Etiology of inattention and hyperactivity/impulsivity in a community sample of twins with learning difficulties

A community sample of 373 8 to 18 year-old twin pairs in which at least one twin in each pair exhibited a history of learning difficulties was utilize d to examine the etiology of inattention and hyperactivity/impulsivity (hyp /imp). Symptoms of attention-deficit/hyperactivity disorder (ADHD) were ass essed by the DSM-III Diagnostic Interview for Children and Adolescents. Ina ttention and hyp/imp composite scores were created based on results of a fa ctor analysis. Results indicated that extreme ADHD scores were almost entir ely attributable to genetic influences across several increasingly extreme diagnostic cutoff scores. Extreme inattention scores were also highly herit able whether or not the proband exhibited extreme hyp/imp. In contrast, the heritability of extreme hyp/imp increased as a linear function of the numb er of inattention symptoms exhibited by the proband. This finding suggests that extreme hyp/imp may be attributable to different etiological influence s in individuals with and without extreme inattention. If this result can b e replicated in other samples, it would provide evidence that the hyp/imp s ymptoms exhibited by individuals with Combined Type ADHD and Predominantly Hyp/Imp Type ADHD may be attributable to different etiological influences.