I'm not an expert, but in my many years have noted:

1) I used to visit an educational consultant in wealthy Marin County (to enhance math and writing skills for my boys). He said that most of his business was getting underperforming Marin kids a "diagnosis" so that schools would have to give them special treatment (ie a kind of excuse for their perceived academic shortcomings).

2) K-12 education is notoriously skewed towards "good behavior" and not necessarily true intelligence (which starts being rewarded in late high school, college, or even careers). As such, ADHD etc really is an early handicap.

3) Some posit that ADHD, Bipolarism, Aspergers are extreme expressions of what is in us all (impatience, moodiness, narrow focus). Some, like geniuses, learn to use to their advantage. My late older brother was on Retalin as a kid, but turned out to be one of these geniuses. Poor guy was so misunderstood.

4) As such, perhaps those with excellent behavioral skills are mediocre in creativity. I suspect many geniuses may be in the moderate ADHD, Bipolar, Aspergers (or even schizo) realm while learning how to adjust.

5) You're probably right in implying that these are less "growing problems" than previously undiagnosed things. Our Nation has gone from 5% attending college in 1940 to about 40% currently, so academic maladjustment is showing up whereas previously in history people of all sorts were hidden on farms, not having to worry about educational rubrics.

6) Extreme cases of ADHD, PTSD, Bipolar (plus Schizophrenia and drug/alcohol abuse) may be large part of homeless population, as it has always been in history?

7) Are kids labelled ADHD simply bored in the classroom (or misplaced, fated for non-academic futures such as professional sports?) My experience is that if young, fidgety learners can be labelled student-athletes (in which the subject material comes at them simple and fast), they can actually be great learners. This works especially well with math, and also implies some of our K-12 curriculum is taught too slowly with some lack of relevance, not tailored to impatient/quick-twitch brains.