Meta-analyses of behavioral ratings of ADHD symptoms in the general population have concluded that variation is largely genetic (60% to 76%).1–3 Different methodologic approaches to meta-analysis may account for the differences across the three studies. In 2005 an influential review of behavioral ratings scale data concluded that 76% of the variance was due genetic influences, with the rest being attributable to E.2 However, this review used an average of “broad sense” heritability estimates across just 20 studies, unweighted for sample sizes. A more recent weighted meta-analysis using SEM methodology to combine the results of individual biometrical genetic studies drew a similar conclusion: 70% of the variance was due to A + D, and the rest to E. This result was somewhat surprising because all other behaviors associated with childhood psychopathology indicated a significant influence of C.1 It is, however, possible that this finding was affected by methodologic limitations of meta-analytic biometrical genetics, including a lack of power to detect sibling interaction, the confounding of C and D in genetic models using only MZ and DZ twins reared together, and the
