Neuropsychology of Adults With ADHD meta analityc rev

Neuropsychology of Adults With Attention-Deficit/Hyperactivity Disorder:

A Meta-Analytic Review

Aaron S. Hervey

Duke University

Jeffery N. Epstein and John F. Curry

Duke University Medical Center

A comprehensive, empirically based review of the published studies addressing neuropsychological
performance in adults diagnosed with attention-deficit/hyperactivity disorder (ADHD) was conducted to
identify patterns of performance deficits. Findings from 33 published studies were submitted to a
meta-analytic procedure producing sample-size-weighted mean effect sizes across test measures. Results
suggest that neuropsychological deficits are expressed in adults with ADHD across multiple domains of
functioning, with notable impairments in attention, behavioral inhibition, and memory, whereas normal
performance is noted in simple reaction time. Theoretical and developmental considerations are dis-
cussed, including the role of behavioral inhibition and working memory impairment. Future directions for
research based on these findings are highlighted, including further exploration of specific impairments
and an emphasis on particular tests and testing conditions.

Attention-deficit/hyperactivity disorder (ADHD) is among the

most frequently diagnosed childhood psychological disorders,
with an estimated prevalence of 2%–7% (Barkley, Fischer,
Newby, & Breen, 1988). The disorder is characterized by symp-
toms of hyperactivity, impulsivity, and inattention (American Psy-
chiatric Association, 2000). Prospective, longitudinal follow-up
studies strongly support the idea that ADHD continues into adult-
hood. Estimates of the portion of children with ADHD who con-
tinue to meet full ADHD criteria as adults vary widely from 4% to
75% (Hechtman, 1992; Klein & Mannuzza, 1991; Mannuzza,
Klein, Bessler, Malloy, & Lapadula, 1993; Thorley, 1984; Weiss
& Hechtman, 1986; Wilens, Biederman, & Spencer, 2002). A
recent study suggests that this wide variation can be partially
accounted for by the source of the symptom reporting and the
definition of the disorder and that previous studies may have
underestimated the persistence of the disorder into adulthood (Bar-
kley, Fischer, Smallish, & Fletcher, 2002).

Although most prior research on ADHD has focused on chil-

dren, there is an emerging body of research examining ADHD in
adults. The past 10 years have seen the development of assessment
tools for adult ADHD (Conners, Erhardt, & Sparrow, 1999; Ep-
stein, Johnson, & Conners, 2001; Ward, Wender, & Reimherr,
1993), clinical medication trials for adults with ADHD (Spencer et
al., 1995), and a significant number of studies examining the
neuropsychology of ADHD in adult populations. Most of this
research has built upon the child ADHD literature by applying

techniques, methodologies, and findings documented in the child
literature to adults. To date, it appears that similar methodologies
translate well from child to adult samples. For example, rating
scales and interviews work well for diagnosing ADHD in adults,
and stimulants appear to be the treatment of choice for adults with
ADHD, consistent with the child literature.

The translation of neuropsychological findings from the child

ADHD literature to adult samples has not proven to be as easy as
translating assessment and treatment strategies. One reason for this
may be the lack of consensus regarding what neuropsychological
deficits actually exist in children with ADHD and what are the best
measures for assessing those deficits. The extensive child ADHD
literature presents several reviews of the neuropsychological per-
formance of children with ADHD (Barkley, Grodzinsky, & Du-
paul, 1992; Pennington & Ozonoff, 1996). Although these reviews
indicate a variety of neuropsychological impairments, they do not
unanimously implicate specific and agreed upon domains of neu-
ropsychological impairment. However, convergence in several ar-
eas, such as attention, working memory, and motoric inhibition,
have been noted, and a unifying theory of ADHD placing behav-
ioral inhibition at the crux of the dysregulation has been suggested
by Barkley (1997b).

The investigation of the neuropsychology of ADHD is a devel-

oping process, and neuropsychological studies of adults with
ADHD have only relatively recently emerged (see Figure 1). The
adult literature in this early stage of development consists mainly
of studies that are descriptive in nature, rather than studies de-
signed to evaluate specific hypotheses. Although this limits the
types of inferences that can be drawn from a meta-analysis of the
literature, it is important to take stock of the literature at this point
to determine what neuropsychological deficits appear to be most
commonly or most strongly associated with adult ADHD. Future
studies can then focus on the testing of hypotheses regarding the
developmental neuropsychology of ADHD.

There are several reasons to begin to examine and to synthesize

the results of neuropsychological studies of adults with ADHD.
First, at a theoretical level similarities and/or differences between
childhood deficits and adult deficits would provide potential in-
sight into the neural mechanisms operating in the disorder across

Aaron S. Hervey, Department of Psychology, Duke University; Jeffery

N. Epstein and John F. Curry, Department of Psychiatry, Duke University
Medical Center.

Additional materials are on the Web at http://dx.doi.org/10.1037/0894-

4105.18.3.485.supp.

Correspondence concerning this article should be addressed to Jeffery N.

Epstein, Child and Family Studies Center, Duke University Medical Cen-
ter, 718 Rutherford Street, Durham, NC 27705, or to Aaron S. Hervey, who
is now at Division of Cognitive and Behavioral Neurology, Brigham and
Women’s Hospital, 221 Longwood Avenue, Boston, MA 02115. E-mail:
epste002@mc.duke.edu or ahervey@partners.org

Neuropsychology

2004, Vol. 18, No. 3, 485–503

0894-4105/04/$12.00

DOI: 10.1037/0894-4105.18.3.485

485

development. This link would also help to establish an endpoint to
a developmental neuropsychological pathway that might eventu-
ally contribute to the validity of ADHD as an adult diagnosis.
Second, at an applied level this line of adult research has several
advantages over parallel child work. Most neuropsychological
tests are conceptualized, developed, and normed for use with
adults. Thus, there is a much broader range of available instru-
ments with well-documented norms available to help accurately
determine performance levels, to determine levels of impairment
when they are present, and to localize domains with deficits.
Finally, results from adult neuropsychological studies can be an-
alyzed without reference to variables associated with brain matu-
ration, a developmental process often mentioned as a potential
contributor to performance differences in ADHD (Tannock, 1998).

Currently, there is no consensus regarding the construction of

the neuropsychological profile of adults with ADHD. A compila-
tion of the existing data from studies identifying and evaluating
adults with ADHD would provide a much needed guide for clinical
and research endeavors. To date, one conceptually based review of
the neuropsychology of adult ADHD has been published (Woods,
Lovejoy, & Ball, 2002). Although comprehensive in its inclusion
of available studies on the topic, this review is limited in utility
because of its methodology. The review by Woods et al. (2002) did
not take into account the variations in sample sizes of ADHD and
control groups across studies. Instead it relied entirely on statisti-
cally significant differences as the primary index of group discrim-
ination across studies. This approach can potentially obscure large
effects between ADHD and control groups in small samples. The
authors highlighted the importance of statistical sophistication and
recommended the use of individual and collective effect sizes as
aspects necessary to better use the research findings in this area.

The present article includes a review and a compilation of the

existing neuropsychological data on adults with ADHD. We at-
tempt to develop a meaningful interpretation of their performance,
highlighting what appear to be the core neuropsychological deficits
with the intention of providing information and conceptualization
with implications both for research and for clinical purposes. In
general, most reviews of clinical neuropsychological data focus on
a single performance domain (e.g., attention). We decided to

perform a comprehensive and empirical review that examines and
synthesizes the entire body of published neuropsychological test
results on adult ADHD. Seven performance domains are specified
for interpretation of the data. Developmental considerations within
the context of this data are discussed. Methodological limitations
of current studies are identified. Finally, theoretical considerations
are discussed.

Method

Sample of Studies

Neuropsychological studies of adults with ADHD were identified

through computerized searches of MEDLINE (1966 –2002) and PsycINFO
(1966 –2002) using combinations of the following terms: ADHD, ADD,
adults, attention deficit disorder, attention-deficit/hyperactivity disorder,
executive function, hyperactivity, minimal brain dysfunction, and neuro-
psychology. References from studies retrieved using these searches were
reviewed to identify additional articles. To be included in this review,
studies (a) needed to be in the context of a published article that included
a group of participants with a minimum mean age of 18 years who were
diagnosed with ADHD and (b) had to have included some form of control
group. The articles also needed to include a neuropsychological assessment
other than or in addition to a version of the Wechsler Adult Intelligence
Scale (WAIS; Wechsler, 1981). Case studies were not included.

General Description and Limitations of the Studies

In all, 33 studies were identified for this review. Overall, the studies

ranged in publication date from 1979 to 2002, with the bulk of these (76%)
published after 1996 (see Figure 1). Participants in the ADHD groups in
these studies were usually acquired through specialty clinics and university
settings. Thus, they do not represent epidemiological samples, and because
clinical samples are more likely to have higher rates of comorbidity than
epidemiological samples, study samples to date may have more comorbid
disorders than would adults with ADHD not coming to clinical attention
(Curry, March, & Hervey, 2004). Study control groups generally consist of
individuals without clinical diagnoses. However, seven of the studies
include clinical control groups either in addition to or instead of a non-
clinical control group. These clinical control groups are varied in degree
and type of pathology. All findings within this review refer to comparisons
between ADHD and nonclinical control groups unless otherwise specified.

There is not complete consensus on criteria or methodology for deter-

mining a diagnosis of ADHD in adults, either in the field at large or in the
studies we reviewed. Most investigators have relied on the criteria set forth
in versions of the Diagnostic and Statistical Manual of Mental Disorders
(DSM; American Psychiatric Association, 2000) or some variant or mod-
ification of the DSM criteria. A minority of other investigators have
adopted the Wender-Utah criteria for ADHD (Wender, 1995). DSM criteria
are usually implemented similarly for diagnosing childhood ADHD and
adult ADHD. To determine symptom criteria, adults are asked about the
presence of the listed DSM symptoms. These include or have included
symptoms of inattention (e.g., often has difficulty sustaining attention),
hyperactivity (e.g., often fidgets or squirms in seat), and impulsivity (e.g.,
often interrupts or intrudes on others). Similarly, Wender symptom criteria
(Wender, 1995) require the presence of persistent motor hyperactivity and
attentional difficulties. In addition, at least two of five criteria must be
present, including affective lability, disorganization or inability to com-
plete tasks, hot temper or explosive short-lived outbursts, impulsivity, and
emotional overreactivity. Only 3 of the 33 studies included in this review
relied solely on Wender criteria (Holdnack, Moberg, Arnold, Gur, & Gur,
1995; Walker, Shores, Trollor, Lee, & Sachdev, 2000; Zametkin et al.,
1990; see Web Table 1 at http://dx.doi.org/10.1037/0894-4105.18.3.485
.supp for diagnostic criteria), with all others using DSM criteria alone or in
addition to Wender criteria.

Figure 1.

Number of empirical studies (by years of publication) compar-

ing neuropsychological performance of adults with attention-deficit/hyper-
activity disorder with a comparison group.

486

HERVEY, EPSTEIN, AND CURRY

The methods for determining ADHD symptom presence vary across

studies. In general, most studies include at least one behavioral question-
naire and an interview with a licensed practitioner (see Web Table 1 at
http://dx.doi.org/10.1037/0894-4105.18.3.485.supp). A number of authors
use questionnaires developed specifically for their studies; however, most
questionnaires query about the presence of DSM–IV (i.e., DSM, 4th ed.;
American Psychiatric Association, 1994) or previous-edition DSM ADHD
symptoms commensurate with the article publication date. In the studies
we reviewed, occasional modifications were made to the DSM criteria so
that symptoms were more relevant to adults. For example, questions
regarding DSM criteria for ADHD are occasionally asked in the past tense
to target symptom history, leading up to an adult diagnosis.

In addition to symptom criteria, patients must also meet age of onset

criteria. ADHD is a developmental disorder that emerges in childhood;
thus, there must be a history of ADHD symptomatology during childhood.
The DSM–IV criteria state that there must be evidence of symptom onset
by 7 years of age. However, several authors have noted that there is no
empirical or historical evidence to show that this criterion distinguishes
valid cases from invalid cases (e.g., Barkley & Biederman, 1997; K. R.
Murphy, Barkley, & Bush, 2001). As a consequence, a number of studies
in our sample used historical cutoff ages greater than 7 years of age.

This heterogeneity in criteria and assessment methodologies likely adds

to the variance in results across these studies. Because investigators often
do not provide high levels of detail in their recruitment and assessment
procedures, attempting to group studies on the basis of sample character-
istics or methodological sophistication or to interpret results on the basis of
samples is difficult. We opted to conduct this meta-analysis despite the
methodological limitations of studies to date. By doing so we can point to
areas of apparent differences between adults with ADHD and adults
without ADHD, thus providing a guide for future, more methodologically
sophisticated and theoretically driven research. For the purposes of this
review, results across studies and samples are grouped together as an adult
ADHD composite and are not weighted or classified according to subject
description.

A final limitation is that there may be differences in the type and degree

of comorbidity in the ADHD samples, which may have an impact on the
results of these studies. For example, a number of studies did not evaluate
the presence of learning disabilities in individuals with ADHD. On the
basis of comorbidity studies of children with ADHD, one can assume that
adults with ADHD have greater comorbid learning disabilities than adults
without ADHD. Thus, the degree to which the presence of a learning
disability might influence performance on neuropsychological tests is
unclear.

Among the 33 studies that compared performance between an ADHD

group and a nonclinical control group, 11 matched their ADHD and
nonclinical control samples. Matched variables across studies include age,
IQ, race, sex, and education. The most frequent matching variables are age
and sex. The remaining 22 studies did not use a matched sample procedure.
Therefore, ADHD and control participants vary slightly on several demo-
graphic variables, including age, sex, and education. With few exceptions,
studies report detailed information about these variables. Although age
range is not reported in all studies, the mean age for participants in those
studies reporting it varied from 19 to 41 years. Males constituted the
majority of the study participants in both the diagnostic and control groups.
Male representation in the ADHD group ranged from 40% to 100%, and in
the control group from 22% to 100%. Overall, ADHD participants reported
a lower range of education than controls, from 10.7 to 16.1 years compared
with a range of 11.4 to 17.0 years for controls. The number of participants
in each study varied dramatically for both the ADHD and control groups.
ADHD groups ranged in size from 6 to 105 participants, whereas control
groups ranged from 6 to 207. In all but 4 studies, participants who normally
took medication were asked to refrain from doing so for at least 24 hr.

Using the mean values across the demographic variables, a representa-

tive study would have included 37 ADHD participants approximately 31.6

years old with 14 years of education and 38 controls approximately 32.2
years old with 14.7 years of education. Among the participants, 68% of the
ADHD participants and 58% of the controls would have been male.

Data Analyses and Organization

A wide variety of neuropsychological tests were used across the

studies included in this review. In all, results from more than 25
different tests contributed to the data to be discussed. Although
some of these tests are not widely used, most are either well-
recognized or variations of well-recognized instruments in the field
of neuropsychology. In this review, we converted all neuropsy-
chological results into effect sizes to examine each measure’s
effectiveness at discriminating between ADHD and control
groups. In an effort to summarize and review similar data across
studies, we present weighted mean effect sizes as recommended by
Hedges and Olkin (1985). They suggested that when considering
only two groups per experiment, assuming equal population vari-
ances, an estimate of effect size (d) is obtained by computing the
mean group differences between the experimental and control
groups divided by a pooled estimate of the standard deviation:

⫽ 共Y៮

⫺ Y៮

兲/

where Y៮

and Y៮

represent the means for the experimental and

control groups, respectively. Here s is defined by the pooled
sample standard deviation:

⫽

冑

共n

⫺ 1兲共s

兲

⫹ 共n

⫺ 1兲共s

兲

⫹ n

⫺ 2

where n

and n

represent the sample sizes for the experimental

and control groups, respectively, and s

and s

represent the

standard deviations for the experimental and control groups, re-
spectively. To sum the effect sizes from multiple studies, each
study’s relative sample size needs to be taken into account. Hedges
and Olkin suggested weighting estimators on the basis of sample
size when more than one study is included in that estimate so that
studies with larger samples are more heavily weighted when
pooling effect sizes:

冘

⫽ 1

Here d represents the effect size for each study and w represents
approximate weights for each corresponding study’s effect size
defined as

⬵

n˜

冘

⫽ 1

n˜

or better stated as the single study’s sample size divided by the
total sample size across all studies.

Both individual and, when multiple studies are included,

weighted effect sizes were interpreted according to Cohen’s (1988)
recommendations of cutoffs of 0.80, 0.50, and 0.20 for large,

487

NEUROPSYCHOLOGY OF ADULTS WITH ADHD

medium, and small effect sizes, respectively. Descriptions of dif-
ferences between groups rely on effect sizes rather than statistical
significance.

Note that several issues are relevant to pooling and reviewing

data across these neuropsychological studies. First, the majority
of the tests yield multiple dependent variables, or measures,
each of which may reflect different abilities and be categorized
in different domains. For example, continuous performance test
(CPT) data provide several measures including reaction time
and errors of commission. Whereas the measure of errors of
commission would best fall into the domain of response inhi-
bition, reaction time would fall under processing speed. At the
same time, multiple cognitive domains may be assessed by a
single measure within a test. For example, the color–word
measure on the Stroop test may measure both attention and
inhibition. Similarly, Trail Making Test—A (TMT–A) of the
Halstead–Reitan Neuropsychological Test Battery (Reitan,
1958) measures motor functioning, whereas Trail Making
Test—B (TMT–B) measures motor functioning along with an
additional set switching ability. These examples also highlight
the point that although a test or test measure may be included
within a particular domain, this does not imply that the test
findings in and of themselves adequately depict performance.
Clinical neuropsychological measures are generally not pure
markers of processing in a single, isolated domain. Indeed, the
reader is encouraged to exercise judicious interpretation of the
findings presented in this meta-analysis, including careful con-
sideration of the limitations of the tests, as it is beyond the
scope of this article to evaluate the construct validity of the tests
included herein. Thus, although a particular performance mea-
sure may be highlighted in one domain in this review for the
sake of simplicity and nonredundancy, it may have relevance to
and should be considered in the context of other domains. This
is discussed where relevant.

There is no broad agreement about the specific domains of

neuropsychological dysfunction related to ADHD. Therefore, we
created a set of broadly defined domains to provide a general
framework. Our purpose was similar to Lezak’s (1995, p. 25)
approach to classes of cognitive functions in that we adopted these
domains because they would prove useful to this review, rather
than for the purpose of reifying such concepts as “attention” and
“memory.” Seven domains are presented through which to view
and classify the findings, acknowledging that this is but one of a
number of classification systems that may be used to help interpret
the summarized information.

Attention: attentional functioning.

Response inhibition: motoric inhibition.

Other tests of executive functioning: tests thought to measure
executive functions other than attention and inhibition, such
as fluency and set alternation.

Memory: memory and learning such as working, short-term,
and

long-term

memory

across

verbal

and

nonverbal

conditions.

Processing speed and motor speed: processing speed or
speeded motor functioning.

Intelligence: intelligence, primarily consisting of measures
from versions of the WAIS.

Other: This category is reserved for tests in which the depen-
dent variables do not primarily reflect an ability covered in
one of the previous six domains. Typically, these tests have
been used sparingly.

Results

Attention

Along with inhibition, attention is the neuropsychological do-

main most closely identified with ADHD (Douglas & Peters,
1979). For more than 20 years, conceptualizations of ADHD or
functionally equivalent disorders (e.g., attention deficit disorder)
have included attentional symptoms (DSM; American Psychiatric
Association, 1980, 1987, 1994). Furthermore, some of the earliest
work examining the neuropsychology of ADHD in children fo-
cused on attention. Recent work addressing the expression of
symptoms of ADHD in adults has suggested that while symptoms
of hyperactivity and impulsivity may wane with age, symptoms of
inattention are more likely to remain stable across the lifespan,
emphasizing the importance and centrality of attentional problems
to this disorder (Biederman, Mick, & Faraone, 2000).

Table 1 (see also Web Table 2 at http://dx.doi.org/10.1037/

0894-4105.18.3.485.supp) presents results across studies on neu-
ropsychological test performance measures that purportedly assess
attention. The range of effect sizes across the variety of measures
is largely in the medium range. Omission errors on CPTs are
traditionally considered to measure inattention because they reflect
instances when the patient is not attending to target stimuli. It is
interesting that the more traditional CPTs (e.g., Gordon, McClure,
& Aylward, 1989; Greenberg & Kindschi, 1996; Rosvold, Mirsky,
Sarason, Bransome, & Beck, 1956), which have a low signal
probability (few signals embedded among many nonsignal stim-
uli), appear to discriminate between groups on the omission errors
variable better than the Conners’ CPT (Conners, 1994), which has
a much higher signal probability (many signals embedded among
few nonsignal stimuli). Furthermore, variance in reaction times has
been used less frequently as a measure of performance on any form
of CPT. Both CPT versions demonstrated a high level of discrim-
ination between people with ADHD and controls through the use
of variability in reaction times. In addition, a reaction time vari-
ability measure on the Shifting Sets Task produced a large effect
size, though this was only a single study. Recently, Leth-Steensen,
Elbaz, and Douglas (2000) suggested that it is variance in reaction
times, particularly the presence of several longer length reaction
times interspersed among normal reaction times (i.e., an exagger-
ated positively skewed distribution), that may be indicative of an
ADHD patient’s performance. The relatively large effect sizes for
variance in reaction times on both forms of the CPT tasks and the
large effect size on a similar measure, the Shifting Sets Task,
attests to the possible utility of reaction time as a promising
outcome measure for assessing performance in ADHD samples.

Another interesting, but not entirely unexpected, result was the

large effect size on the Paced Auditory Serial-Addition Task
(PASAT; Gronwall, 1977). The PASAT was used by Gronwall
(1977) in two studies as a measure of executive functioning related

488

HERVEY, EPSTEIN, AND CURRY

to attention. The PASAT requires the participant to listen to a
series of numbers read one after the other and to add each number
to the number before it, rather than to the total. The PASAT has
four trials across which only the interstimulus interval varies
by 0.4 s, from 2.4 s to 1.2 s. Data were presented from the four
trials combined. The rapid stimulus presentation and complexity of
this task make it a formidable task for adults without ADHD and
a very difficult task for adults with ADHD who have difficulty
organizing verbal information that is presented in a rapid manner.
Because of this task’s requirements, it has not been used with
children. It will be interesting if similar comparable tasks such as
the Children’s Paced Auditory Serial-Addition Task (D. A. John-
son, Roethig-Johnston, & Middleton, 1988) can be implemented
with children with ADHD and whether such a task produces
similar large between-groups differences.

Effects of Distraction on Attention

Distraction of attention is especially relevant to those with

ADHD because it is part of the diagnostic criteria and a frequent
complaint of adults with ADHD (DSM–IV; American Psychiatric
Association, 1994). One test that directly measures distractibility is
the Goldman–Fristoe–Woodcock Test of Auditory Discrimination
(TOAD; Goldman, Fristoe, & Woodcock, 1970). The TOAD has
demonstrated sensitivity to high levels of distractibility in children
(Sanger, Keith, & Maher, 1987). Corbett and Stanczak (1999)
published the only study using the TOAD with an adult sample.
Participants are tested while listening to sound on headphones in
two conditions: in quiet, ideal listening conditions, and in the
presence of noise distraction. This single study using the TOAD
demonstrated a small-to-medium effect size for adults with
ADHD.

Another neuropsychological measure of distraction is the Stroop

Color–Word Interference Test (see Spreen & Strauss, 1991). In
particular, the color–word condition of the Stroop, which assesses
the patient’s ability to name the color of printed words while
ignoring the written text, measures the ability to avoid distracting

aspects inherent to the stimuli. Comparisons between adults with
and without ADHD demonstrated a medium effect size on this
measure. One other attentional variable of the Stroop test partially
dependent on distraction resistance is the interference measure.
The interference variable is obtained by a simple equation that
predicts performance in the color–word condition on the basis of
performance on the color-naming and word-reading conditions.
Interference is represented by the deviation of the predicted per-
formance from the actual performance. There is relatively little
between-groups effect when the interference measure is used to
compare adults with ADHD and adults without ADHD. One
reason for the difference in results stemming from use of the
interference measure versus the color–word measure is that adults
with ADHD may be slower in reading the words or listing the
colors suggesting slower verbal processing speed. Therefore, their
predicted performance on the interference measure is lower rela-
tive to adults without ADHD.

Several conclusions can be drawn from the data presented in this

domain. It appears that attentional functioning in adults with
ADHD is impaired when compared with controls using standard-
ized and commonly accepted measures of attention. The effective-
ness of the traditional CPT and the Conners CPT in differentiating
those adults with ADHD from controls is consistent with research
using children with ADHD. In their meta-analysis of research
using traditional versions of the CPT comparing children with and
without ADHD, Losier, McGrath, and Klein (1996) reported an
effect size of 0.67 for errors of omission, very similar to the 0.76
found in the adult studies presented in this review. That both the
Conners and the traditional CPTs produce medium between-
groups effects despite using somewhat different experimental par-
adigms (see Ballard, 2001) increases confidence that attentional
functioning as measured by omission errors is impaired in adults
diagnosed with ADHD. With respect to the specific influence of
distraction, the TOAD, the color–word measure of the Stroop test,
and the Visual Cancellation Task (Scattered Letters Version) col-
lectively suggest that adults with ADHD are susceptible to the

Table 1
Test Measures and Effect Sizes for Studies Included in the Attention Domain

Test

Measure

No. of

studies

Individual

study

effect size

Weighted

mean

effect size

Conners Continuous Performance Test

Errors of omission

0.51

RT standard error

0.53

⬘

0.65

Traditional continuous performance tests

Omission errors

0.76

RT standard error

0.71

Stroop test

Color–word

0.47

Interference

0.15

Letter cancellation tasks

Omissions

0.34

Disorganized strategy

0.29

Time to complete

0.22

Goldman–Fristoe–Woodcock Test of

Auditory Discrimination

Quiet condition

0.14

Noise condition

0.46

Paced Auditory Serial-Addition Task—

Revised

Total time

0.83

Shifting Sets Task

RT (Trial 4)

1.11

RT variability (Trial 4)

0.88

Note.

⫽ reaction time.

489

NEUROPSYCHOLOGY OF ADULTS WITH ADHD

specific influence of distracting stimuli. The Stroop color–word
measure consistently differentiated between groups of adults with
ADHD and controls. That all seven attentional measures have
distinguished adults with ADHD from adults without ADHD is
encouraging and a finding consistent with a large body of research
in the child literature suggesting attentional impairment (Penning-
ton & Ozonoff, 1996).

Response Inhibition

Response inhibition is considered by many to play a critical role

in ADHD. Influential work by Barkley (1997b), among others, has
theoretically linked the ability to inhibit responses to a host of
neurocognitive abilities impaired in children with ADHD. Many
measures of motoric inhibition have been used to identify deficits
in children and adults with ADHD (Epstein, Johnson, Varia, &
Conners, 2001; Pennington & Ozonoff, 1996). Test performance
measures considered to reflect response inhibition are presented in
Table 2 (see also Web Table 3 at http://dx.doi.org/10.1037/
0894-4105.18.3.485.supp).

Both the Conners and the traditional CPTs provided measures of

response inhibition in the form of commission errors. Commission
errors on these tests occur if participants respond when they are
required to inhibit their response. The Conners CPT appears to be
a reasonably effective measure for distinguishing adults with
ADHD on the basis of percentage of commission errors (weighted
d

⫽ 0.63). The traditional CPT demonstrated a much smaller

weighted effect size (0.26). This disparity in findings across CPT
versions is not surprising considering a potential difference in
response bias established in each of these tests. The Conners
CPT’s high signal probability and rapid response pace may prime
an impulsive response pattern (Epstein, Johnson, Varia, & Con-
ners, 2001). It has been demonstrated to lead to a greater percent-
age of commission errors than traditional CPTs (Ballard, 2001),
thus more directly measuring response inhibition than attention.
Other tests of motoric response inhibition that have been used less
frequently to compare adult ADHD samples to control groups
include the Delayed Oculomotor Response Task, the Matching
Familiar Figures Test (MFFT), and the Stop Signal Test. Not
surprisingly, the Stop Signal Test produced a similar magnitude

effect size (weighted d

⫽ 0.85) as did the commission error rate

from the Conners CPT (weighted d

⫽ 0.63). Both tests are similar

in terms of task parameters.

Ross, Harris, Olincy, and Radant (2000) looked at the Delayed

Oculomotor Response Task as a way to distinguish between prob-
lems with motoric inhibition and working memory. The task
presents participants with dots on a screen, one at a time. Partic-
ipants are instructed to watch a dot when it appears in a fixed
location on a video monitor. While watching the first dot, a second
dot appears momentarily. Participants are instructed not to look at
the second dot, but to remember where it appears, and only to look
at where the second dot appeared after the first dot disappears
(either a 1- or 3-s delay). The dependent measure of the percentage
of trials with premature saccades, or the number of times partic-
ipants looked at the location of the second dot prematurely, rep-
resents an inability to inhibit responding to a prepotent task-
relevant stimulus. Participant responses after both 1- and 3-s
delays produced some of the largest effect sizes observed in this
review. However, these results must be considered in the context
of this being only a single study with 10 research participants per
group.

Overall, published studies indicate measurable deficits in mo-

toric inhibition for adults with ADHD compared with controls. The
Stop Signal Test has been successfully used to identify differences
between children with ADHD and controls. Similar to the child
literature, a mean weighted effect size in the large range based on
two studies warrants further investigation. The MFFT (Kagan,
1964) yielded small differences similar in size to that of the
traditional CPT, though it did not appear to distinguish those with
ADHD from controls to the same degree as the other instruments
in this section. Collectively, the pattern of these results can be
interpreted to suggest that inhibition is particularly impaired when
a prepotent behavioral response is established.

Other Tests of Executive Functioning

Beyond the domains of attention and inhibition, a number of

other tests have been used to study executive functioning in adults
with ADHD. The definition of executive function varies according
to the academic discipline, the specific ability identified, and the

Table 2
Test Measures and Effect Sizes for Studies Included in the Response Inhibition Domain

Test

Measure

No. of

studies

Individual

study

effect size

Weighted

mean

effect size

Conners Continuous Performance

Test

Commission errors

0.63

Continuous performance test

Commission errors

0.26

Delayed Oculomotor Response Task

Percentage of premature saccades

at 1-s delay

1.38

Percentage of premature saccades

at 3-s delay

0.82

Matching Familiar Figures Test

Response latency

0.05

Errors

0.33

Posner Visual Orienting Test

Percentage of impulsive

responses

0.49

Stop Signal Task

RT on stop signal trials

0.85

Note.

⫽ reaction time.

490

HERVEY, EPSTEIN, AND CURRY

author defining the ability. Pennington and Ozonoff (1996) pro-
vided a general, yet comprehensive definition that is particularly
applicable to ADHD. They suggested that executive abilities re-
quire goal-directed behavior, often with competing but erroneous
response alternatives, the dysregulation of which cannot be ac-
counted for by a basic deficit in perception, memory, or language.
Although these tests do not necessarily tap one specific executive
function, performance on them is thought to be primarily mediated
by the frontal lobes (Lezak, 1995). Table 3 (see also Web Table 4
at

http://dx.doi.org/10.1037/0894-4105.18.3.485.supp)

presents

test performance measures thought to reflect these executive
abilities.

Perhaps the most interesting findings from this review are that

one of the classic tests of executive functioning, the Wisconsin
Card Sorting Test (WCST; Heaton, Chelune, Talley, Kay, &
Curtiss, 1993) demonstrated almost no between-groups effect. The
WCST has been used extensively in the child literature to attempt
to document deficits in executive functioning, with mixed results.
The results from this review suggest that adults with ADHD do not
have deficits in aspects of executive functioning measured on the
WCST. One possible explanation is that there may be a ceiling
effect whereby the test is too easy for adults, perhaps as a result of
neurological maturation (Snow, 1998; Welsh, Pennington, &
Groisser, 1991). One study directly investigated WCST perfor-
mance as a function of age during adolescence (Seidman, Bieder-
man, Faraone, Weber, & Ouellette, 1997). However, results indi-
cated that older adolescents performed more poorly, whereas

younger adolescents did not perform differently from age-matched
controls. It is worth noting that differential performance was
apparent for the older adolescents on a majority of the tests
administered in that study, whereas the younger adolescents did
not differ from age-matched controls on any neuropsychological
tests, raising the possibility that the samples used may have been
exceptional in some way.

The Rey–Osterrieth Complex Figure Test (Rey & Osterrieth,

1993) was included in three studies in this review, two of which
provided measures included in this domain. Seidman, Biederman,
Weber, Hatch, and Faraone (1998) did not find between-groups
differences in planning ability using traditional scoring methods.
The other study (Schreiber, Javorsky, Robinson, & Stern, 1999),
however, used the copy condition of the Rey–Osterrieth and found
group differences between adults with ADHD and controls on a
variety of measures derived from the less commonly used Boston
Qualitative Scoring System (BQSS). Schreiber et al. (1999) also
developed the BQSS and reported the measures as reflecting
executive ability (see Table 3).

Among the studies in the meta-analysis, three tests were

administered that measure verbal fluency: The Controlled Oral
Word Association (COWA) test (Benton, Hamsher, & Sivan,
1983), the Animal Fluency Test (Lezak, 1995), and the Cookie
Theft Test from the Aphasia Screening Test (Halstead & Wep-
man, 1959). On the COWA, participants are asked to name
words that begin with specific letters. On the Animal Fluency
Test, participants name as many animals as they can in 60 s.

Table 3
Test Measures and Effect Sizes for Studies Included in Other Tests of Executive Functioning
Domain

Test

Measure

No. of

studies

Individual

study

effect size

Weighted

mean

effect size

Trail Making Test—B

Total time

0.68

Wisconsin Card Sorting Test

No. of categories completed

0.02

Failure to maintain set

0.17

Perseverative errors

0.12

Nonperseverative errors

0.12

Controlled Oral Word Association test

Total words

0.60

Animal Fluency Test

Total words

0.93

Cookie Theft Test

No. of occurrences named

0.08

Luria’s Recurring Figures Drawing and

Reciprocal Alternation Motor Task

No. of errors

0.69

Rey–Osterrieth Complex Figure Test

Organization delayed

0.10

Cluster accuracy (BQSS)

0.37

Configural accuracy (BQSS)

0.61

Detail presence (BQSS)

0.28

Neatness (BQSS)

0.72

Planning (BQSS)

0.67

Fragmentation (BQSS)

0.38

Vertical expansion (BQSS)

0.12

Horizontal expansion (BQSS)

1.41

Perseveration (BQSS)

0.00

Progressive Planning Test

Not provided

0.12

Tower of Hanoi

Time to first move

0.39

No. of moves

1.09

No. of errors

0.55

Time to solution

0.68

Note.

BQSS

⫽ Boston Qualitative Scoring System.

Effect size favoring performance in the attention-deficit/hyperactivity disorder-only sample.

491

NEUROPSYCHOLOGY OF ADULTS WITH ADHD

Finally, on the Cookie Theft Test, the participant is presented
with a picture and is instructed to name as many things as
possible that are occurring in the picture. Adults with ADHD
performed much more poorly than adults without ADHD when
they had to either list words from memory or list words based
on category (COWA and Animal Fluency). However, when
adults with ADHD were able to consult a picture to derive
words (Cookie Theft Test), their performance was comparable
to that of adults without ADHD. These differences between
tasks may reflect a cognitive deficit such as unprompted word
retrieval or they may reflect a problem with cognitive inhibition
such that patients may have problems inhibiting thoughts of
previously reported words without visual prompting. Further
examination of this phenomenon using multiple experimental
task conditions is necessary to determine the locus of deficit.

Finally, TMT–B from the Halstead–Reitan Battery (Reitan,

1958), the Tower of Hanoi task (Pennington, Groisser, & Welsh,
1993), and memory measures from the Rey–Osterrieth Complex
Figure Test (Rey & Osterrieth, 1993) have been used to distinguish
groups of adults with ADHD from nonclinical control groups. All
three tasks require the participant to perform a complex, yet
nonverbal, series of prescribed motions to complete the task. They
also require advanced planning. TMT–B and the Tower of Hanoi
also require set shifting. On all of these tests, adults with ADHD
appear to perform worse than controls. Results from the Tower of
Hanoi test suggest that between-groups differences are not attrib-
utable to processing speed or motoric speed but are more likely
due to the propensity for ADHD adults to make more errors and
use an inefficient strategy for completing these tasks.

Overall, several tests of executive functioning that primarily

address abilities other than attention and inhibition have iden-
tified performance differences between adults with ADHD and
controls. Verbal fluency as measured by the COWA appears to
be a well-documented weakness. Without exception, all seven
published studies using the COWA documented poorer perfor-
mance in adults with ADHD in terms of average mean scores.
Likewise, animal fluency also distinguished a group with
ADHD from a group of controls with a large effect size. Using
the BQSS, the Rey–Osterrieth Complex Figure Test provides an
interesting opportunity for further investigation. It may be that
process-oriented approaches allow for test scoring to be tailored
to particular theoretically predicted deficits for those with
ADHD. It is interesting to note that the WCST, considered by
many to be the quintessential test of executive functioning and
a moderately successful delimiter of children with ADHD, did
not appear to distinguish the two groups with even a small
effect size on any variables. The Progressive Planning Test
similarly appears to have limited discriminant utility. The data
suggest that executive functioning involving verbal mediation
or complex motor movements requiring planning and set shift-
ing components are abilities more likely to be impaired in
adults with ADHD than in controls.

Memory

Theoretical and empirical research has suggested that there are

a variety of factors that may lead to memory impairment in
children with ADHD, including difficulty attending and difficulty
processing memories, possibly involving strategy selection based

on executive impairment. Similarly, these same factors may play a
role in memory performance in adults with ADHD (Barkley,
1997b; Seidman et al., 1998). Various aspects of memory in adults
with ADHD have been examined in a select number of studies (see
Table 4 for results from test performance measures of memory; see
also Web Table 5 at http://dx.doi.org/10.1037/0894-4105.18.
3.485.supp). Overall, it appears that across most tests, adults with
ADHD had memory deficits relative to controls. These deficits
may be directly tied to memory processes or may be associated
with related cognitive processes such as attention or even inhibi-
tion. What is clear is that whatever deficits underlie memory
performance, adults with ADHD appear impaired compared with
controls.

With the wide range and large number of memory tests used

across studies, the consistency in finding effects is quite remark-
able. Perhaps the most effective use of these data is to begin by
examining the measures on which ADHD patients did not present
with deficits in order to understand what processes or abilities may
not be impaired in these patients. One trend that can be seen in
these results is that ADHD patients did not appear to have diffi-
culty with memory when presented with visual stimuli in the form
of a figure. Both on the Wechsler Memory Scale—Revised (Wechs-
ler, 1987) tests of Visual Reproduction and on memory measures
from the Rey–Osterrieth Complex Figure Test, the effect sizes
representing differences between adults with ADHD and adults
without ADHD were small to minimal. Note that this is consistent
with the set of executive functioning data on verbal fluency in
which adults with ADHD performed poorly in response to verbal
cues but appeared to have no differences compared with adults
without ADHD if they were allowed to use a visual picture as a cue
to prompt verbal responses (Benton et al., 1983; Halstead &
Wepman, 1959; Lezak, 1995).

One way to interpret this performance pattern has to do with

the components of working memory as proposed by Baddeley
and Hitch (1974, 1994). Their influential conceptualization
identifies three primary components: the visuospatial sketch-
pad, the phonological loop, and the central executive. The
visuospatial sketchpad is responsible for temporarily storing
non-verbally based information in memory for immediate use,
whereas the phonological loop is responsible for the temporary
storage of verbally based information for subsequent immediate
use. The central executive is the controlling component of
working memory and is responsible for the allocation of atten-
tion between these two “slave” components. The data from this
review are consistent with a deficit in the functioning of the
phonological loop but not the visuospatial sketchpad, perhaps
because of the poor allocation of attention to the phonological
loop— or more specifically, those aspects of the loop that relate
to verbal material. As Gathercole and Baddeley (1993) dis-
cussed, the components of the phonological loop that deal with
verbal material are critical to word recognition, language de-
velopment, and reading skills. For example, phonological re-
coding occurs during judgments of phonology and prosody,
rhyme, and syllabic stress structure. Evidence suggests that
disruption of articulation results in impairments in these judg-
ments, presumably as a result of disrupted phonological recod-
ing. It is possible that verbally based memory tasks are nega-
tively affected by a disruption in recoding to the point that

492

HERVEY, EPSTEIN, AND CURRY

memory abilities such as recognizing meaningful semantic re-
lationships between words or phonological similarities are sac-
rificed, resulting in poor performance either directly or through
poor memory strategy selection. Deficits in the processing of
the phonological loop leading to poorer performance when
demands exceed ability might also lead to more general exec-
utive function and memory impairments to the extent that these
areas depend on this aspect of working memory.

On the California Verbal Learning Test (CVLT; Delis, Kramer,

Kaplan, & Ober, 1987), there were notable performance differ-
ences between adults with ADHD and control adults on six of the
seven measures reviewed. This is consistent with the only study
using the child version of this test with children with ADHD
(Loge, Staton, & Beatty, 1990). Serial Clustering was the only
measure on which the ADHD group actually performed better than
the control group. On the other hand, on the Semantic Clustering
measure, the difference between groups was in the medium effect
size range (d

⫽ 0.59). Like the Semantic Clustering measure, the

Serial Clustering measure represents a strategy. Superior perfor-
mance on one of these two measures over the other suggests a
memory strategy preference. The data indicate that adults with
ADHD tend to adopt a less efficient but more immediately obvious
serial clustering strategy than do adults without ADHD, who are
more likely to adopt a semantically related memory strategy.
Although it is not possible to determine the influence of this
preference from the data at hand, it is possible that it accounts for
a portion of the overall performance disparity across all of the
CVLT variables.

Adults with ADHD appear to have memory deficits, though

these deficits vary as a function of the test used to measure them.
On the basis of the results of six studies, one can state with some

degree of confidence that working and short-term memory as
measured by tests such as the Digit Span subtest of the WAIS
appear to be mildly impaired. It is impossible to determine from
the available data whether these two aspects of primary memory
are differentially impaired on the Digit Span subtest because it
combines performance from both aspects to determine a score.
Furthermore, differential performance across tests that present
stimuli verbally as opposed to visually suggest that adults with
ADHD do not have difficulty with visuospatial memory. The
CVLT is the most extensively used memory test for adults with
ADHD, with four documented studies. The findings vary but
generally revolve around deficits in retrieval, encoding, and the
ability to recognize and organize words via semantic clustering in
the ADHD samples. It appears that adults with ADHD used serial
clustering over semantic clustering compared with controls. Based
on the data presented, there is evidence to suggest that memory
impairments, whether tied directly to memory or to meta-memory
processes, are a part of the profile of ADHD in adulthood, con-
sistent with DSM–IV diagnostic criteria (American Psychiatric
Association, 1994).

Processing Speed and Motor Speed

Processing and motor speed are two related areas important for

several cognitive abilities. Intuitively, evaluating motor ability in a
disorder for which the defining criteria include overactivity makes
sense. It would be useful to determine whether reaction time or
motor speed differences can at least partially account for poorer
performance on neuropsychological tests. To these ends, selected
variables thought to reflect processing and motor abilities from

Table 4
Test Measures and Effect Sizes for Studies Included in the Memory Domain

Test

Measure

No. of

studies

Individual

study

effect size

Weighted

mean

effect size

Wechsler Adult Intelligence Test—Revised

Digit Span

0.31

California Verbal Learning Test

Learning Trial 1

0.51

Learning Trials 1–5

0.91

Free Recall Short Delay

0.59

Free Recall Long Delay

0.60

Recognition

0.90

Semantic Clustering

0.59

Serial Clustering

0.14

Wechsler Memory Scale—Revised

Logical Memory I

0.37

Logical Memory II

0.46

Visual Reproduction I

⬍ 0.01

Visual Reproduction II

0.12

Rey–Osterrieth Complex Figure Test

Accuracy Score Delayed

0.25

36-point score

0.15

Simon

Longest sequence

0.47

Auditory Consonant Trigrams

No delay

0.56

3-s delay

0.86

9-s delay

0.54

18-s delay

0.37

Delayed Oculomotor Response Task

Percentage of spatial errors

at 1-s delay

0.43

Percentage of spatial errors

at 3-s delay

0.03

Effect size favoring performance in the attention-deficit/hyperactivity disorder-only sample.

493

NEUROPSYCHOLOGY OF ADULTS WITH ADHD

several tests are presented (see Table 5 for test performance
measures; see also Web Table 6 at http://dx.doi.org/10.1037/
0894-4105.18.3.485.supp).

Collectively, the effect sizes on these processing and motor

speed outcome measures are much lower in size than those seen
in other cognitive domains. Most are in the small effect size
range. One trend that can be seen in these data is that when
increasing cognitive processing demands co-occur with motor
activity, there appears to be a relative increase in response time
as a function of those processing demands. Reaction time on the
Conners CPT, for which there is minimal processing demand,
showed no performance differences between adults with ADHD
and controls. On the traditional CPT there was a small differ-
ence in reaction time, but that difference increased when dis-
traction stimuli were introduced. On the Delayed Oculomotor
Task it may be that a 3-s delay allows for ample time to process
the information necessary for a quick response in those with
ADHD, whereas the 1-s delay does not, resulting in a greater
latency to respond. Small effects are seen in the Stroop, which
requires a degree of verbally mediated processing. Still a
greater effect is seen on TMT–A, which requires both verbal
and fine motor processing.

Intelligence

Certainly the psychological test most widely administered to

adults with ADHD is the WAIS (Wechsler, 1955, 1981, 1994).
Published studies of adult ADHD use versions of the WAIS for a
variety of reasons, such as using IQ as an outcome measure, as a
baseline measure of cognitive performance, or as a sample descrip-
tion. Of the 33 studies included in this review, 20 included at least
one subtest from a version of the WAIS, and 12 of these reported
estimated full scale IQs. In addition to the WAIS, the Kaufman
Brief Intelligence Test (Kaufman & Kaufman, 1983), the National
Adult Reading Test—Revised (Crawford, 1992), and the Shipley
Institute of Living Scale (Shipley, 1939) have each been admin-

istered in 1 study to estimate intellectual functioning. Specific
subtests from the WAIS were also investigated. Table 6 (see also
Web Table 7 at http://dx.doi.org/10.1037/0894-4105.18.3.485
.supp) reports performance across these measures. All tests high-
lighted poorer performance for adults with ADHD. Whereas most
of the measures of intelligence demonstrated a general intelligence
deficit for adults with ADHD, the Arithmetic (weighted d

⫽ 0.50)

and Digit Symbol subscales (weighted d

⫽ 0.62) of the WAIS had

effect sizes in the medium range, indicating that they may measure
cognitive processes related to ADHD deficits. These two sub-
scales, in addition to the Digit Span task reviewed previously,
place high demands on verbal working memory.

Other

Most of the neuropsychological measures administered to adults

with ADHD were amenable to one of the six previously discussed
categories. However, a small group of other tests and measures
have been administered to adult ADHD samples that purport to test
alternative cognitive abilities (see Table 7 for test performance
measures; see also Web Table 8 at http://dx.doi.org/10.1037/0894-
4105.18.3.485.supp). In general, these measures have been admin-
istered in a limited number of studies. Barkley, Murphy, and
Kwasnik (1996) developed two brief tests to measure creativity in
terms of verbal and spatial usage fluency. Neither test appears
useful for discriminating adults with ADHD from controls. Small
performance differences between adults with ADHD and controls
were apparent on the Embedded Figures Test (Spreen & Benton,
1969). Though it is unclear what broad cognitive ability this test
taps, visual search and tracing abilities are important.

Studies Including Clinical Control Groups

All of the aforementioned studies in this review have used

nonclinical controls as a comparison group to assess ADHD-
related deficits. The inclusion of clinical control groups has an

Table 5
Test Measures and Effect Sizes for Studies Included in the Processing Speed and Motor Speed
Domain

Test

Measure

No. of

studies

Individual

study

effect size

Weighted

mean

effect size

Conners Continuous Performance Test

0.04

Continuous performance test

Vigilance RT

0.21

Distraction RT

0.36

Trail Making Test—A

Completion time

0.53

Stroop test

Color condition

0.30

Word condition

0.23

3RT Test

Simple task

0.08

Choice task

0.23

Conditional task

0.04

Delayed Oculomotor Response Task

Response latency at 1-s delay

0.49

Response latency at 3-s delay

0.03

Finger tapping

Right hand

0.06

Left hand

0.67

Shifting Sets Task

1.11

Stop Signal Test

RT on nonstop signal trials

0.58

Note.

⫽ reaction time.

Effect size favoring performance in the attention-deficit/hyperactivity disorder-only sample.

494

HERVEY, EPSTEIN, AND CURRY

added advantage over nonclinical comparison groups because be-
tween-groups neuropsychological differences between ADHD and
clinical comparison groups are more likely attributable to specific
ADHD-related problems rather than attributable to general psy-
chopathology. Unfortunately, only a small number of studies have
been conducted with adult ADHD patients that have also used
clinical comparison groups. The clinical control groups used in
these few studies generally fall into two categories: (a) clinical
controls with a nonrelated DSM diagnosis or (b) clinical controls
with ADHD and a comorbid diagnosis. Overall, there appears to be
a general pattern to the results of these studies (see Web tables at
http://dx.doi.org/10.1037/0894-4105.18.3.485.supp). When non-
ADHD clinical controls are used, differences that were observed
between participants with ADHD and nonclinical controls appear
to be evident when comparing ADHD patients to clinical controls.
For example, it appears that adults with ADHD perform worse on
tasks that measure attention, response inhibition, and other tests of
executive functioning than do clinical controls. This was true when
the clinical controls consisted of patients with single Axis I dis-
orders (e.g., depression, anxiety) or were classified as psychiatric
outpatients. In the single study (Ross et al., 2000) in which a
clinical control group of schizophrenia patients was used, how-
ever, the ADHD patients performed better on a task of visually
based short-term memory and response inhibition.

When adults with ADHD and a comorbid condition were used

as clinical controls, the general effect was that the comorbid group
performed worse on most neuropsychological tests. This suggests
that ADHD-related neuropsychological deficits may be exacer-

bated by the presence of comorbid psychopathology. It is unclear
by what mechanism this exacerbation in neuropsychological func-
tioning may occur. It may be that the presence of another psycho-
logical disorder, with its inherent deficits, produces an additive
negative neuropsychological effect on patients, or it may be that
there is some form of interaction that takes place between ADHD
and other disorders that produces greater impairment. Finally, it
may be a simple relation that greater levels of psychopathology
produce greater neuropsychological impairment.

Very few studies have used different ADHD subtypes as con-

trols. The advantage of this strategy is that neuropsychological
deficits can be attributed more specifically to certain classes of
symptoms. In the only study to use the University of Pennsylvania
Smell Identification Test, Gansler et al. (1998) compared adults
who had inattentive ADHD with adults who had hyperactive–
impulsive ADHD and found that the inattentive group performed
much better than the hyperactive–impulsive group. These results
are intriguing but should be considered preliminary given that this
is the only study to use this measure with adults with ADHD.

Discussion

The data presented in this review suggest that there are a wide

variety of general and specific performance differences that indi-
cate neuropsychological deficits associated with adult ADHD. In
each of the cognitive domains presented for discussion, deficits
were revealed in a portion of the measures reviewed, suggesting
some degree of impairment. Thus, the results do not indicate a
domain-specific neuropsychological deficit. That is, all domains
revealed at least some degree of impairment on at least a subset of
the tests considered within each domain. There were certainly
some domains (e.g., attention) in which adults with ADHD ap-
peared to differ more from adults without ADHD than in other
domains as evidenced by more consistent medium-to-large effect
sizes across tests. This is largely consistent with the findings of
Woods et al. (2002), who suggested that their review of the
literature indicates a pattern of frontal-subcortical dysfunction.
However, by using a meta-analytic approach, the present review
also reveals impairments in nonexecutive functioning aspects of
memory, processing speed, and motor speed, suggesting that fron-
tal-subcortical dysfunction only partially characterizes the deficits
seen in adults with ADHD. What remains unclear, partially as a

Table 6
Test Measures and Effect Sizes for Studies Included in the Intelligence Domain

Test

Measure

No. of

studies

Individual

study

effect size

Weighted

mean

effect size

Wechsler Adult Intelligence Test—Revised

Estimated Full Scale IQ

0.39

Freedom From Distractibility

index

0.33

Vocabulary subtest

0.29

Arithmetic subtest

0.50

Digit Span subtest

0.31

Block Design subtest

0.35

Digit Symbol subtest

0.62

National Adult Reading Test—Revised

Total no. correct

0.54

Kaufman Brief Intelligence Test

Overall estimated IQ

0.08

Shipley Institute of Living Scale

Overall score

0.80

Table 7
Test Measures and Effect Sizes for Studies Included in the Other
Domain

Test

Measure

No. of

studies

Individual

study

effect size

Weighted

mean

effect size

Creativity Test

Object usage

0.08

Spatial objects

0.04

Embedded Figures

Test

Completion time

0.45

No. correct

0.45

No. of errors

0.49

495

NEUROPSYCHOLOGY OF ADULTS WITH ADHD

result of measures that are not “process pure,” is how much a
deficit in one cognitive domain can be responsible for causing poor
performance on test measures across other domains. For example,
can attentional dysfunction cause problems in performance on tests
of memory, inhibition, processing speed, and so forth? Or can the
observed pattern of results be explained by a more general, pos-
sibly centralized deficit in cognitive processing that may affect all
cognitive domains? This would certainly explain the finding that
adults with ADHD have lower levels of psychometrically mea-
sured intelligence than do nonclinical controls. Clearly, a compre-
hensive theory of ADHD needs to account for this general, non-
specific pattern of neuropsychological deficits seen in adults with
ADHD.

An alternative strategy for analyzing the pattern of deficits

observed in the ADHD patients is to examine the performance
measures on which adults with ADHD performed most differently
from adults without ADHD in an attempt to determine whether a
consistent underlying process was present across tests. One general
pattern that appeared to emerge was that adults with ADHD
performed more poorly on tests in which there was a verbal
presentation of stimuli rather than a visual presentation. Further, if
distracting stimuli were added to the verbal presentation, whether
task or non-task specific, the performance of adults with ADHD
appeared to deviate more from that of nonclinical controls. On
tests with verbal presentation, such as the CVLT, TOAD, Auditory
Consonant Trigrams, and the Arithmetic subtest of the WAIS,
effect sizes comparing adults with ADHD and nonclinical controls
were in the medium effect size range. All of these tasks require
listening to a series of stimuli or data and performing some mental
operation with these data. Further, three measures that challenged
auditory processing by presenting distracting stimuli in unison
with target stimuli produced some of the largest effect sizes seen
across all performance measures: the Recognition subtest of the
CVLT, which assesses the participant’s ability to differentiate
target stimuli from previous distraction stimuli; the TOAD, which
measures auditory attention in a quiet and in a noisy condition; and
the PASAT, which presents numbers and requires participants to
process (i.e., add) only the last two numbers presented while
ignoring previous stimuli. A deficit in processing verbal informa-
tion is consistent with the clinical complaints of many of these
adult patients and is also consistent with some specific DSM–IV
symptom criteria (e.g., “often does not listen when spoken to
directly, is often easily distracted by extraneous stimuli”; Ameri-
can Psychiatric Association, 1994, p. 84).

The other apparent consistency across tests on which adults with

ADHD performed most poorly is that as the task demands in-
crease, ADHD performance appears to deviate more and more
from that of nonclinical controls. In general, the neuropsycholog-
ical tests across domains varied in terms of four conditions: com-
plexity, time requirements, processing speed, and motor function-
ing. It appears that as the number or level of these conditions
increase, they combine for a cumulative or summative effect. This
is similar to but more specific than the conclusions made by D. E.
Johnson et al. (2001) suggesting that performance decrements in
adults with ADHD are more likely to be seen with tests involving
greater cognitive demands or increased complexity. In the present
review, with the exception of memory evaluation, those perfor-
mance measures that include elements of each of these conditions
generally tend to yield greater effect sizes than performance mea-

sures that include only one or two conditions. That is not to say
that measures that differentiate performance between those with
and without ADHD will always include all of these conditions.
Rather, it suggests that in addition to identified deficits such as
those in motor response inhibition or simple verbal fluency, one
can appreciate that adults with ADHD appear to have difficulty
when the neurocognitive system is taxed by multiple demands to a
point at which performance is sacrificed to a greater extent than it
is for adults without ADHD. Essentially, the point at which adults
with ADHD appear to operate at maximum cognitive capacity may
be reached, on average, sooner or more easily than for adults
without ADHD. Although reasons for this trend are unclear, it may
be that under these conditions cognitive processing is impaired,
leading to poorer performance. For example, the WCST is con-
sidered a complex instrument measuring aspects of memory, set
switching, and perseveration. However, the test has no time limit,
and therefore speeded processing is not critical. Adults with
ADHD appeared to perform no differently than adults without
ADHD on this test. Compare this to tests such as the PASAT or
TMT–B, for which learning a new skill is required under timed
conditions. In the PASAT, the individual is required to learn a
complex addition skill under exaggerated time demands, where
processing speed is critical. In TMT–B, the individual is also
required to learn a slightly complex motor sequencing task under
timed conditions, requiring efficient and speeded processing. On
each of these tests, adults with ADHD differ from adults without
ADHD, with effect sizes ranging from medium to large, consistent
with the idea of impaired learning under multiple demand condi-
tions as a factor in performance.

Theoretical Considerations

Comprehensive theories of ADHD, especially those that predict

neuropsychological performance, are rare. Douglas (1983) posited
a theoretical model in which poor sustained attention or vigilance
was the core deficit of ADHD. Subsequent studies specifically
examining sustained attention and vigilance in ADHD children
have produced inconsistent results and failed to support this the-
ory. Another prominent theory that was originally proposed by
Quay (1988a, 1988b) and later modified by Barkley (1997a,
1997b) proposes that the core deficit associated with ADHD is
poor behavioral inhibition. Barkley (1997a, 1997b) suggested that
this deficit leads to difficulties in other major cognitive abilities
related to executive functioning, reflected in neuropsychological
impairment, and ultimately in impaired behavioral expressions.
These major cognitive abilities include (a) nonverbal working
memory and the capability to hold events in mind so as to use them
to control a response (e.g., the behavioral sequence of event,
response, and outcome); (b) the internalization of speech (verbal
working memory) and the critical role that it plays in the devel-
opment of self-control; (c) self-regulation of affect, motivation,
and arousal; and (d) reconstitution— defined as the analysis and
resynthesis of the component parts of the sequences of events or
messages. Barkley (1997a) admirably synthesized and retrospec-
tively predicted expected neuropsychological outcomes in the
child literature on the basis of the existing findings. However,
because children and adults with ADHD appear to perform poorly
across almost the entire gamut of neuropsychological tests, any
theory, including Barkley’s (1997a, 1997b), which predicts wide-

496

HERVEY, EPSTEIN, AND CURRY

spread ADHD-related neuropsychological deficits and no ADHD-
related neuropsychological strengths is likely to be supported by
the existing and future neuropsychological literature.

A cursory evaluation of consistency between the observed pat-

tern of neuropsychological results and Barkley’s (1997a, 1997b)
theory does suggest that the two are compatible. The data reviewed
here are consistent and convincing in documenting deficits in
behavioral inhibition in ADHD adults. In evaluating Barkley’s
theory more closely, one needs to consider several methodological
factors that Barkley (1997a) highlighted as important when inter-
preting empirical findings of neuropsychological performance in
ADHD. First, because impairments in inhibition are at the crux of
executive dysfunction in ADHD, deficits are predicted to exist in
other executive functions as a consequence of inhibition, though it
is predicted that impairment should not be as severe as the core
inhibitory deficit itself. This was not entirely true in this review, as
deficits in inhibition were largely in the medium effect size range,
which was similar in magnitude to observed neuropsychological
deficits in other domains (e.g., attention, memory). Barkley
(1997a) also suggested that executive function deficits represent
delays in maturation and therefore appropriate measures should
capture the maturational lag. The implications of such a suggestion
make it somewhat difficult to account for deficits in adults with
ADHD as maturational delays.

Another way to interpret the data in this review involves con-

sidering the relationship between neuropsychological functioning
and working memory, specifically verbally based working mem-
ory. Using Baddeley and Hitch’s (1974, 1994) development and
current conceptualization, one can address the concept of working
memory as the three component system mentioned earlier, con-
sisting of the phonological loop (verbal working memory), the
visuospatial sketchpad (nonverbal working memory), and the cen-
tral executive. Baddeley and Hitch (1974, 1994) suggested that the
duration of time that information is stored in the phonological loop
can be accounted for by a model of decay where loss of informa-
tion is partly a result of the passage of time. They went on to
suggest that the loop is also vulnerable to disruption via distrac-
tion. That is, irrelevant items add noise to the store of the phono-
logical loop making recall more difficult.

One might consider the DSM–IV criterion for ADHD of “often

easily distracted by extraneous stimuli” (American Psychiatric
Association, 1994, p. 84) as representing a direct link to verbal
working memory, and an inherent contributor to neuropsycholog-
ical performance deficits in adults with ADHD. Consider that one
role of working memory is to hold information so that one can act
on that information. In the case of one who is easily distracted,
those distractions may be represented as irrelevant items that add
noise to the temporary store of the phonological loop. In this way
distraction serves as a disruption in working memory. Although
this disruption may not cause impairment in situations in which
demand on verbal working memory is low, situations in which
demand is high or in which verbal working memory is overloaded
may magnify the disruption, leading to behavioral manifestations
such as an inability to “follow through on instructions” or to be
“forgetful in daily activities” (p. 84), or perhaps additional diag-
nostic criteria set forth in the DSM–IV.

Indeed, other conceptualizations of working memory also detail

the deleterious process of distraction in working memory (Hasher
& Zachs, 1988) and highlight the critical role that inhibitory

mechanisms play in the efficient functioning of working memory,
as well as in various aspects of attention (Neumann, 1987; Posner,
Inhoff, Friedrich, & Cohen, 1987). The data garnered from this
review are consistent with a differential susceptibility to the effects
of distraction on verbal working memory suggesting increased
susceptibility in adults with ADHD.

The pattern of widespread neuropsychological deficits in adults

with ADHD does not immediately point to selective and specific
brain impairments. Nevertheless, performance patterns as they
relate to neuroanatomy may provide useful information. The pat-
tern of performance highlighting deficits in multiple domains is
suggestive of a generalized effect. That is, rather than identifying
damage in particular regions of the brain that directly lead to
neuropsychological impairment, the data do not convincingly iden-
tify one region over another. Instead, they suggest impairment in
brain functioning as a whole, where deficits in processing speed
and the ability to contend with more complex processing are
revealed. However, generalized impairments are often closely re-
lated to deficits in subcortical and frontal functioning expressed as
primary impairments in processing, as well as decreased atten-
tional functioning (Lezak, 1995). Strong evidence for difficulties
with learning and memory, particularly working memory, where
consistent and effortful attention is essential, is in accordance with
this idea. Moreover, an argument can be made for the critical role
that consistent and effortful attention plays in the majority of
neuropsychological tests included in this review, thereby at least
partially accounting for a generalized impairment. In fact, substan-
tial evidence identifying the importance of frontal lobe functioning
in mediating susceptibility to memory interference exists (Badde-
ley, 1986; Goldman-Rakic, 1993; Stuss et al., 1982). Several
studies using both neurological scanning instruments (e.g.,
positron emission tomography, fMRI, and event-related potentials;
Casey et al., 1997; Filipek et al., 1997) and neuropsychological
testing (Aman, Roberts, & Pennington, 1998) have implicated
these particular areas as likely contributors to observed deficits in
children with ADHD.

Developmental Considerations

In general, similar patterns of neuropsychological deficits were

observed in adults as have been reported in children. Some specific
neuropsychological tests have been used in both children and
adults (e.g., CPT). In most cases, these tests revealed similar
results across age groups, though, again, the point must be made
that both children and adults demonstrate neuropsychological def-
icits across almost the entire gamut of neuropsychological tests.
On a test such as the CPT, the similarities in results across
populations are striking with respect to omission errors (Losier et
al., 1996). Certainly these similarities in results across children and
adults imply continuity of the ADHD phenomenon across devel-
opment though it is based on a quasi-cross-sectional review.

There were some inconsistencies in performance across children

and adults. Some tests that have demonstrated group differences in
children were not effective at discriminating ADHD from non-
ADHD groups in adults. Two examples of such tests are the
WCST and the MFFT. Both tests have discriminated children with
ADHD from children without ADHD (Pennington et al., 1993;
Shue & Douglas, 1992). However, in this review of the adult
literature, the effect sizes representing between-groups differences

497

NEUROPSYCHOLOGY OF ADULTS WITH ADHD

on these tests were close to zero. Several explanations may clarify
this pattern of results. Both tests measure executive functioning,
per se. Executive functioning deficits in children may be more
pronounced because of a maturational lag in brain development.
As children with ADHD become adults with ADHD, brain matu-
ration as related to executive functioning catches up to normal
brain development, and deficits on these tests may diminish. An
argument against this explanation is that other tests that are highly
related to executive functioning (e.g., TMT–B, Tower of Hanoi,
COWA) demonstrated medium-to-large between-groups effect
sizes. Thus, an alternative explanation appears warranted. It may
be that these specific executive functioning tests measure different
skills in children as compared with adults. For example, in children
the WCST may measure the ability to form concepts, whereas in
adults the WCST may measure the ability to shift learning sets.
Thus, developmental level may affect what the test is measuring.
Yet another explanation for this discrepancy in findings is that
some of the neuropsychological tests that have been used to
demonstrate differences in child samples may be too easy for
adults. That is, adults may demonstrate a ceiling effect on some
tasks. Although it is recognized that tests such as the WCST and
MFFT have been used in adults with brain lesions, adults with
ADHD do not present with severe localized brain lesions and can
present as fairly normally functioning adults. Thus, it is entirely
possible that tests such as the WCST and MFFT do not challenge
their deficits to such a degree for them to differentiate themselves
from nonclinical control samples.

There were some tests that were used with adult samples that

have not been used with children. The primary reasons some tests
have not been used with children are lack of norms in younger
samples and/or task difficulty. One test that was used in two adult
studies was the PASAT. This test has not been utilized in child
ADHD samples, though a child version of the test exists. These
adult studies produced between-groups effect sizes for the PASAT
in the medium and large range (ds

⫽ 0.74 and 1.15). These were

some of the largest effect sizes seen in this review and are con-
sistent with the overall finding that overloading verbal working
memory produces much poorer performance in ADHD patients. A
similar task that has not been used in any child studies and has
been used in one adult study with a very large effect size is the
Shifting Set Task. A future goal should be to translate the intrigu-
ing results from these tests down to child samples, recognizing that
there may be a need to modify the task parameters. Along the same
lines, there are some tasks that have been used extensively with
children that have only been used minimally with adults, such as
the Stop Signal Task. There is a voluminous literature with chil-
dren using this task; however, only two studies have reported
results on this test using adults, both with encouraging results.
More studies are needed that use the Stop Signal Task and other
tasks with large effect sizes to examine performance patterns
across tests and within specific subpopulations of ADHD patients.

Limitations

The meta-analytic review process can summarize findings

across a range of studies, but it cannot remove the limitations of
the summarized studies. Before presenting our findings we men-
tioned several significant limitations of these investigations. Here
we return to consider these limitations and several others. Al-

though data were compiled from all studies in which they were
presented, there were several variables that could not be controlled
for when computing effect sizes. A number of studies used ADHD
and control samples matched on important demographic variables,
and for these the matched variables do not affect the interpretation
of the results. The majority of studies, however, did not use
matched samples or control for certain potential confounding
variables. Among the most critical of these variables are education
and intelligence. It is widely understood that these two variables
play an important role in predicting performance on many neuro-
psychological tests (Lezak, 1995). The differences in education
between the ADHD and control samples as a whole was 0.7 years,
in favor of the latter. It is impossible to predict accurately the
potential overall effect that this difference represents in test per-
formance in the present selection of studies, but the possible
influence should not be overlooked. The same is true of intelli-
gence. Intelligence is positively correlated with neuropsychologi-
cal test performance. In fact, the WAIS itself is considered by
many to be a neuropsychological test. Generally speaking, studies
include the WAIS as a demographic variable, but often present
findings with and without controlling for intelligence as a demo-
graphic variable. Several authors have made the theoretical argu-
ment that the underlying causal factors involved in ADHD result in
lower IQ scores and may be the same as those that contribute to
poorer performance on other neuropsychological measures (e.g.,
Mariani & Barkley, 1997; K. R. Murphy et al., 2001). In addition,
it is plausible that the underlying factors leading to the behavioral
symptoms of ADHD may be the same factors involved in the
neuropsychological correlates of ADHD. Removing variability
associated with differential IQ performance may be removing part
of what constitutes ADHD. This is the perspective that was
adopted in this review. Intelligence is presented as an outcome
variable rather than a demographic variable.

Comorbidity and sample selection are interrelated factors that

could not be controlled for in this review. These are important
factors because adults who present with ADHD often have a
comorbid psychiatric condition (Wilens et al., 2002). The studies
in this review used clinic-based samples and therefore have limi-
tations because these samples are likely to have greater psycho-
pathology and comorbidity then would be found in population-
based samples. The minimal number of studies that used comorbid
clinical control groups in this review suggests that the presence of
a comorbid disorder with ADHD can affect neuropsychological
functioning. Most studies in this review did not conduct compre-
hensive assessments of comorbid conditions to control or to test
for the effects of these comorbid diagnoses. For example, evidence
from childhood ADHD studies suggests that the presence of learn-
ing disabilities, especially mathematically related disabilities, am-
plifies performance deficits (Seidman, Biederman, Monuteaux,
Doyle, & Faraone, 2001). The majority of the studies in this review
did not control for the presence of learning disabilities in their
samples (24 of the 33 studies). However, one study specifically
compared performance before and after accounting for the vari-
ance associated with the presence of a learning disability (Seidman
et al., 1998). Although the portion of the sample with a learning
disability was small (12.5%), the comorbid condition had no
impact on the results for any of the tests, including the CVLT and
the Rey–Osterrieth Complex Figure Test.

498

HERVEY, EPSTEIN, AND CURRY

Although the exact effects of the varying types of comorbid

conditions on neuropsychological performance are unknown, they
may influence performance and should be carefully assessed and
their effects analyzed in future studies. It is encouraging to note
that two studies (Barkley, Murphy, & Bush, 2001; K. R. Murphy
et al., 2001) have specifically addressed effects due to comorbidity
in their samples. On the basis of a cohort control strategy, these
authors concluded that the additional presence of depressive, anx-
ious, or oppositional symptomatology did not influence neuropsy-
chological performance in adults with ADHD. These findings
contrast with the pattern of performance seen in the present review,
in which ADHD patients with comorbid disorders usually demon-
strated greater neuropsychological deficits than those ADHD pa-
tients without comorbid diagnoses. Methodological differences
may at least partially explain this. First, the present review presents
a summary of findings based on effect sizes and does not limit
itself to findings of statistically significant group differences. Co-
morbid and noncomorbid ADHD group performances may not be
statistically significant in small sample studies, yet the relevant
effect sizes associated with comorbidity may be substantial, and
these would be identified in the present review. Also, although
Barkley et al. (2001) and K. R. Murphy et al. (2001) deserve credit
for their attempt to assess the influence of comorbidity, they used
a method that may have limited statistical power by splitting a
continuous variable. An alternative method of analysis taking
advantage of the continuous nature of the data might more clearly
reveal the role that comorbidity plays in neuropsychological
performance.

A critical point that needs to be addressed with some detail in

future work is the information that can be gathered from compar-
isons in performance made between adults with ADHD and clin-
ical controls. Without this differentiation between ADHD and
clinical control groups, it cannot be determined whether the ob-
served performance patterns are specific to an ADHD population
or merely reflective of psychopathology in general.

The use of medication in the treatment of ADHD is widespread

(Wilens, Biederman, Spencer, & Prince, 1995). Although the ma-
jority of our reviewed studies (20 of 33) requested participants to
refrain from taking medications for at least 24 hr prior to their
assessment, thereby controlling for most of the immediate clinical
effects, the treatment history of adult ADHD patients was not
controlled. Little is known regarding the long-term influence of
medication treatment, stimulant or otherwise, on neuropsycholog-
ical performance. Currently, there are no studies adequately track-
ing this potential influence. Therefore, the possibility that long-
term medication treatment may result in structural and/or perma-
nent changes in neuropsychological functioning should not be
overlooked. Rather, this potential influence should be considered
when interpreting the results of the included studies, as well as
when planning for future studies of the neuropsychology of
ADHD. To control for this potential influence, current and future
empirical work might include a priori group comparisons and
longitudinal tracking of medication treatment. In addition, with
adequate and detailed history, retrospective studies can be con-
ducted to help elucidate this issue.

A final variable that should be taken into account when consid-

ering the data presented is sex. Overall, there were fewer women
in the studies reviewed. Although there is little work addressing
the sex differences among adults with ADHD, the child literature

has proposed and documented differences in several areas, includ-
ing subtype manifestation (Biederman et al., 2002). In light of
these findings, and the disparity in sex representation in the present
review, the implications of this review may be more applicable to
men with ADHD than to women with ADHD.

Though measures were taken to ensure that this review was

comprehensive, limitations inherent in literature reviews should be
considered, as well. There is no doubt that the largest problem in
this review is heterogeneity across and within the ADHD samples.
Because there is no agreed-upon diagnostic strategy for diagnosing
or describing adult ADHD, different diagnostic methods were used
across studies to define the ADHD subgroups. For example, most
studies used DSM–IV criteria, which focus on those behaviors that
are expected in the childhood expression of the disorder, whereas
three studies used the Wender criteria. The latter focus on many
symptoms similar to those of the DSM–IV, but also include symp-
toms related to stress intolerance and interpersonal relationship
difficulties. As a result, it is unclear whether those adults identified
using the Wender criteria would be the same adults identified using
DSM criteria.

To date, no psychometric properties have been reported for any

interviews of adult ADHD used in the studies presented. Although
many adults express symptoms of ADHD, diagnostic criteria for
adults have not been validated and are clearly needed.

An additional challenge in interpreting these data comes from

the inclusion of the various ADHD subtypes in most studies. This
renders findings difficult to interpret in the context of continuity
across development when compared with childhood ADHD stud-
ies that do target particular subtypes and their neuropsychological
correlates. It is unclear whether the adults with different ADHD
subtypes would perform differently on neuropsychological
measures.

Future Directions

Given the observed performance patterns as well as the factors

that influence the data discussed in this review, there are a variety
of directions in which to pursue future research in the neuropsy-
chology of adult ADHD. Noting the limitations of previous stud-
ies, the field will benefit from several methodological consider-
ations to better evaluate individual study hypotheses and contrib-
ute to the knowledge in the field using standard methods. Study
samples need to be carefully selected. In terms of diagnosis, the
current standard in the literature uses DSM criteria. In verifying
diagnosis, studies should report the process used for diagnosis,
specify the type of interview used, and include a quantifiable
measure of severity. A history of ADHD symptoms also needs to
be documented to verify the presence of the disorder in childhood.
Finally, because ADHD is highly comorbid with a number of other
disorders, measures should be taken to assess the presence of these
concomitant conditions both in individuals with ADHD and in
comparison controls.

Beyond diagnosis, studies need to include adequate sample sizes

for statistical power to determine the presence of performance
differences between ADHD and control groups should they exist.
A number of studies in this review identifying large effect sizes did
not statistically differentiate adults with ADHD from controls with
traditional statistical significance because of small sample sizes.
Studies attempting to discern subtle neurocognitive differences

499

NEUROPSYCHOLOGY OF ADULTS WITH ADHD

need to utilize appropriate sample sizes to examine adequately
these differences. In addition, when possible studies should test
patients free of medication influence, using an appropriate washout
period if needed.

Studies more closely addressing specific neurocognitive abil-

ities including verbal versus nonverbal fluency, memory strat-
egy utilization, and domain-specific memory abilities including
verbal, visual, and motor memory would help to clarify the
issues raised by the data reviewed here. Another area that would
particularly benefit from further research is the investigation of
the summative effect of testing conditions on task performance.
This might be achieved by modifying existing tests or devel-
oping new ones that use a consistent paradigm but vary the
number of demand conditions in which the participant is placed.
In this way steps may be taken to advance the current state of
the field beyond the level of descriptive studies, by incorporat-
ing and testing specific and competing theories. There are also
a number of neuropsychological tests presented in this review
that have been infrequently used to test adults with ADHD but
that provide encouraging findings. Replication of studies using
these tests is needed. Finally, it is critical that longitudinal
research specifically tracing the developmental progression of
neurocognitive deficits associated with ADHD be conducted to
better understand clinical outcomes in treated and untreated
participants with ADHD.

Collectively, research in these areas contributes to a growing

body of work that may aid in investigations of ADHD etiology. In
addition, this type of work may also contribute to better diagnosis
in the future. We agree with Woods et al. (2002) that “neuropsy-
chological dysfunction is an integral component of the constella-
tion of symptoms experienced by adults diagnosed with ADHD”
(p. 29), and we recognize the value in the use of current neuro-
psychological instruments in assessment of functional impairment.
However, we hesitate to recommend their use as required clinical
tools in the diagnosis of ADHD. Woods et al. aptly pointed to the
need for research assessing the contribution and validity of neuro-
psychological assessment to the diagnostic process. We concur and
adopt a more conservative approach to diagnosis, recognizing the
as-yet-undetermined degree of sensitivity and specificity of current
neuropsychological measures when assessing adult ADHD.

Conclusion

The neuropsychology of adult ADHD is marked by deficits in

multiple performance domains. These deficits range from subtle to
pronounced in magnitude. To account for this, one needs to con-
sider both the abilities of adults with ADHD and the limitations of
the tests themselves. One way to view the measured deficits in the
ADHD groups is to assume that they directly and proportionately
reflect the actual deficits in adults with ADHD. On the other hand,
one might assume that the actual differences that contribute to
adult ADHD are more significant than the reviewed measures
portray them to be and that these tests are neither sensitive enough
to reveal these differences nor specifically targeted to capture
ADHD idiopathic symptoms. To better address this question and
more clearly establish a pattern of performance consistent and
specific to adult ADHD, research addressing the issues raised in
this review is necessary. Regardless of the view taken, it is clear
that neurocognitive deficits are not always obvious and easily

captured. It is also critical to keep in mind that although this review
highlights areas of neurocognitive deficits in adults with ADHD,
these deficits are sometimes based on subtle differences revealed
using effect sizes between groups. Although this review suggests
valuable information about the developmental progression of neu-
rocognitive abilities in adults with ADHD, and although the es-
tablished data are in line with Barkley’s (1997a, 1997b) theoretical
work, this review does not provide immediate clinical recommen-
dations for diagnosis. Rather, it emphasizes directions in which
future endeavors around this issue may proceed. Neuropsycholog-
ical information is important for many reasons highlighted in this
review. However, individual diagnosis based on performance on
these measures is discouraged in light of the current diagnostic
criteria requiring diagnosis to be made on the basis of behavioral
evidence only. Nevertheless, these deficits help shape the field’s
understanding of ADHD as a disorder across the life span and
provide data that may prove useful for researchers and clinicians
alike.

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Received October 2, 2002

Revision received May 5, 2003

Accepted May 7, 2003

䡲

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