Does the definition of ADHD affect heritability?

Objective: A twin study design was used to examine the genetic validity of attention-deficit hyperactivity disorder (ADHD)-related phenotypes. Method: Questionnaires covering ADHD symptoms were sent to the families of 2,846 school-age twins. Parent-rated symptoms were obtained for 2,082 twin pairs and teacher-rated symptoms were available for 1,470 twin pairs. Results: Broadly defined parent-rated, teacher-rated, and "pervasive" (both parent- and teacher rated) ADHD categories were found to be highly heritable. Significant shared environmental effects were also detected for teacher-rated ADHD. A common genetic factor was found to have a modest influence on both parent- and teacher-rated symptom scores and categories, but additional genetic and environmental influences were also found far teacher-rated ADHD. Consistent with previous findings, ADHD symptom scores were again found to be highly heritable. Maternal contrast effects were found far the Rutter A scale items but could not be detected for the DuPaul ADHD rating scale. Conclusions: Broadly defined pervasive ADHD appears to be as heritable as ADHD behaviors defined by maternal reports alone. A common genetic factor influences maternally rated and teacher-rated ADHD but does not account for all of the genetic variance for teacher-rated ADHD. ADHD symptom scores are highly heritable, and maternal contrast effects appear to vary for different measures.