Twin study of the etiology of comorbidity between reading disability and attention-deficit/hyperactivity disorder

This study utilized a sample of 313 eight- to sixteen-year-old same-sex twi n pairs (183 monozygotic, 130 dizygotic) to assess the etiology of comorbid ity between reading disability (RD) and attention-deficit/ hyperactivity di sorder (ADHD). RD was assessed by a discriminant function score based on th e Peabody Individual Achievement Test, a standardized measure of academic a chievement. The DSM-III version of the Diagnostic Interview for Children an d Adolescents was used to assess symptoms of ADHD, and separate factor scor es 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 wa s larger for inattention than hyp/ imp, Behavior genetic analyses indicated that the bivariate heritability of RD and inattention was significant (h(g (RD/Inatt))(2) = 0.39), whereas the bivariate heritability of RD and hyp/im p was minimal and nonsignificant (h(g(RD/Hyp))(2) = 0.05). Approximately 95 % of the phenotypic covariance between RD and symptoms of inattention was a ttributable to common genetic influences, whereas only 21% of the phenotypi c overlap between RD and hyp/imp was due to the same genetic factors, Am. J , Med. Genet, (Neuropsychiatr. Genet,) 96:293-301, 2000, (C) 2000 Wiley-Lis s, Inc.