What can ADHD without comorbidity teach us about comorbidity?

► In a clinically-referred ADHD cohort, age range was younger in the ADHD without comorbidity and older in ADHD + internalizing disorders. ► ADHD with internalizing disorder has a significantly greater association with paternal psychiatric conditions. ► After matching by age, gender, IQ and SES, ADHD with externalizing disorders had significantly higher total ADHD, hyperactivity/impulsivity score and single item score of difficulty awaiting turn than ADHD without comorbidity and ADHD with internalizing disorders.