Importantly, much of the genetic analysis for AD/HD defined the phenotype from a qualitative perspective; the presence or absence of an AD/HD diagnosis. A more recent trend in the study of AD/HD genetics has been to focus on endophenotypes or intermediate phenotypes of AD/HD. As proposed originally by Gottesman and Gould (2003). endophenotypes for neuropsychiatric conditions like AD/HD are appealing because the qualitative phenotypes are often fraught with phenotypic heterogeneity resulting in a loss of power to identify genetic associations. There are several benefits from using an endophenotype: (i) the trait is quantitative which should provide more power over a dichotomous trait, (ii) both unaffected and affected individuals can be included in the analysis which increases the sample size, and (iii) it is an intermediate phenotype which should be closer to the primary cause(s) of the disorder and thus may help to address phenotypic heterogeneity. Endophenotypes for neuropsychiatric conditions can take a variety of forms, including rating scales, biomarkers, and neuroanatomical features. Our group has previously used and discussed the rationale for selecting Conners’ Continuous Performance test (CPT) measures as
