children received more “liked least” nominations than control children and ADD/WO children
received fewer “liked most” nominations than control
children. When children with co-diagnoses were included (primarily conduct disorder), both ADD groups
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received more “liked least” nominations and fewer “liked
most” nominations compared with the control group.
Additionally, ADD/H children were more likely to be
nominated as “fights most.”
Gaub and Carlson (1997) used teacher ratings to assess
the social functioning of ADHD/I, ADHD/HI, ADHD/
C, and normal control children. The control group was
rated as more liked, less disliked, and less regarded neutrally than all ADHD groups. The ADHD/C group was
rated as less liked and more disliked than the ADHD/I
group. Additionally, the children with ADHD/C were
rated higher on a social problems scale than the children
with ADHD/I, who in turn were rated higher than the
control group.
Edelbrock et al. (1984) compared the social functioning of ADD/H, ADD/WO, and clinic control boys using
teacher ratings on the Child Behavior Checklist-Teacher
Report Form. ADD/H boys were rated as more unpopular than ADD/WO boys. However, ADD/WO boys
were rated as more socially withdrawn. Hynd et al. (1991)
used parent ratings on the Personality Inventory for Children to assess ADD groups’ social functioning. It was
found that ADD/H children were rated as having more
inappropriate social skills than ADD/WO children.
Maedgen and Carlson (2000) compared combined and
inattentive groups on measures of social knowledge and
performance, as well as on their behavior following a disappointment. Parents rated the combined children as
more disliked and less liked than the inattentive group,
whereas teachers rated the combined group as less liked.
On a measure of social knowledge, both groups showed
deficits relative to controls, but the combined group
showed more aggressive solutions to social problems,
whereas the inattentive group showed more passive
behavior. Finally, there was a tendency for the combined
group to display more negative behavior following disappointment than children in the inattentive group.
Distinct behavioral differences between the inattentive
and combined subtypes were reported in a peer nomination and observational study by Hodgens, Cole, and Boldizar (2000). Although both subtypes of boys with
ADHD received lower social preference scores (“liked
most” nominations minus “liked least” nominations) than
controls, the two ADHD subtypes were observed to
engage in different patterns of behavior in playgroups.
The inattentive boys were observed to engage in high levels of solitary, on-looking behavior and low levels of sus-
tained interactions compared to the control boys and boys
with the combined subtype. In terms of peer nominations, the inattentive group received significantly more
“shy” nominations than the combined group, whereas the
latter received more “starts fights” nominations than did
the other two groups. These findings document that even
though both the inattentive and combined subtypes have
significant peer problems, these difficulties appear to
reflect different behavioral etiologies.
Together, the findings from studies assessing ADHD
children’s social functioning indicate that, although both
ADHD/I and ADHD/C groups exhibit social impairment, ADHD/C children appear to display more social
problems, such as fighting (Hodgens et al., 2000), and are
more likely to be actively rejected by their peers than
ADHD/I children (Wheeler & Carlson, 1994). In contrast, there is some evidence to suggest that the inattentive
group is more likely to exhibit passive, withdrawn behavior (Hodgens et al., 2000; Maedgen & Carlson, 2000).
This pattern of group differences is consistent with findings from the peer relations literature (Rubin, Bukowski, & Parker, 1998), which suggests that disruptive,
aggressive behavior is associated with both low popularity
and active rejection, whereas inattentive, passive behavior
is associated only with low popularity.
Demographics and Epidemiology
An important indicator of diagnostic distinctiveness can
be found in the epidemiology of ADHD, especially
differential gender ratios and differences in population
prevalence. A number of studies examine such differences
for the combined and inattentive groups.
Prevalence. In comparing the reported prevalences of the
combined and inattentive groups, an interesting differential pattern emerges depending on whether population or
clinic samples are examined. For the community samples,
ADHD/I is the more prevalent subtype (Carlson &
Mann, 2000), with the ADHD/I subtype approximately
twice as common as the ADHD/C subtype (Baumgaertel
et al., 1995; Gaub & Carlson, 1997; Wolraich et al., 1996).
In contrast, in clinical samples, ADHD/C is approximately 1.5 times more prevalent than ADHD/I (Eiraldi
et al., 1997; Faraone et al., 1998; Lahey et al., 1994; Morgan et al., 1996). Thus, although ADHD/I appears to be
more prevalent in the population, ADHD/C children are
much more likely to be referred to clinics for treatment.
This most likely reflects the greater amount of disruptive
behavior found in ADHD/C children.
Gender, Race, and Socioeconomic Status. Males are more
prevalent in all ADHD subtypes throughout both clinical
and community samples (Carlson & Mann, 2000; Lahey
et al., 1997). However, there is an even greater proportion
of males in the ADHD/C subtype than in the inattentive
subtype in both clinical and community samples. In community studies, the male-to-female ratio for the ADHD/
C subtype ranges from 2.8:1 (Gaub & Carlson, 1997) to
3.5:1 (Wolraich et al., 1996), with an average of 3.2:1. In
clinical studies, the ratio for the ADHD/C subtype ranges
from 1.2:1 (Faraone et al., 1998) to 8.4:1 (Lahey et al.,
1994), with an average of 4.1:1. For the ADHD/I subtype, ratios in the community studies range from 2:1
(Baumgaertel et al., 1995) to 2.3:1 (Gaub & Carlson,
1997), with an average of 2.1:1. Ratios in the clinical studies range from 1.5:1 (Faraone et al., 1998) to 3.7:1 (Lahey
et al., 1994), with an average of 2.5:1.
Race within ADHD subtypes was examined by only
one community-based study (Gaub & Carlson, 1997) and
by two clinic-based studies (Eiraldi et al., 1997; Lahey et
al., 1994). In all cases, no difference between subtype
groups was found. Socioeconomic status was only
reported in three clinic-based studies. Faraone et al. (1998)
found that ADHD/I children were of higher socioeconomic status than other ADHD subtypes. Paternite et al.
(1996) found a similar trend among ADHD groups,
although the results did not reach significance. In contrast,
Barkley, Grodzinsky, and DuPaul (1992) found no differences in socioeconomic status between the combined and
inattentive groups.
Overall, ADHD/C and ADHD/I groups can be distinguished on several important demographic variables,
especially gender and prevalence. Alternatively, they
appear to be indistinguishable on the demographic variables of race and socioeconomic status. Children with
ADHD/C are more prevalent in clinical samples, while
children with ADHD/I are more prevalent in community
samples. This most likely reflects the greater disruptiveness and aversiveness of the behavior of the combined
group, leading to a greater likelihood of being referred
and is consistent with the earlier age of onset for this
group noted earlier. Children with ADHD/C are also
more likely to be male, which raises interesting questions
about the possible underlying mechanisms (e.g., genetic,
biological, referral biases) that may differentiate the combined and inattentive groups.
Laboratory Studies
There are a relatively large number of studies that have
examined differences between the combined and inattentive groups in terms of laboratory measures. Most of these
studies have examined various forms of cognitive functioning or performance on neuropsychological measures,
although a few have looked at more direct measures of
brain functioning. Despite the relatively large number of
studies, this is a difficult area to review for several reasons.
First, it is difficult to identify reliable findings because
there is often little consistency across studies in the measures used. Thus, there are numerous cases in which a specific measure was used in only a single study. Second,
many of the studies appear to take an atheoretical
approach to differentiating the two groups, so that it is
difficult to know which findings may reflect chance
differences. In a related vein, several of the studies examine results from batteries of measures, with no attempt to
correct for the large number of analyses, further calling
into question the reliability of any significant findings that
may emerge. Third, many of these studies suffer from
small sample sizes, so that it is possible that the frequent
failure to obtain significant group differences may merely
reflect insufficient statistical power. Finally, these studies
frequently do not control for differences in IQ. Thus, it is
not always clear whether the results reflect group differences in the specific construct being measured or in overall intellectual functioning.
Bearing in mind these cautionary concerns, we
attempt to review this disparate literature in a somewhat
parsimonious fashion. Therefore, we will merely mention
the numerous studies and measures for which group
differences were not found, recognizing that low power
may well be compromising many of these studies. We
summarize the measures for which significant group
differences were obtained, with a special focus on measures for which reliable differences across studies are
A number of studies have assessed the possibility of
different levels of intelligence between ADHD/C and
ADHD/I groups. The consistent finding from the studies
is that there is no difference in intelligence between these
V8 N4, WINTER 2001
subtypes (Barkley et al., 1990, 1992; Faraone et al., 1998;
Frick et al., 1991; Holcomb, Ackerman, & Dykman,
1985; Hynd et al., 1991; King & Young, 1982; Morgan
et al., 1996; Nigg et al., 2000; Paternite et al., 1996;
Schaughency, Lahey, Hynd, Stone, & Piacentini, 1989).
Two studies found significant group differences in IQ, but
the results are not consistent across these studies. Carlson
et al. (1986) found that ADD/WO and control groups
obtained higher IQ scores than the ADD/H group. Additionally, they found that the ADD/WO group obtained a
higher verbal IQ score than the ADD/H group. Although
Marshall et al. (1997) found no difference in full-scale IQ
or verbal IQ among DSM-III-R ADHD and DSM-III
ADD/WO groups, they did find that the ADD/H group
obtained higher performance IQ scores than the ADD/
WO group. Among studies comparing ADHD/C and
ADHD/I groups with control groups, most have found
that both of the ADHD groups score lower on IQ measures than do controls (Barkley et al., 1990; Faraone et
al., 1998; Holcomb et al., 1985; King & Young, 1982;
Schaughency et al., 1989; Paternite et al., 1996).
In addition to measures of intelligence, a few studies
have examined group differences on extensive batteries of
neuropsychological measures. Schaughency et al. (1989)
administered the Luria-Nebraska Neuropsychological
Battery-Children’s Revision (LNNB-CR) to groups of
ADD/H, ADD/WO, and clinic control children. When
full-scale intelligence was included as a covariate, no significant differences emerged among the three groups on
the LNNB-CR battery scales. Similarly, Lorys, Hynd, and
Lahey (1990) failed to find ADD/H and ADD/WO
differences on several neurocognitive measures. The
groups were administered selected variables from the
LNNB-CR and reaction-time and speeded classification
tasks. Analysis of covariance using age, full-scale IQ, and
number of conduct disorder symptoms as covariates revealed virtually no differences among groups on the
neurocognitive measures.
In contrast to these null findings, Frank and Ben-Nun
(1988) studied differences between boys with ADD/H
and boys with ADD/WO using a neuropsychological
battery. Children with ADD/H were more likely to have a
history of perinatal or neonatal abnormality. Furthermore,
they were more likely to have abnormal findings on neurological examinations of motor skills. While both ADD/
H and ADD/WO groups performed below normal on
almost every cognitive psychometric test administered,
the ADD/H group performed below the ADD/WO
group on test of visual perception, visual sequential memory, and writing performance.
For a number of years now, ADHD has been theorized
to be associated with deficits in frontal lobe functioning
(Barkley, Grodzinsky, & DuPaul, 1992). Several recent
studies have examined whether these deficits also extend
to the inattentive group. Barkley et al. (1992) compared
12 boys with ADD/H and 12 boys with ADD/WO on
a variety of neuropsychological measures of frontal lobe
functioning, including pegboard performance, trailmaking test, Porteus Mazes, the Stroop, and the Wisconsin Card Sorting Test. There were no group differences
on any of these measures. Similarly, Nigg et al. (2000) did
not find a deficit for the ADHD group on the Tower of
London, a measure of executive functioning. Nigg et al.
found a deficit for the ADHD/I group on this measure,
but this was accounted for by IQ differences.
In contrast to the null results for measures of executive
functioning, Klorman et al. (1999) compared 102 children
with ADHD/I and 207 children with ADHD/C on two
measures of executive functioning: the Tower of Hanoi
and the Wisconsin Card Sorting Test. The ADHD/C
group exhibited deficits in executive functioning on both
tasks compared to the ADHD/I group. The former group
made more nonperseverative errors on the Wisconsin
Card Sorting Test and solved fewer puzzles and broke
more rules on the Tower of Hanoi. Further, these differences were not accounted for by comorbid ODD or reading disorder. Finally, Nigg et al. (2000) found the ADHD/
I group to exhibit a deficit in set shifting on the trailmaking test.
Several studies have attempted to examine group
differences on measures of automatic and effortful processing. Ackerman, Anhalt, Dykman, and Holcomb
(1986) compared ADD/WO, ADD/H, reading disorder,
and normal control groups on tasks assumed to measure
effortful processing. These tasks included semantic and
acoustic encoding, list learning of high- and low-imagery
words, memory of two printed words, rapid addition and
subtractions, and paper-and-pencil arithmetic tests. No
differences were found between ADD/WO and ADD/H
groups. The control group generally performed better on
the tasks than all three clinical groups.
Using the same sample of children, Ackerman, Anhalt,
Holcomb, and Dykman (1985) studied differences involving innate and acquired automatic processing. The sample
of children completed tasks involving frequency of occurrence sensitivity, naming and temporal sensitivity, speed of
writing O’s and one’s name, rapid addition and subtraction, and paper-and-pencil arithmetic test. Once again,
no differences in processing were found between ADD/
WO and ADD/H groups. However, on a task of arithmetic computation, the two groups showed delayed automatization compared with controls.
In contrast, group differences were found by Hynd et
al. (1991), who assessed 10 ADD/WO and 10 ADD/H
children using measures of rapid alternating naming
(RAN) and rapid alternating stimulus (RAS) naming.
These are generally assumed to assess automatized processes underlying confrontational naming. Results indicate than ADD/WO children were slower than ADD/H
children on these tasks, suggesting a deficit in automatized processing.
Other studies have examined other aspects of cognitive
functioning. Conte et al. (1986) compared performances
of ADD/H, ADD/WO, and control groups on paired
associate tasks. The tasks were presented under both fast
and slow presentation rates and under both mixed and
fixed rates. Although the control group made more correct matches during paired associate learning than the two
ADD groups, these latter two groups did not differ from
one another. Similarly, Carlson et al. (1986) did not find
differences between ADD/H and ADD/WO groups on
a variety of cognitive measures, including a rapid naming
task, a visual match-to-sample task, Detroit Visual Attention Span for Objects, and Stroop Color Distraction test.
Hynd et al. (1989) compared differences in reaction time
and cognitive processing speed among ADD/H, ADD/
WO, and clinical control groups. The groups did not
differ on simple reaction time, mean physical match classification time, and mean name-match classification time.
There were also no differences among groups on error
rates for these tasks.
Finally, one study examined group differences on
direct measures of brain functioning. Holcomb et al.
(1985) examined the differences in event-related brain
potentials among ADD/H, ADD/WO, reading disabled,
and normal control males. No differences were found
between ADD groups.
Overall, the studies of neurocognitive differences
between ADHD/C and ADHD/I do not allow definitive
conclusions to be drawn. Although the studies consistently support the conclusion that neurocognitive deficits
exist within both ADHD groups, few reliable differences
were found between groups. A few studies indicated possible differences between groups that need to be further
explored. Specifically, Klorman et al. (1999), using an
appropriately powerful design, did find executive functioning deficits to be specific to the combined group. This
is consistent with previous theorizing (see Barkley, 1997)
as well as research on behavioral inhibition (Nigg et al.,
2000), but no other study found such a differentiation. A
few studies pointed to processing deficits in the inattentive
group, including set shifting on the trail-making test
(Nigg et al., 2000) and automatized processing on the
RAN/RAS tasks (Hynd et al., 1991). Finally, Frank and
Ben-Nun (1988) found that ADD/H children had a
greater history of perinatal or neonatal abnormality as well
as higher amounts of abnormal motor skills than ADD/
WO children. Despite these few positive findings, it is disappointing that there is not one reliable group difference
in all of the studies examining cognitive and neuropsychological differences between the inattentive and combined groups.
Family History
Although researchers have explored the family psychiatric
history of children with ADHD fairly extensively (see,
e.g., Faraone, Biederman, Chen, Milberger, Warburton, & Tsuang, 1995), only a few studies have explored
differences between ADHD subtypes on this variable. In
1990, Barkley et al. compared ADD/H, ADD/WO,
learning disabled, and normal control groups in terms of
parental reports of psychopathology among maternal and
paternal relatives. It was found that ADD/H children
were more likely than all other groups to have paternal
relatives with attention deficits and hyperactivity as well as
maternal relatives with substance-abuse problems. ADD/
WO children were more likely than all other groups to
have maternal relatives with anxiety disorders. Additionally, both ADD/WO and learning disabled children were
more likely than the other two groups to have siblings
with a learning disorder. The mothers of the four groups
did not differ, in self-reported depression, although the
mothers in all three clinical groups reported greater psychological distress than did the mothers in the control
The results of the Barkley et al. (1990) study offer intriguing group differences in terms of family history of psychopathology, differences consistent with earlier reported
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evidence that the combined group is more associated with
externalizing problems, whereas the inattentive group is
more associated with internalizing problems. However,
this differential family history has not been replicated in
two other studies that reported such data (Faraone, Biederman, & Friedman, 2000; Frank & Ben-Nun, 1988).
Specifically, Faraone et al. failed to find that the subtypes
“bred true”—in other words, that there were significant
associations between the child’s disorder and the type of
ADHD present among first-degree relatives. The relatives
of the two subtypes both showed higher rates of ADHD/
C than did the control relatives, but the three groups did
not differ in terms of rates of ADHD/I among the firstdegree relatives. Thus, it is impossible to draw any firm
conclusions about differences in psychopathology among
relatives of probands of the inattentive and combined
groups. Future studies in this area are highly desirable
because the results of these investigations could be especially informative concerning possible genetic differences
among the subtypes, as well as implications for understanding the differential mechanisms that may underlie
these disorders.
Treatment Response
Only one study has compared of the response ADD
groups to medication. No studies have compared the
response of ADD groups to other forms of treatment,
such as behavior management or psychotherapy. Barkley,
DuPaul, and McMurray (1991) compared the responses
of ADD/H and ADD/WO groups to three doses of
methylphenidate (5-, 10-, or 15-mg doses). Using a tripleblind, placebo-controlled crossover design, medication
effects were assessed using parent and teacher ratings, laboratory tests, and direct observations during academic
tasks. An examination of all of the dependent variables
revealed that both the combined and inattentive groups
showed positive medication effects, with no variable
showing a significant group-by-dose interaction. This
would suggest that the groups are equally sensitive to the
medication across the range of doses. However, a somewhat different conclusion was drawn when medication
recommendations were made. Based on all of the drugresponse data available, a psychologist and a pediatrician
determined whether each child should be maintained on
medication at the end of the study and, if so, at what dosage. More ADD/WO children (24%) were judged nonresponsive to methylphenidate compared with ADD/H
children (5%). Additionally, more ADD/WO children
responded best to the lowest dose (35%), while more
ADD/H children responded best to the moderate (52%)
or highest (19%) dose of methylphenidate.
Barkley et al. (1991) suggest that the differential
response of the two groups to the medication are most
likely due to the greater severity of symptoms exhibited
by the combined group, allowing more room for the
higher doses of medication to have an effect. Given that
this is the only study examining medication responsivity
for the two groups, it is impossible to know whether
symptom severity accounts for this difference or whether
this may reflect some underlying biological difference.
Clearly this is an area where greater research is necessary.
Prognosis and Follow-up Studies
Numerous long-term follow-up studies of children with
ADHD have been reported in the literature. However, we
could find no study that has followed-up a sample of children with the inattentive subtype for an appreciative
length of time. This no doubt is due to some degree to
the relatively new status of this disorder. However,
it probably also reflects the fact that most studies in this
area have been concerned with establishing the validity
of this subtype, so that follow-up studies might appear
premature. A few studies do suggest that the hyperactive/
impulsive dimension is a greater predictor of negative
outcomes than is the inattention dimension (see Lynam,
1996), suggesting that the combined group would be at
greater risk. Such a conclusion would also be consistent
with the greater association of the combined group with
aggression and conduct disorders. However, without
definitive follow-up studies it is premature to draw any
firm conclusions about the possible differential outcomes
of the combined and inattentive groups.
A number of review papers have appeared of late summarizing the evidence concerning the validity of the inattentive group (Carlson & Mann, 2000; Carlson et al., 1999;
Lahey et al., 1997). However, these reviews generally have
started with the assumption that the inattentive subtype
belongs in the ADHD category, and the authors have concluded that this disorder is a valid subtype of ADHD. In
contrast, this review started with a different basic assumption: the possibility that the inattentive group is a valid
disorder but perhaps is not a member of the ADHD fam-
ily. When viewed from this perspective, the evidence
seems quite consistent: ADHD/C and ADHD/I are distinct and unrelated disorders.
According to the traditional psychiatric approach, the
validity of a psychiatric disorder is substantiated by demonstrating that it can be distinguished from other disorders
by a clear clinical description of essential and associated
features, neurological and physical studies, laboratory
studies, family psychopathology studies, follow-up studies, and treatment studies. Although there is no definitive
way to determine when a distinct disorder has been found,
the validity of a disorder increases as the body of evidence
grows in support of it.
The review of literature comparing ADHD/C and
ADHD/I groups reveals that the preponderance of evidence supports the conclusion that they are best conceptualized as distinct disorders rather than as subtypes of the
same disorder. ADHD/C and ADHD/I can be distinguished from each other by the essential features used to
differentiate the two disorders (i.e., hyperactive/impulsive
symptoms), and even in terms of the features they supposedly share (i.e., inattention symptoms). From this perspective, the two disorders have no defining features in
common, which challenges the basic assumption that the
disorders share some common core deficit. This conclusion is enhanced when one examines the symptoms used
to describe the disorders. For the inattentive type, “sluggish,” “hypoactive,” and “daydreaming, lost in space” are
the symptoms associated with this disorder (Carlson &
Mann, in press), whereas for the combined group the
relevant symptoms are “disinhibited,” “hyperactive,” and
The distinction is enhanced when the evidence regarding associated features and demographics is examined.
There is reliable evidence that the combined group, compared with the inattentive group, is much more likely to
be male, to have an earlier age of onset or referral, to be
actively rejected by their peers, and to have comorbid
externalizing disorders. This last finding is also supported
by the limited family history data available. In contrast, the
inattentive group is reliably more likely to have a math
disability, suggesting that there is a difference in the core
processing deficits in the two groups. In addition, there is
suggestive, albeit inconsistent, evidence that the inattentive group is more likely to have internalizing problems
and to have relatives with internalizing problems. Similarly, the inattentive group is more likely to be shy and
withdrawn in their peer relations rather than outright
rejected. This last finding is also consistent with the
increased prevalence of internalizing problems in the inattentive group. Finally, there is suggestive evidence that the
inattentive group may be less responsive to stimulant
medication and exhibit an effective response at a lower
dose than the combined group.
Looking at these findings in their entirety, without
being wed to the belief that the combined and inattentive
groups are subtypes of the same disorder, it is difficult to
imagine how one could conceive of them as variants of
the same disorders. Not only do they have virtually nothing in common, but in many ways they appear to fall at
opposite ends of a continuum-disinhibited versus inhibited, overactive versus hypoactive, externalizing versus
internalizing, energetic versus sluggish. In fact, there are
several other disorders (e.g., CD, ODD) that would
appear to have much more in common with ADHD/C
than does ADHD/I.
Removing the inattentive group from the ADHD family
of disorders clearly would cause some initial problems and
confusion. A new name for the disorder would be necessary, preferably one that did not suggest any association
with the ADHD disorders. Thus, attention deficit disorder (ADD) has been used occasionally to refer to the inattentive group, but this may be so similar, both historically
and currently, to the name for the combined group that it
would still carry connotations of the two disorders being
somehow related. Thus, a completely distinct name
would be desirable.
Deleting the inattentive group from the ADHD diagnosis would also cause some confusion and problems for
the general public. For example, according to the regulations of the Department of Education, ADD is referred to
as qualifying for special services, and this term is used for
both the inattentive and combined groups. Obviously, the
Department of Education can ignore any changes made
in the DSM, just as they have ignored the switch to the
ADHD terminology. Nevertheless, problems may arise if
educators and clinicians are using completely different terminology to describe the inattentive group. Parents may
also experience some confusion, believing their child has
an attention deficit disorder only to discover that their
child has a disorder with a different name that has no relation to ADHD.
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Despite these relatively minor confusions and inconveniences, redefining the inattentive group as a separate disorder can only have long-term benefits in terms of adding
clarity to the field. This is true for practical, theoretical,
and methodological reasons. Practically, redefinition will
help eliminate some of the unnecessary debate about
whether ADHD constitutes a disruptive disorder, as
claimed by the DSM. Parent support groups (e.g.,
CHADD) raised concerns about considering ADHD as a
disruptive disorder, primarily because many of the parents
in this organization had children with the inattentive type,
and thus they objected to characterizations of their children as having a disruptive disorder. Children meeting criteria for the combined group clearly engage in disruptive
behavior, so such a characterization is not a problem.
Theoretically, this change in the diagnostic system
should bring greater clarity to our understanding of the
underlying mechanisms involved in both disorders. Right
now it is difficult, if not impossible, to come up with a
unifying theory that can account for the problems experienced by both types of children. For example, Barkley’s
(1997) model for explaining ADHD in terms of problems
in inhibition does a nice job of accounting for many of
the problems experienced by children with ADHD/C.
However, Barkley (1997) explicitly states that his behavioral inhibition model of ADHD refers only to the
ADHD/C and ADHD/HI subtypes. This is because he
presumes their primary impairment to be response inhibition, while he presumes the primary impairment of
ADHD/I to be inattention. Thus, his theory is not appropriate in attempting to account for the problems of children with ADHD/I. Alternatively, there may be models
or theories that would better account for the problems
associated with the inattentive group. Viewing ADHD/I
as a distinct disorder will encourage the growth of theoretical literature regarding the development and dysfunction of children with ADHD/I. No strong theoretical
model exists that provides a framework for understanding
this disorder. Not only would such a theoretical model
inform our conceptualization of the subtype, but it would
inspire research to evaluate its validity. It probably is
impossible to develop a unifying theory to account for
both disorders. Recognizing the distinct nature of these
two disorders makes this a moot point.
Methodologically, removing the inattentive group
from the ADHD category should bring greater clarity to
the research being undertaken on both disorders. First, it
will encourage differentiating ADHD/C and ADHD/I in
research studies. Currently, the subtypes of ADHD are
often regarded as part of one homogeneous group and not
differentiated within studies; but they are, in fact, heterogeneous. This may be especially problematic for research
on the molecular genetics of ADHD and neuroimaging
studies, which rely on relatively small samples; both are
predicated on the notion that the phenotype or group
under study is relatively pure. Removing ADHD/I from
the ADHD umbrella will refine the phenotype and presumably help move this important research forward.
Of greater significance, perhaps, this change should
bring about a more focused research agenda concerning
the inattentive group. To date, all validity studies concerning this group have involved, as a comparison group,
ADHD/C. Further, almost all of the relevant research
questions addressed have arisen from what is known about
ADHD/C and how the two groups may differ along these
dimensions. For example, studies have been undertaken
to see if the two groups may differ in terms of measures
of behavioral disinhibition. However, under the current
reconceptualization of the inattentive group, there is no
reason to assume that behavioral disinhibition is a relevant
construct to study for this group. Similarly, when treatment studies for the inattentive group are planned, we
need no longer be locked into the mind set of identifying
potential treatments that have worked for the combined
group. Instead, viewing ADHD/I as a distinct disorder
should positively impact the quality of treatment provided
to children with ADHD/I. Specifically, it will encourage
the development of treatments, such as psychotherapy and
pharmacotherapy, that are specifically designed for the
unique needs of ADHD/I children. It will also encourage
physicians, clinicians, and educators to provide more tailored interventions for children with ADHD/I. The
problems these children experience may begin to be more
specifically addressed than they are currently, leading to
greater improvements for these children, both behaviorally and academically.
Finally, perhaps the best example of the benefits associated with this diagnostic reconceptualization is related to
the attention symptoms used in the DSM-IV criteria. As
noted earlier, the work group was so intent on using the
same criteria for attention problems for all of the subtypes
that they eliminated potentially some of the most relevant
attention problems for the inattentive group. However, if
the inattentive group is thought of as a separate disorder,
then investigators can focus on which specific attention
problems are most relevant for this group, regardless of
their relevance for the combined group.
Reconceptualizing the inattentive group as a separate
disorder does raise one classification problem: Where in
the nomological net does this disorder fall? Unfortunately,
researchers have not addressed this issue. It is possible that
the inattentive group may be better conceptualized as a
form of a learning disability. It is even possible, based on
the comorbidity and family history findings, that the inattentive group may have more in common with internalizing disorders. It may even be that it will need to find its
own niche. What does appear to be true from this review
is that it does not belong in the ADHD family.
In reviewing the literature on the inattentive group, one
can identify a number of problems that make it difficult to
draw firm conclusions about the inattentive disorder and
its relation to ADHD. In this section we review the many
difficulties in the hope that this will help guide and
improve future studies in this area.
Diagnostic Confusion. Although the inattentive disorder
is only 20 years old, the diagnostic criteria have already
undergone two major changes. The disorder disappeared
with DSM-III-R and reappeared with DSM-IV. Although
the evidence reviewed suggests that the correspondence
between DSM-III and DSM-IV versions of the inattentive
group is acceptable, problems exist in trying to derive
inattentive groups from samples obtained under the
DSM-III-R criteria. Thus, it is difficult to know how well
the samples identified in earlier studies correspond to
those samples being identified under the current diagnostic criteria.
Of greater significance is the confusion concerning the
appropriate criteria for determining attention problems.
The DSM-IV work group used identical attentional
symptom criteria for the combined and inattentive
groups, despite suggestive evidence that the nature of the
attention problems exhibited by these two groups may
differ. The result is that the inattentive group as currently
defined is heterogeneous and actually reflects at least two
different manifestations of the disorder; a sluggish inattentive group and a subthreshold combined group (Carlson & Mann, in press). Much of the inconsistency in the
literature may well reflect the different distributions of
these groups in the inattentive samples used.
Sampling Issues. In addition to diagnostic problems, we
also found that the results can be affected by sampling procedures, particularly whether clinical or community
samples are used (Carlson & Mann, 2000). The effect of
these sampling differences was most evident in the prevalence rates reported, in which the combined type is more
prevalent among clinical samples but the inattentive
group appears to be more prevalent in community
samples. Although perhaps not as dramatic, sampling also
appears to affect the findings concerning gender ratios and
the prevalence of comorbid disorders. Dramatic or not,
sampling differences among studies add noise to the
research literature.
Comorbidity. Another methodological factor to consider
in assessing the results of ADHD subtype studies is the
presence of comorbid psychiatric diagnoses among children with ADHD. Both internalizing and externalizing
disorders have been found to co-occur with ADHD, and
there is fairly consistent evidence that the inattentive and
combined groups may differ in the degree to which they
exhibit comorbid externalizing, internalizing, and learning disability disorders. On the one hand, this can be seen
as a valuable lead in identifying underlying differences
between the groups. On the other hand, comorbid conditions can be viewed as unfortunate confounds that make
interpretation of group differences difficult if not impossible. Most articles in this review did not differentiate
between ADHD children who did and did not have a psychiatric codiagnosis. However, a few of the studies examined ADHD groups with and without comorbid
psychiatric disorders. Carlson et al. (1987) found that
when children with codiagnoses (primarily conduct disorder) were removed from analyses of peer sociometric
nominations, children with ADD/H were no longer
nominated more than children with ADD/WO for
“fights most.” When Lahey et al. (1987) removed children
with a codiagnosis of CD from analyses of teacher ratings,
ADD/H children were no longer found to have more
conduct problems or to be more impulsive than ADD/
WO children. Similarly, Nigg et al. (2000) noted that
several of their results and conclusions about group differences changed when comorbid conditions were statistically controlled. These results suggest that some of the
impairment associated with particular ADHD subtypes
may be more related to co-occuring diagnoses than to
ADHD. Although such results are open to many interpretations (Angold, Costello, & Erkanli, 1999), at the very
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least they underscore the importance of taking comorbidity into account in all studies. Angold et al. (1999)
present an excellent discussion of both the problems
inherent in disentangling effects associated with comorbid
conditions, as well as the ways in which analyses of
comorbidity patterns can inform us about both the potential etiology of disorders and the validity of subtypes of
these disorders (see also Caron & Rutter, 1991).
Paucity of Studies. Although a considerable number of
studies have appeared over the past 20 years examining
the validity of the inattentive diagnosis, the studies are not
evenly distributed across the classification criteria used in
the present review. Thus, for example, although there are
a number of studies examining issues relating to comorbidity or cognitive functioning, there are few if any studies
in the areas of treatment response, family history, genetics,
and long-term outcome. Why these areas have been
ignored is unclear. Certainly long-term follow-up studies
are more difficult, especially given the relative recency of
the disorder, but their value is extremely high. Similarly,
family history and genetic studies are expensive and labor
intensive, but they probably bring us closer to understanding etiology than any other type of study. However,
treatment-response studies are not nearly so difficult, and
hundreds of such studies exist concerning ADHD/C.
Whatever the explanation for the dearth of studies in these
areas, such studies will yield important information
regarding the distinctiveness of the inattentive group.
Type 1 and Type 2 Errors. One of the more disappointing
problems in the literature is the consistently small sample
sizes. It was not unusual to find studies samples with 10
or 12 children per group. When other factors adding to
group heterogeneity are included (e.g., comorbid status,
gender, age, diagnostic heterogeneity), it is clear that
many of the studies reviewed were grossly underpowered.
Few studies ever corrected for the large number of comparisons undertaken. This was especially a problem in the
studies using laboratory measures, in which entire batteries were administered and examined with no correction
for inflated alpha levels. Thus, most of the studies
reviewed suffered in some fashion from high Type 1 and/
or Type 2 error probabilities, making it impossible to
know if unreplicated findings reflect chance findings or
weak statistical power in subsequent studies.
These methodological problems were compounded by
the fact that most of the studies reviewed were not
engaged in theory-driven hypothesis testing (see Nigg et
al., 2000, for an exception) in examining group differences. Instead, a variety of purportedly relevant measures
were collected to see which might differentiate the two
groups. One can have more confidence in the results
when they are consistent with well-thought-out theoretical hypotheses than when they reflect a small number of
significant effects resulting from the analysis of a battery
of measures.
Confirmatory Bias. In addition to the atheoretical nature
of much of the available literature, the research also
reflects a confirmatory bias. Specifically, the overwhelming majority of studies reviewed accept the views offered
by DSM-IV that the inattentive group is a subtype of
ADHD and that the proposed diagnostic criteria (especially, the inattention symptoms) are appropriate for both
groups. Different lines of investigation would have been
generated if, for example, the inattentive subtype were
assumed to reflect a developmental disorder rather than a
disruptive disorder. However, because assumptions such as
these were never considered, these lines of investigations
were not pursued. Further, studies in which differences
between ADHD/C and ADHD/I are found are interpreted only from the DSM subtype perspective, despite
the fact that the results are consistent with the distinct disorders position advanced here.
Future Directions. Although there are compelling reasons
to view ADHD/I and ADHD/C as distinct disorders,
additional research is needed to continue to build support
for the validity of these groups as distinct and unrelated
disorders. As noted earlier, there are many gaps in the literature concerning the inattentive group. One area to be
addressed is the formulation of a theoretical model that
addresses the development and dysfunction of ADHD/I.
While theoretical models for ADHD exist, the ADHD/I
subtype is not generally included. A theoretical model of
ADHD/I based on its deficit in cognitive control has yet
to be well developed. Several lines of research could help
in developing such a theoretical model. Specifically,
behavioral genetic and family history studies can be especially useful in identifying underlying, etiological mechanisms that may account for the problems experienced by
the inattentive group. In addition, studies using advanced
technology, such as neuroimaging methods, to examine
differences in brain metabolic processes or brain morphology can be very valuable in this regard. Further, family,
genetics, and epidemiological studies of comorbidity (see
Angold et al., 1999) will help determine where in the diagnostic nomenclature the inattentive group really belongs.
As noted earlier, much more information is needed on
the specific attentional problems associated with the inattentive group. Most of what we know about this comes
from studies examining responses on rating scales (see
Carlson & Mann, in press), rather than from systematic
laboratory investigations. Although the rating scale studies
offer some promising leads in describing the types of
attention problems these children experience (e.g., the
sluggish tempo described earlier), these instruments are
too global and too broad to offer specificity concerning
the actual patterns of attention exhibited by these children. Systematic investigations using well-validated laboratory and cognitive processing measures are needed to
precisely identify the specific deficits experienced by these
children. In a similar vein, such studies may offer valuable
insights to the diagnostic criteria employed for the inattentive and combined groups. Expanding and/or altering
the diagnostic criteria to identify attention problems for
the inattentive group may serve to increase the accuracy
of identifying true cases of this disorder.
Effective treatments and long-term outcomes are two
other areas where research is badly needed concerning the
inattentive group. To date, only one treatment study
involving ADHD/I children has appeared (Barkley et al.,
1991), and this involved medication only. It is interesting
to note that the large-scale, multisite treatment study for
ADHD (Arnold et al., 1997) actually excluded the inattentive group from the study, so we cannot assume that
the results reported would generalize to this group. Further, as noted above, it is not clear that the treatments
identified to be effective for the combined group would
automatically be the treatments of choice for the inattentive group. For example, the fact that this group reliably
exhibits greater math deficits suggests that more focused
academic interventions may be necessary for effective
intervention. By identifying the inattentive group as a distinct disorder, this will enable researchers and clinicians to
approach the problem of effective interventions from a
fresh perspective and not be limited by what is known
about the combined group.
Longitudinal studies assessing long-term outcomes for
the inattentive group also need to be conducted. These
investigations would help us better understand the developmental trajectories of these children. Gaining knowl-
edge in this area is necessary for developing appropriate
prevention plans and targeting children who are at risk for
particular problems, such as academic difficulties, underemployment, and interpersonal difficulties.
In sum, we believe that the field would benefit from a
shift in perspective. ADHD/C and ADHD/I are not variants of a single disorder, but should be considered distinct
disorders. Extant evidence supports this claim, and we are
confident that future evidence will as well, if we only
begin to look for it.
1. Given the paucity of research on the predominantly
hyperactive-impulsive subtype, issues relating to this subtype
will not be discussed further in this review.
We thank Caryn Carlson for her helpful suggestions and feedback concerning the manuscript.
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