Parents completed the CBCL at baseline and follow-up. The attention problems subscale, which served as our primary clinical outcome measure because questions and scoring procedures were similar at both time points, consists of several items reflecting difficulties with attention, motor activity, impulsivity and other ADHD-related behaviors. The CBCL-AP effectively discriminates ADHD cases from non-ADHD controls (Hudziak, Copeland, Stanger & Wadsworth, 2004), and correlates well with the clinical DSM-IV assessment of ADHD (Derks, Hudziak, Dolan, Ferdinand & Boomsma, 2006). Importantly, scores on the CBCL-AP have proven to be highly heritable and genetically stable over time such that genetic effects explain approximately 75% of the heritability developmentally from age 7 years and older (Rietveld, Hudziak, Bartels, Beijsterveldt & Boomsma, 2004). Raw scores on the CBCL were transformed to age-corrected T-scores.
