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Introduction-Attention-deficit/hyperactivity disorder (ADHD) and reading disability (RD) are complex childhood disorders that frequently co-occur, but the etiology of this comorbidity remains unknown.Method-Participants were 457 twin pairs from the Colorado Learning Disabilities Research Center (CLDRC) twin study, an ongoing study of the etiology of RD, ADHD, and related disorders. Phenotypic analyses compared groups with and without RD and ADHD on composite measures of six cognitive domains. Twin analyses were then used to test the etiology of the relations between the disorders and any cognitive weaknesses.Results-Phenotypic analyses supported the hypothesis that both RD and ADHD arise from multiple cognitive deficits rather than a single primary cognitive deficit. RD was associated independently with weaknesses on measures of phoneme awareness, verbal reasoning, and working memory, whereas ADHD was independently associated with a heritable weakness in inhibitory control. RD and ADHD share a common cognitive deficit in processing speed, and twin analyses indicated that this shared weakness is primarily due to common genetic influences that increase susceptibility to both disorders.Conclusions-Individual differences in processing speed are influenced by genes that also increase risk for RD, ADHD, and their comorbidity. These results suggest that processing speed measures may be useful for future molecular genetic studies of the etiology of comorbidity between RD and ADHD.

We report preliminary behaviour genetic analyses of reading and listening comprehension from The Colorado Learning Disabilities Research Center. Although the twin sample with these new measures is still of limited size, we find substantial, and significant, genetic influences on individual differences in both reading and listening comprehension. In addition, word recognition and listening comprehension each accounted for significant independent genetic influences on reading comprehension. Together, they accounted for all the genetic influence on reading comprehension, indicating a largely genetic basis for the 'simple model' of individual differences in reading comprehension proposed by Hoover and Gough (1990).When we began our behaviour genetic studies of 'dyslexia', or reading disability, with identical and fraternal twins in 1982, it was commonly assumed that the difficulties in reading comprehension that children with dyslexia have are largely, if not completely, because of their deficits in phonological skills and word decoding (Liberman, Shankweiler, Fisher & Carter, 1974;Perfetti, 1985;Shankweiler, 1989). For example, in Perfetti's 'verbal efficiency theory', the higher processing demands in word decoding in poor readers were thought to result in fewer resources for processing the meaning of text, resulting in deficits in their reading comprehension. However, in recent years, evidence has been accumulating to support some dissociation between decoding and comprehension problems. This raises the question of the extent to which deficits in decoding and comprehension share a common aetiology. The purpose of this report is to provide preliminary findings in our attempt to address this question by examining the genetic and environmental aetiologies of individual differences in comprehension. Relation between word decoding and comprehension problemsEarly research by Curtis (1980) found that while word decoding and reading comprehension are very highly correlated in beginning readers, this correlation declines

We examined reading comprehension in children with ADHD by assessing their ability to build a coherent mental representation that allows them to recall central and peripheral information. We compared children with ADHD (mean age 9.78) to word reading-matched controls (mean age 9.89) on their ability to retell a passage. We found that even though children with ADHD recalled more central than peripheral information, they showed their greatest deficit, relative to controls, on central information – a centrality deficit (Miller & Keenan, 2009). We explored the cognitive underpinnings of this deficit using regressions to compare how well cognitive factors (working memory, inhibition, processing speed, and IQ) predicted the ability to recall central information, after controlling for word reading ability, and whether these cognitive factors interacted with ADHD symptoms. Working memory accounted for the most unique variance. Although previous evidence for reading comprehension difficulties in children with ADHD have been mixed, this study suggests that even when word reading ability is controlled, children with ADHD have difficulty building a coherent mental representation, and this difficulty is likely related to deficits in working memory.

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Numerous studies have examined the etiology of the association between reading difficulties and attention-deficit hyperactivity disorder (ADHD) in school-age children, but little is known about the relation between prereading skills and ADHD behaviors prior to the beginning of formal reading instruction. A population-based sample of 809 pairs of preschool twins completed an extensive battery of prereading measures, and the parent of each twin completed an ADHD rating scale. Phenotypic analyses revealed small but significant correlations between DSM-IV inattention ratings and six prereading composite measures, whereas hyperactivity-impulsivity symptoms were not independently associated with any of the prereading scores. Multivariate twin analyses indicated that virtually all of the phenotypic correlation between inattention and prereading performance is attributable to common genetic influences, consistent with results obtained in studies of older twins. Although additional research is needed to test alternative causal models in children younger than five years old, these results are most consistent with the hypothesis that reading difficulties and inattention symptoms are attributable to common genetic influences.

ABSTRACT— The majority of children who receive special education services meet criteria for reading disability (RD) or attention‐deficit/hyperactivity disorder (ADHD), but additional research is needed to understand the long‐term academic outcome of children in these groups. Individuals with RD only (N = 71), ADHD only (N = 66), both RD and ADHD (N = 51), or neither disorder (N = 118) were identified through the ongoing Colorado Learning Disabilities Research Center twin study and retested 5 years later. Results of the follow‐up testing indicated that, in addition to ongoing reading difficulties, individuals with RD exhibited higher rates of academic difficulties, depression, and adolescent‐onset conduct disorder. Initial ADHD status was associated with academic and social difficulties and elevated rates of nearly all comorbid disorders 5 years later. The group with comorbid RD and ADHD had more stable reading deficits than the group with RD without ADHD and exhibited greater impairment than groups with either disorder alone on outcome measures of academic functioning and social difficulties. These results suggest that individuals with both RD and ADHD are at increased risk for negative outcomes as adolescents and young adults and that when RD and ADHD co‐occur, interventions should be provided for both disorders.

Longitudinal twin data were analyzed to investigate the etiology of the stability of genetic and environmental influences on word reading and reading comprehension, as well as the stability of those influences on their relationship. Participating twin pairs were initially tested at a mean age of 10.3 years, and retested approximately five years later. Both word reading and comprehension were found to be highly stable, and genetic influences were primarily responsible for that stability. In contrast to studies with younger participants, no unique genetic influences were observed at follow-up testing in this older sample. High genetic correlations were obtained between word reading and reading comprehension at both ages, indicating common genetic influences. However, significant genetic influence on comprehension was also observed, independent of that on word reading. Although the phenotypic relation between the two measures appeared to decline across time, the genetic etiology of this relation was highly stable.