How do you know whether you got ADHD or not?
Doctors have a set criteria a person must meet to be diagnosed, since there really is no way other to confirm a child with ADHD. Neurologically you could find signs of ADHD, but it isn't as definitive as, say, finding cancer or diabetes in the body. This is the reason scientists, doctors, and psychiatrists argue to whether or not ADHD actually exists and to what extent it does.
There are parts of the brain, however, that do not function properly when you do have actually ADHD. But typically, psychiatrists will diagnose a person without running a CAT scan. Well, depending on where you live, what you can afford, who you go to, etc. This is why probably it is over-diagnosed. But just because a child can't pay attention and or is hyperactive doesn't mean they have ADHD. :s