Earlier quoted context omitted.
People with ADHD have reduced brain growth in areas associated with executive control. ADHD is very real. So is the 50%+ of untreated people with ADHD having substance abuse problems. So is the higher suicide rate. So is the reduced rate of high school and college graduation. So are the dramatically increased rates of depression. Then there is the incredibly high rate of co-morbidity of ADHD with other psychological…
Those are interesting facts, but I don't think they prove anything about existence of ADHD as an actual disease. Lower-than-average height is also correlated with many diseases but it isn't a disease on its own.
Now obviously proving the entire causation thing here is pretty hard, but, if someone is missing their eyes, and that person is blind, there might just be a causative relationship happening.
To put it in perspective (http://www.medscape.org/viewarticle/549973), 32% of people with ADHD suffer from depression. 21% bipolar. That isn't some statistical oddity.
(The % is higher in the untreated population, no citation on hand right now sorry)
Over 50% of people with ADHD have substance abuse problems.
Imagine taking a population and saying "oh yeah half those people are alcoholics, but there is no strange underlying cause, nope, we should just let them be."[1]
ADHD isn't the popularized image of "little Johnny can't do his boring old homework that the mean teacher assigned", but rather "Jenny can't make herself go to sleep even though if she is late one more day for work she will be fired and have no money for food or rent."
ADHD is like having your entire sense of self control ripped away from you, no matter how much you want to do something, you cannot force yourself to do it. Pleading and begging doesn't work. Breaking down in tears over one's complete lack of ability to accomplish the simplest of tasks doesn't help any. The neurochemical responsible for self control quite literally isn't there.
[1] Well ignoring that America has basically done that for multiple cultures already...