The dimensionality of ADHD in adults has been studied on the basis of self-reports such as Conners' Adult ADHD Rating Scales (CAARS) [21], a popular instrument containing the key domains (inattention, hyperactivity, and impulsivity) and an “overall” index of ADHD symptoms. ADHD self-reports such as the CAARS are considered to be reliable and valid to estimate the symptomatology of the disorder [22–25], and scores covary with keys of ADHD such as boredom and sustained attention [26] and with comorbid problems such as depression, anxiety, and stress [27, 28]. Electrophysiological and MRI studies showed that ADHD self-reports, especially the dimension of inattention, are associated with compromised response monitoring [29] and reduced total brain volume [30]. These findings in particular underscore the neurobiological dimensionality of self-reported ADHD symptoms.