I think it's over-diagnosed, but I also think that it's becoming more prevalent at the same time. I think it's a manifestation of maladaptive traits spreading through the gene pool, traits that would have been selected against in previous centuries. Our easy, post-industrial living has removed all of the harsh Darwinian selection pressures that made is strong of body and mind.

I think that each generation from here on out will be more autistic, more mentality ill, have lower IQ and be more physically frail. Eventually we won't have enough smart and sane people to keep the lights on and food in the grocery stores. Then society will break down, the harsh selection pressures will return, the weak will die, and strong will breed and over the ensuing centuries re-populate the genome with pro-adaptive traits again.