This use of the dimensional model in the study of ADHD has facilitated the collection of data from larger samples, because face-to-face interviews are not required. However, without recourse to a clinical category, defining the ADHD phenotype for genetic studies remains a contemporary and controversial issue.18,19 Interrater correlations for ADHD behaviors between parents are teachers are typically around 0.3.18,20,21 This low correlation has been attributed to many sources, and problems arise for genetic studies when differences in these assessments reflect sources of variance other than the ADHD trait of interest. These alternative sources of variance may include conflation with other correlated behaviors (such as oppositional behaviors, known as halo effects).22–24 Also possible are “contrast effects,” where the behavior of one child affects the perception and rating of the other child within a twin pair.25–28 Contributions from these other sources of variance to ADHD behavioral assessments decrease the precision with which ADHD symptoms measure the underlying genetic liability for ADHD. In association and linkage studies, statistical power increases with the effect size of the quantitative trait locus.29,30 Thus a precise definition
