Etiology of the comorbidity between RD and ADHD: exploration of the non-random mating hypothesis.

The present research sought to test the validity of the non-random mating hypothesis in a community sample in order to determine whether it contributes significantly to the comorbidity between attention-deficit/hyperactivity disorder (ADHD) and reading disability (RD) and to test whether the two disorders segregate independently. Data were gathered from 394 twin participants of the Colorado Learning Disabilities Research Center (CLDRC) and their biological parents. Parents were asked to complete retrospective questionnaires regarding their own ADHD and RD symptomatology as children, as well as rate their children on DSM-IV ADHD symptomatology before age 12. Twin RD was assessed by a composite score consisting of several reading measures. Chi-square and correlational analyses assessed the presence of non-random mating, as well as whether the two disorders were transmitted independently. Non-random mating between those with ADHD and those with RD did not significantly contribute to the comorbidity of the two disorders. Furthermore, there was some evidence that these two disorders did not segregate independently in this sample. These results are consistent with recent evidence that ADHD and RD are often comorbid because of significant genetic overlap. There was little evidence to support that non-random mating also contributes to this comorbidity in the present sample.