The influence of genetic factors in the comorbidity of spelling disability and hyperactivity was investigated in two samples of 190 and 260 same sex twin pairs. The method of bivariate group heritability was used to estimate the genetic correlation for spelling disability and hyperactivity. A similar though not statistically significant value for the genetic correlation was obtained for the two samples (0.29 and 0.42). It was estimated that approximately 75% of the co-occurrence of these two conditions was due to shared genetic influences.
In order to assess the genetic etiology of attention deficit hyperactivity disorder (ADHD), the basic regression model for the analysis of selected twin data (DeFries & Fulker, 1985, 1988) was fitted to questionnaire data (DICA: Diagnostic Interview for Children and Adolescents; Herjanic, Campbell, & Reich, 1982) for 37 identical and 37 fraternal twin pairs tested in the Colorado Reading Project. Results of this analysis suggest that ADHD is highly heritable. Moreover, adjusting DICA scores for either IQ or reading performance differences did not substantially change parameter estimates. In future analyses of larger data sets, distinguishing between possible subtypes of attentional problems (e.g., ADD with or without hyperactivity) may facilitate tests of more searching etiological questions.
our years after the inception of the American Psychological Asso-F ciation, W. Pringle Morgan (1896) used the term congenital wordblindness to describe the case of an intelligent 14-year-old boy who was incapable of learning to read. Within a decade, the familial nature of this condition was described by Thomas (1905) as follows: 121
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