The probands were required to have ADHD according to the standards of the current diagnostic and statistical manual of mental health (DSM-IV-TR) and had a semi-structured diagnostic interview based on KSADS-PL or Kinder-DIPS, the Child Behavior Checklist, the Conners parent and teacher Scales or the German Teachers Report on children aged 6–18 years. Probands also had an intelligence quotient above 70, a birth weight >2,000 g, no major medical events during their mother’s pregnancy and no drug abuse in their mother during pregnancy. They were also free of single-gene disorders known to be associated with ADHD and free of neurological disease and damage, and they did not meet the criteria for autism, Asperger’s syndrome, schizophrenia, bipolar disorder, primary major depressive episode, anxiety disorder or Tourette’s Syndrome.
