is highly hereditable.
The incidence of adhd is about 5% of population. Identical twins have a concordance of >80% (if one twin has adhd, it's >80% likely the other will). Full blood siblings have a rate of 35%, parent to child rate is 20%, if both parents are adhd, the child rate is closer to 50%. Faraone sv, biederman j, milberger s. An exploratory study of adhd among