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This study utilized a sample of 313 eight- to sixteen-year-old same-sex twin pairs (183 monozygotic, 130 dizygotic) to assess the etiology of comorbidity between reading disability (RD) and attention-deficit/hyperactivity disorder (ADHD). RD was assessed by a discriminant function score based on the Peabody Individual Achievement Test, a standardized measure of academic achievement. The DSM-III version of the Diagnostic Interview for Children and Adolescents was used to assess symptoms of ADHD, and separate factor scores were computed for inattention and hyperactivity/impulsivity (hyp/imp). Individuals with RD were significantly more likely than individuals without RD to exhibit elevations on both symptom dimensions, but the difference was larger for inattention than hyp/imp. Behavior genetic analyses indicated that the bivariate heritability of RD and inattention was significant (h(2)(g(RD/Inatt)) = 0.39), whereas the bivariate heritability of RD and hyp/imp was minimal and nonsignificant (h(2)(g(RD/Hyp)) = 0.05). Approximately 95% of the phenotypic covariance between RD and symptoms of inattention was attributable to common genetic influences, whereas only 21% of the phenotypic overlap between RD and hyp/imp was due to the same genetic factors.
Willcutt et al. (Sat,) studied this question.