ORIGINAL ARTICLE

Comorbid ADHD and mental health disorders: are these childrenmore likely to develop reading disorders?

Florence Levy • Deidra J. Young • Kelly S. Bennett •

Neilson C. Martin • David A. Hay

Received: 11 April 2011 / Accepted: 20 September 2012 / Published online: 4 November 2012

� Springer-Verlag Wien 2012

Abstract While attention-deficit/hyperactivity disorder

(ADHD) has been associated with both internalizing and

externalizing childhood behaviour disorders, the specific

relationship of these comorbid disorders to ADHD and

reading problems is less well defined. The present study

analysed data from the Australian Twin ADHD Project,

which utilized DSM-IV-based ratings of ADHD, separation

anxiety disorder, generalized anxiety disorder, depression,

conduct disorder, and oppositional defiant disorder for twins

and siblings aged 6 to 18 years. While differences between

children with and without ADHD were demonstrated for

those with separation anxiety disorder, generalized anxiety

disorder, depression, conduct disorder, oppositional defiant

disorder and a reading disorder, for all age groups, regres-

sion analysis of ADHD diagnostic subtypes by age and

reading disorder showed that only generalized anxiety dis-

order remained significant after controlling for ADHD

subtypes. Analysis of the mean reading disorder scores in

children with and without ADHD showed that children with

conduct disorder had significantly more reading problems,

as did children with multiple comorbid disorders. In sum-

mary, both age and ADHD diagnosis were associated with

variations in these comorbid disorders, and multiple

comorbid disorders were associated with greater reading

impairment.

Keywords Reading disorder � Attention-deficit/

hyperactivity disorder � Comorbidity � Psychometrics �Genetics � Heritability

Introduction

The best available estimates of attention-deficit/hyper-

activity disorder (ADHD) prevalence are around 5.29 %

for children and adolescents and 4.4 % in adulthood

(Polanczyk et al. 2007; Polanczyk and Rohde 2007).

ADHD is highly comorbid with many mental health dis-

orders, including anxiety, and other disruptive behaviour

disorders. The rate of comorbid mental health disorders

amongst children with ADHD is reported to be over 80 %,

with reading disorder also commonly reported (Faraone

et al. 1998; Willcutt and Pennington 2000a, b). In an

Australian study, children with ADHD consistently dem-

onstrated more comorbid mental health disorders, indicat-

ing a strong relationship between high rates of

This paper is based on a poster by Professor Florence Levy and

colleagues presented at the 14th Scientific Meeting of the

International Society for Research in Child and Adolescent

Psychopathology, Seattle, Washington, 17–20 June 2009.

F. Levy

School of Psychiatry, Prince of Wales Hospital,

University of New South Wales, Sydney, Australia

F. Levy (&)

Child and Family East, Prince of Wales Hospital,

Randwick, NSW 2031, Australia

e-mail: F.Levy@unsw.edu.au

D. J. Young � K. S. Bennett

School of Psychiatry and Clinical Neurosciences,

University of Western Australia, Perth, WA, Australia

D. J. Young (&)

Western Australian Centre for Mental Health Policy Research,

Graylands Hospital, Brockway Road, Mount Claremont,

WA 6010, Australia

e-mail: Deidra.Young@health.wa.gov.au

N. C. Martin � D. A. Hay

School of Psychology and Speech Pathology,

Curtin University of Technology, Perth, WA, Australia

123

ADHD Atten Def Hyp Disord (2013) 5:21–28

DOI 10.1007/s12402-012-0093-3

externalizing and internalizing symptoms (Levy et al.

2005). In the Colorado Longitudinal Twin Study of

Reading Disability, twins with a history of reading disorder

were reported to have deficits on school achievement, when

compared with the control twins (monozygous and dizy-

gous), leaving open the question of the relationship

between reading and comorbid mental health disorders

(Wadsworth et al. 2007).

The comorbidity of disruptive behaviour disorders with

ADHD has been reported to be 35 % for oppositional defiant

disorder (ODD) and 30–50 % for conduct disorder (CD)

(Biederman et al. 1991). In genetic analyses of these three

disorders, the genetic correlations were found to be very high

(rADHD�ODD = 0.73; rADHD�CD = 0.82; rODD�CD = 0.79),

suggesting that there are a common set of genes influencing

the three disruptive behaviours (Waldman et al. 2001).

Willcutt et al. (2000) demonstrated that children with

reading disorder are more likely, than those without, to

meet criteria for ADHD, and this association is stronger for

symptoms of inattention, than symptoms of hyperactivity

and impulsivity. Further, Willcutt and Pennington (2000a,

b) reported, in a study of comorbid mental health disorders

and reading disabilities, that individuals with reading dis-

order exhibited significantly higher rates of internalizing

and externalizing disorders, than those without reading

disorder. There appeared to be no significant gender dif-

ferences in comorbidity (Levy et al. 2005).

Previous studies have reported an association between

reading achievement and antisocial behaviour, but as yet

causation has not been fully explained. This association

was investigated in the UK using the Environmental Risk

Longitudinal Twin Study and appeared to result from

environmental factors common to both reading disorder

and antisocial behaviour and was stronger in boys (Trzes-

niewski et al. 2006). Environmental factors were also

suggested as influencing the relation between reading dis-

order and conduct disorder. Leading candidate environ-

mental risk factors weakly mediated the association. For

boys, the best explanation was a reciprocal causation

model: poor reading led to antisocial behaviour, and vice

versa. In contrast, the relation between reading achieve-

ment and attention-deficit hyperactivity disorder was best

explained by common genetic influences (Martin et al.

2006).

The possibility that reading problems are comorbid with

a number of mental health disorders, in addition to ADHD,

was first suggested by Cantwell and Baker (1991), and

more recently investigated by Martin et al. (2009). It is not

clear whether reading disorder is an outcome or an etio-

logical factor in relation to other mental health disorders.

An understanding of processes involved in optimal reading

development should provide further insights into comor-

bidity with other mental health disorders.

According to Shaywitz and Shaywitz (2008), the pos-

terior parietal cortex plays an important role in attention,

presumably via connections between the posterior parietal

cortex and PFC. The authors describe studies which indi-

cated that children with dyslexia exhibited reduced acti-

vation in a left hemisphere network, involving the inferior

parietal lobule. Nakamura postulated that attentional sys-

tems in the PFC activate the more dorsal system, because

brain activation triggered by conscious perception of word-

like stimuli should be subject to top–down attentional

amplification by the prefrontal cortex, providing a distrib-

uted activation of the fronto-temporal-parietal network

(Swanson et al. 1998).

Dehaene (1998) points out that the visual cortex is divi-

ded into two functional streams—a ventral pathway which

focuses on invariant object recognition (identity, shape and

colour), and a second visual pathway, the dorsal route

through the parietal cortex, which is primarily concerned

with space and action (Goodale and Milner 1992).

According to Dehaene, patients with ‘mirror blindness’ do

not see any difference between mirror shapes. Dehaene

suggests that, in the early stages of reading, we initially need

our dorsal system to distinguish letters such as ‘‘b’’ and ‘‘d’’.

Progressively, the ventral system learns to break with

symmetry and perceive asymmetry and consider ‘‘b’’ and

‘‘d’’ as two views of the same object. Ultimately, it assigns

them distinct neural populations that cease to generalize

across mirror reversals. In this way, the occipito-temporal

cortex acquires an asymmetrical neuronal hierarchy for

visual word recognition. Unlike its neighbouring cortical

regions for object or face recognition, which continue to

generalize over left–right changes, our reading architecture

ceases to confuse mirror images (Cantwell 1998).

Thus, according to Dehaene (1998), good decoding skills

do not arise from associations between letters and speech

sounds alone—letters must also be perceived in their proper

orientation, at the appropriate spatial location, and in their

correct left–right order. According to Dehaene, this inte-

gration is limited prior to the full development of the pre-

frontal and occipito-temporal cortex. Thus, the development

of reading capacity may be fundamentally linked to the

development of wider prefrontal capacity and behaviour.

The present study investigated the hypothesis that

reading problems are likely to be associated with symp-

toms of behaviour disorder that may also relate to pre-

frontal maturation problems. Also, more severe reading

delays may be associated with multiple behavioural sym-

ptomatologies, reflecting underlying developmental delays.

Specific aims/hypotheses of the present study were:

1. Children with ADHD symptoms are more likely to

have a reading disorder, when compared with children

without ADHD;

22 F. Levy et al.

123

2. Children with symptoms of ADHD and one mental

health disorder are more likely to have a reading

disorder, when compared with children without a

mental health disorder;

3. Children with ADHD and symptoms of multiple

comorbid mental health disorders (two or more) are

more likely to have a reading disorder, when compared

with children without such comorbidity.

Methods

The Australian Twin ADHD study sample

The Australian Twin ADHD Study commenced in 1991.

One of the aims of this study was to follow a group of twins

and their siblings in order to describe the developmental

pattern of ADHD.

There have been four phases of data collection as

detailed in Bennett et al. (2006). In each of the data col-

lection phases, parents completed a questionnaire about

their twins and similar aged siblings, using the Twin and

Sibling Questionnaire (Hay and Levy 1999; Levy and Hay

1991). This questionnaire covers areas including develop-

ment, behaviour, and education. Phase 4 commenced in

2004, with the Twin and Sibling Questionnaire data col-

lected from parents of twin children families throughout

Australia. This phase of the data collection continued

through to 2007, resulting in 3,060 twin families partici-

pating in this phase of the study. Some families had just the

twins in the study and some had up to two siblings par-

ticipating in addition to the twins. The 3,060 families

participating in this study consisted of 5,303 twins and

1,855 siblings (n = 7,158).

A mail-out questionnaire included parental ratings of

ADHD (DSM-IV), as well as DSM-IV-based measures of

separation anxiety disorder, generalized anxiety disorder,

depression, conduct disorder, and oppositional defiant

disorder for twins and their siblings aged 6–18 years.

Reading difficulties were rated on a parental questionnaire

developed by Willcutt and Pennington (2000a, b). There

are no children under the age of 6 in this study.

ADHD

In the present study, ADHD was measured using a DSM-

IV-based parent rating of ADHD symptoms, the Australian

Twin Behaviour Rating Scale. This scale consisted of 18

items derived from the DSM-IV in wording (American

Psychiatric Association, 2000). These items consisted of

nine inattentive and nine hyperactive/impulsive measures.

The DSM-IV ADHD items were rated on a response set: not

at all [0], just a little [1], often [2], and very often [3]. These

items were recoded into a binary measure: (0, 1 = 0) and

(2, 3 = 1). According to the DSM-IV, a diagnosis of Inat-

tentive ADHD requires at least six out of the nine inattentive

symptoms to be coded 1, while a diagnosis of Hyperactive/

Impulsive ADHD requires at least six out of the nine

hyperactive symptoms to be coded 1. Where a person or

child has at least six inattentive and six hyperactive symp-

toms, this subtype was categorized as combined ADHD.

The variable ADHD consisted of the four categories: inat-

tentive ADHD, hyperactive/impulsive ADHD, combined

ADHD and no ADHD. The questionnaire was a parent

reported rating scale and did not include age of onset of

ADHD symptoms, so that definitive ADHD diagnoses, as

distinct from ADHD symptomatology, could not be made.

Reading disorder

Five mental health disorder symptomatologies, comorbid

with ADHD, were compared for reading problems using

mixed multilevel analyses. Reading was measured using a

scale of six items developed by Willcutt and colleagues

(personal communication) and now described in the first

6 items of Table 2 in Willcutt et al. (2011) for the Colorado

Longitudinal Twin Study of reading disorder. This reading

scale forms part of the Colorado Learning Difficulties

Questionnaire (CLDQ) and scores on this scale range from

0, ‘‘not at all/never’’ to 3 ‘‘very much/very often’’. The

CLDQ Reading Difficulty scale has been reported by

Willcutt and colleagues to be highly correlated with the

Peabody Individual Achievement Test (PIAT). In their

study, the CLDQ Reading Difficulty scale demonstrated

high validity and reliability. Before the CLDQ Reading

Difficulty scale was analysed, all measures were trans-

formed so that a positive correlation (Pearson correlation

coefficient) indicated a significant relationship between

higher ratings on the CLDQ Reading Difficulty scale and

more severe difficulties on the PIAT reading measure,

regardless of the original scaling of the measure. The

strong correlation with the Peabody Individual Achieve-

ment Tests for Reading (r[re] = 0.64; CI = 0.60, 0.68;

p \ 0.001) was also confirmed by the effect size for 945

cases diagnosed with reading disorder (d[re] = 1.81;

CI = 1.42, 2.20; p \ 0.001) (Willcutt et al. 2011). The

questionnaire was normed on an American population

(Colorado) and has demonstrated high reliability and

validity in other studies as well (Martin et al. 2006).

This scale consists of the following items:

1. Does this child have difficulty with spelling?

2. Did this child have difficulty learning letter names?

3. Did this child have difficulty learning phonics (sound-

ing out words)?

Comorbid ADHD and mental health disorders 23

123

4. Does this child read more slowly than other children of

the same age?

5. Does this child read below grade or expectancy level?

6. Has this child required extra help in school because of

problems in reading or spelling?

The response set for reading disorder items was: not at

all [0], just a little [1], often [2], and very often [3]. Each

item was rescored into a binary measure: (0, 1 = 0) and

(2, 3 = 1). The variable Read was prepared from the six

binary items and ranged from 0 to 1. The mean score for

the whole population was 0.09 with a 95 % CI of [0, 0.32].

Mental health problems

For disorder-related symptomatologies, the authors dis-

cussed and agreed on those symptoms in the questionnaire

which best approximated the DSM-IV wording and total

number of symptoms to achieve the best approximation to

the DSM-IV criteria. The questionnaire included DSM-IV

based diagnostic measures of the following childhood

mental health disorders: Separation Anxiety, generalized

anxiety disorder, conduct disorder, oppositional defiant

disorder and depression. For example, at least four out of

eight symptoms were required for a diagnosis of opposi-

tional defiant disorder (American Psychiatric Association

2000). The response scale for these items was the same as

for reading disorder: not at all [0], just a little [1], often [2],

and very often [3], with each item rescored into a binary

measure of 0 or 1.

Data analysis

Of particular interest to this study is the comorbidity of

reading problems and the DSM-IV-based mental health

symptoms. With the children grouped within families, the

mixed models method was used in order to account for the

family group effects. There were 7,089 children and 2,702

families, with an average of 2.6 children per family. Mixed

models are characterized as containing both fixed and

random effects. The fixed effects are analogous to standard

regression coefficients and are estimated directly. The

random effects were not directly estimated, but were

summarized according to their estimated variances and

covariances. Random effects in this analysis were the

random intercepts, and the grouping structure of the data

consisted of family groups. The error distribution of the

linear mixed model was assumed to be Gaussian. In these

analyses, the dependent variable was the composite mea-

sure Reading Disorder (Read), with the estimate allowed to

vary between families. The children were classified as

being a twin or sibling and this variable (twin) was inclu-

ded in all analyses in order to account for possible

differences in reading disorder between the two types of

children. The other explanatory variables included the

categorical variables: age group (agegp) and ADHD sub-

type (ADHD); and the dichotomous variables: depression

(DXDE), separation anxiety (DXSA), generalized anxiety

disorder (DXGAD), conduct disorder (DXCD) and oppo-

sitional defiant disorder (DXODD).

The procedure xtmixed from Stata for this type of esti-

mation (Stata 2007) was next applied. This procedure, a

linear mixed models method, accounted for the variability

between families (ID), while estimating the reading dis-

abilities of the children (Read). With most explanatory

variables in this study being categorical, the xi technique

was used to compare categories within each variable (e.g.,

i.twin allowed comparison of twins’ and siblings’ reading

disabilities). Each variable had a baseline category for

comparative purposes. For example, the variable ADHD

had the baseline set at the category non-ADHD cases.

Using the post-estimation procedure, adjust, the linear

predictions of xb were then estimated for each categorical

variable, along with standard errors and the 95 % CIs.

Finally, reading disorders were estimated for pairs of

comorbid mental health disorders.

Results

The prevalence of ADHD in this cohort when using DSM-

IV criteria was found to be: 5.49 % ADHD, 3.26 % inat-

tentive, 0.85 % hyperactive and 1.38 % combined

(Table 1). Higher mean scores for reading problems were

found for those cases with ADHD, particularly the com-

bined and/or inattentive subtypes, which were four times

higher than the average reading disorder for all cases. This

was consistent for the DSM-IV diagnoses of ADHD and

further analyses were carried out using DSM-IV diagnoses.

For each of the six mental health symptomatologies

investigated in this paper, all cases had significantly higher

reading disorder mean scores (l = 0.22–0.53), compared

with those without a mental health disorder (l = 0.09)

(Table 2). Specifically, those who had ADHD and one

other mental health problem were even more likely to have

a reading problem. For example, a child with ‘depression’

had a mean reading problem score of 0.38, while a child

with both ‘depression’ and ‘ADHD’ had a mean reading

problem score of 0.61. The following analyses incorporated

both the twin effect and the family grouping effect.

Mixed multilevel analyses for reading problems

revealed firstly that twins were more likely to have reading

problems, when compared to siblings (b = 0.122; 95 % CI

[0.114, 0.129]) (Table 3). With few age-related differ-

ences, there was only a small effect for 15–18-year-old

24 F. Levy et al.

123

children (b = 0.095; 95 % CI [0.082, 0.109]). They

showed less reading problems than the youngest age group.

There were significant differences in mean reading

problem scores for those with inattentive (b = 0.415; 95 %

CI [0.383, 0.445]) and combined ADHD (b = 0.505; 95 %

CI [0.455, 0.552]), compared with children without ADHD

(Table 3). This effect was true for twins, DZ twins, MZ

twins and siblings, whether or not the child was a twin.

Further, children with generalized anxiety disorder symp-

toms were more likely to have a higher mean reading

problem score, irrespective of whether they were a twin or

not. Similarly, children with conduct disorder or opposi-

tional defiant disorder symptoms were more likely to have

a high reading problem score (l[ 0.5).

Finally, the reading problem mean score (l) was

investigated for children with symptoms of more than one

comorbid mental health disorder. For this test, the age

group 6–9 years was excluded to allow for normal varia-

tions and schooling, in the development reading skills. The

highest reading problem scores were found for children

with inattentive ADHD symptoms and/or combined ADHD

symptoms (Table 4). When inattentive ADHD symptoms

were comorbid with depression (l = 0.60), separation

anxiety (l = 0.61) or generalized anxiety disorder symp-

toms (l = 0.63), those children were significantly more

likely to have higher reading disorder scores. Similarly,

when combined ADHD subtype symptoms are comorbid

with depression (l = 0.65), separation anxiety (l = 0.63)

or generalized anxiety disorder symptoms (l = 0.57),

reading problem scores were high. Finally, conduct disor-

der and Inattentive or combined ADHD subtypes showed

significantly high reading problem scores.

Table 1 Prevalence of ADHD by sub-type, gender

Gender ADHD sub-type (%) Non-ADHD

n = 6,314Inattentive

n = 233

Hyperactive

n = 61

Combined

n = 99

Total ADHD

n = 393

Males 4.65 1.09 2.15 7.89 92.11

Females 1.78 0.60 0.57 2.96 97.04

Total 3.26 0.85 1.38 5.49 94.51

Reading disorder (l) 0.43 0.20 0.53 0.42 0.10

Table 2 Mean reading disorder score by mental health disorder (MHD) symptoms

Mental health disorder (MHD) symptoms n % of Total % of MHD Mean reading

difficulty l (SD)

Depression 46 0.64 5.94 0.38 (0.43)

Separation anxiety 226 3.16 29.16 0.22 (0.36)

Generalized anxiety disorder 87 1.22 11.23 0.36 (0.40)

Conduct disorder 103 1.44 13.29 0.29 (0.36)

Oppositional defiant disorder 324 4.53 41.81 0.28 (0.38)

Total ADHD (DSM-IV) 393 5.49 50.71 0.42 (0.41)

Inattentive ADHD 233 3.26 30.06 0.44 (0.41)

Hyperactive ADHD 61 0.85 7.87 0.20 (0.31)

Combined ADHD 99 1.38 12.77 0.53 (0.42)

One mental health disorder and reading 775 10.83 100.00 0.29 (0.38)

ADHD and depression 27 0.38 3.48 0.61 (0.42)

ADHD and separation anxiety 51 0.71 6.58 0.49 (0.45)

ADHD and generalized anxiety disorder 30 0.42 3.87 0.56 (0.44)

ADHD and conduct disorder 59 0.82 7.61 0.38 (0.39)

ADHD and oppositional defiant disorder 139 1.94 17.94 0.44 (0.43)

ADHD and one mental health disorder 176 2.46 22.71 0.46 (0.43)

ADHD and any two mental health disorders 86 1.20 11.10 0.49 (0.43)

No. of mental health disorders 6,314 88.21 0.09 (0.23)

Total cases 7,158 100.00 0.12 (0.26)

Comorbid ADHD and mental health disorders 25

123

When two or more non-ADHD comorbid symptom

problems were investigated, high mean reading problem

scores were seen for: depression and Separation Anxiety

(l = 0.31), generalized anxiety disorder (l = 0.38), con-

duct disorder (l = 0.54) or oppositional defiant disorder

symptoms (l = 0.42), with high mean reading problem

scores. Similarly for Separation Anxiety and generalized

anxiety disorder (l = 0.31), conduct disorder (l = 0.39)

or oppositional defiant disorder symptoms (l = 0.34),

reading problem scores were high. Where generalized

anxiety disorder symptoms were comorbid with conduct

disorder (l = 0.44) or oppositional defiant disorder

symptoms (l = 0.36), reading problem scores were also

high, as were conduct disorder and oppositional defiant

disorder symptoms (l = 0.30).

The total population of twins and siblings had a very low

reading problem scores (l = 0.12). Where there were no

measureable mental health disorders symptoms, the read-

ing disorder score was even lower (l = 0.09).

Discussion

The present findings replicate the Willcutt findings of an

increased association of reading problems with Inattentive

ADHD symptoms. The above association of reading dis-

order, with ADHD and two or more mental health disorders

could indicate that either ADHD or reading disorder is the

primary deficit or that comorbidities reflect associated

stages of development. The significance of multiple

Table 3 Mixed multilevel analyses for reading disorder

Reading disorder predictor variables Predicted coefficient Standard error 95 % CI

Twins and

siblings

Twins

only

MZ twins

only

DZ twins

only

Lower

boundary

Upper

boundary

Twins/siblings 0.122*** – – – 0.004 0.114 0.129

Age group

6–8 years 0.123 0.119 0.107 0.128 0.008 0.106 0.136

9–11 years 0.134 0.140 0.125 0.148 0.006 0.121 0.145

12–14 years 0.109 0.115 0.118 0.114 0.006 0.097 0.119

15–18 years 0.095* 0.107 0.088 0.120 0.007 0.082 0.109

ADHD

None 0.098 0.104 0.098 0.110 0.004 0.091 0.105

Inattentive 0.415*** 0.415*** 0.406*** 0.453*** 0.016 0.383 0.445

Hyperactive/impulsive 0.177 0.170 0.257*** 0.160 0.031 0.112 0.232

Combined 0.505*** 0.527*** 0.395*** 0.525*** 0.025 0.455 0.552

Depression 0.329 0.380 0.345 0.355 0.004 0.106 0.120

Separation anxiety 0.222* 0.228 0.206 0.263* 0.017 0.189 0.255

Generalized anxiety disorder 0.313*** 0.361*** 0.336*** 0.390*** 0.008 0.206 0.237

Conduct disorder 0.261* 0.353 0.266 0.366 0.009 0.267 0.301

Oppositional defiant disorder 0.262** 0.286* 0.199 0.295 0.006 0.216 0.242

*** p \ 0.001; ** p \ 0.01; * p \ 0.05

Table 4 Mean reading disorder scores by two comorbid mental health disorders

Mental health disorder symptoms

(age range 9–18 years)

Comorbid mental health disorder symptoms and reading disorder symptoms

ADHD—

inattentive

ADHD—

hyperactive

ADHD—

combined

Depression SA GAD CD ODD

Depression 0.58 – 0.65 –

Separation anxiety 0.61** – 0.57 0.32 –

Generalized anxiety disorder 0.62** 0.11 0.60 0.36 0.32* –

Conduct disorder 0.34 – 0.50 0.54* 0.34 0.44 –

Oppositional defiant disorder 0.47 0.19 0.53 0.41 0.33** 0.35 0.29 –

*** p \ 0.001; ** p \ 0.01; * p \ 0.05: significant differences between MHD and not MHD (row) for each condition (column)

26 F. Levy et al.

123

comorbidities has had limited investigation. Willcutt and

Pennington (2000b) found that children with reading dis-

abilities were more likely to have elevated internalizing

symptoms. Similarly, the present study found that reading

problem scores were higher for children with both ADHD

and depression or anxiety disorder symptoms and has

extended this finding in relation to combinations of two or

more comorbid symptomatologies. As the degree of

comorbid symptomatologies increases, reading problems

appear to be more severe, but the etiological direction of

this association is unclear.

Levy (2010) has discussed the ‘paradox’ of comorbid

internalizing and externalizing behavioural phenotypes in

terms of excess communication between subcortical cir-

cuits, possibly as a result of insufficient cortical inhibition,

as well as excessive reliance on stimulus-based subcortical

systems. A neuroanatomical understanding of reciprocal

and non-reciprocal connections between modular circuits

have provided a basis for understanding previously puz-

zling aspects of comorbidity between internalizing and

externalizing childhood syndromes, previously regarded as

orthogonal. The present study suggests that the develop-

ment of reading skills is fundamentally associated with the

maturation of cortical/subcortical neural circuits involved

in the control or modulation of behaviour.

Advances in functional brain magnetic resonance

imaging has provided greater understanding of the neural

mechanisms involved in reading and how these differ in

children with reading disabilities (Shaywitz and Shaywitz

2008). Recent findings demonstrate that dual cognitive

processes are involved in reading, including attentional

mechanisms. Disruption of these attentional mechanisms is

thought to play a causal role in reading difficulties. Pre-

liminary studies have suggested that stimulants tradition-

ally used to treat ADHD may prove to be beneficial for

improving reading in children with reading disorder

(Grizenko et al. 2006; Keulers et al. 2007; Richardson et al.

1988). This may be due to the medications easing symp-

toms of inattention, which then facilitates the child’s ability

to read.

According to Shaywitz and Shaywitz (2008) within a

dual route model, the critical process of phonological

recoding (translating letters to sound) can occur via two

different mechanisms: a lexical mechanism that addresses

the phonological code directly from the stored lexical

representations, and a sublexical mechanism, that assem-

bles the phonological code serially letter by letter. The

authors discuss recent evidence suggesting that attention is

necessary for the computation of phonology, resulting in

reading the word aloud. They suggest that further consid-

eration of the role of attentional mechanisms in support of

reading is necessary. The present data raise the further

question of association of reading with both internalizing

and externalizing comorbid disorders, suggesting that

reading disorder may be marker for a number of develop-

mental problems.

Clinical implications

The study suggests that where multiple comorbidities are

present, particularly with ADHD, clinical attention should

be paid to the presence of reading disorder. Not only should

clinicians be aware of the possible presence of reading

problems, but also that the problem may be more severe as

the amount of comorbid symptomatologies increases.

Concurrent treatment of both the ADHD and the reading

disorder could have beneficial effects for both these dis-

orders, as well as other comorbidities, suggesting that a

multi-disciplinary team is needed to address these issues, in

such children.

Limitations of the current study

This study is based upon a large cohort of mainly twins

with some singletons (siblings). The findings are therefore

limited to the extent that the cohort may not be represen-

tative of a large population of singletons. Further, there

were no younger children in this study to report on, so the

conclusions are limited in applicability to those aged

6–18 years. While the epidemiology of reading disorder

and ADHD is reported in many twin and singleton studies,

the developmental relationship between reading disorder

and other mental health disorders is currently not well

understood, but important for an understanding of comor-

bidity in children.

Acknowledgments The Australian Twin ADHD Study was sup-

ported by a project grant (ID 479217) from the Australian National

Health and Medical Research Council. This research was facilitated

with access to the Australian Twin Registry, an Australian national

research resource supported by an Australian National Health and

Medical Research Council Enabling Grant (ID 310667) from the

Australian National Health and Medical Research Council and

administered by The University of Melbourne, Victoria, Australia.

The support of the Western Australian Child Health Twin survey in

recruiting new twin families to this study is also gratefully

acknowledged. We would also like to thank the many families who

continue to support the Australian Twin ADHD Study.

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