Given our relatively small sample size, the heritability estimates for the subscales according to the CPRS and CTRS, and the CPT measures should be interpreted cautiously. Although it appears at first glance that the CTRSs, but not the CPRSs are significantly heritable in our population, the overall trends are consistent with one another (hyperactive-impulsive h2>total symptom count h2>inattentive h2) and both the hyperactive-impulsive and the total symptom count estimated heritabilities according to the parent ratings are bordering on significance. Several studies have reported higher heritability estimates of AD/HD subscales according to parent ratings than what we have estimated in our study population (Kuntsi and Stevenson, 2001; Price et al., 2001). Possible explanations for the observed discrepancy include differences in the power of the study, study population characteristics, informant characteristics, rating scale, and the analysis method used.
