I'm not sure ADHD is over diagnosed.
Quite often we work with families with children who are not diagnosed until they are in their teens, and are in trouble with the police or excluded from school, and have developed conduct disorders because their self esteem is so low. It is only after things come to a head that somebody thinks it might be helpful for them to see a paedicatrician and it turns out they have ADHD. They had it all along but because everyone just assumed they were naughty and lazy they just grew up being constantly punished for something they couldn't really help, and they get to the point where they don't bother trying to conform anymore.
In girls, who are often missed, it often comes out when they have mental health issues as teens due to their low self esteem.
Last year I was talking to our local police officer responsible for Youth Offending and Crime prevention. He has a fairly large patch of about a third of Herts. He told me that he has figures showing that over 70% of the children in our local Young Offenders Institute have a diagnosis of ADHD, AS or Tourette's Syndrome, but that the majority of them were only diagnosed once they were in the Youth Offenders Institute.
What a tragedy! Who knows how much better things would have been for those kids if their difficulties had been picked up sooner.
You don't get the BBC talking about how badly these kids have been let down, and how much it is costing our society, do you?
OK rant over!