Etiology and neuropsychology of comorbidity between RD and ADHD: The case for multiple-deficit models

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 pro cessing 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 (C) 2010 Elsevier Srl All rights reserved