Although ADHD is a dichotomous diagnostic category in the Diagnostic and Statistical Manual of Mental Disorders (the disorder is considered present or absent), twin studies have focused on the heritability of continuously distributed traits that contribute to the diagnosis (see Wood et al.3 for a summary of measurements used). This dimensional model approach assumes that a diagnosis of ADHD is given when individuals pass a clinically relevant threshold on these behaviors. Under this model, twin studies on such behaviors as “inattentiveness” and “overactivity” in the general population can be generalized to the clinical diagnosis of the disorder. This approach is supported by research studies that have found similar heritability estimates for dichotomous (reflecting the presence or absence of the disorder) and dimensional measurements of ADHD. In addition, using a regression-based method (extreme group),16 it is possible to compare heritability estimates in different parts of the distribution of the trait. ADHD behaviors yield similar heritability estimates regardless of where the threshold is placed to define the two groups.17
