A doctor described this to me as follows: ADHD is not an inability to concentrate; rather, it's an inability to suppress the impulse to concentrate on more interesting things. For interesting stuff, there's no impulse to focus on something else -- indeed, the impulse is to block out everything else.
For dull stuff (such as schoolwork), the impulse is always to focus on something more interesting. The impulsive behavior is moderated by the uptake of dopamine into the brain. Stimulants allow this to happen, which is why they are used to treat ADHD.
As it happens, a little bit of ADHD isn't necessarily bad -- it spurs creativity, and is apparently very common among very bright people. But it can get to the point where it's impossible to focus on anything at all -- in which case mediction helps.
It looks like the tendency to hyper-focus on "interesting" stuff, and be easily distracted from "dull" stuff, can be a plus or minus depending on the context, and what the person considers "interesting". If the "interesting" thing that the person hyper-focuses on is a fly buzzing across a busy street, then it's a problem. If the interesting thing is mathematics, physics, music, or art, and he has high IQ, then you have genius
The real debate here, just under the surface,is not whether a disorder "exists" or not. It certainly exists, because it is observed.
The real debate is just another iteration of the endlessly repeated nature vs. nurture debate. Is it a defect present at birth or is it acquired? And that, like all iterations of nature vs. nurture, soon becomes a logical circle. The fact that medication works on something does not tell us whether or not the condition is congenital or acquired by behavior.
If this "ability" of attention is normally formed in early childhood in response to external demands to pay attention to one thing and not another (read: parenting), and if a child were to simply not form this ability due to a lack of externally imposed discipline, then that lack of "ability" could easily be hardwired into the child as the developing brain bypasses that developmental stage and leaves a cognitive gap behind as it moves on to other developments.
This cognitive gap would look, by observing behavior at school age, just like a congenital defect. And medication would work.
The debate is fierce because the possible conclusions are so disparate: either there is a lot of bad parenting in the first 3 or 4 years of life, or we have discovered a condition which long was attributed to bad character.
Either a terrible injustice is being done on this generation of children by their parents, or a terrible injustice was done on earlier generations of children by their parents.