I think it's a little of everything.
I'm anti Jenny McCarthy "my son had Autism from Mercury poisoning in immunizations but I put him on the gluten-free, casein-free diet and now he's CURED" group. Egad, keep that junk from me.
BUT -
I think some of it is genetics. In our case, my boy's paternal side is loaded with good stuff. Few Bipolar great aunts. Grandmother that has severe Learning Disability (LD)'s. Father that is a drunk/druggie - 40 years old on his 4th DUI, going to jail. Autism - he has a cousin, no speech, tippy toe walker, 18 years old, rages like crazy. Bipolar ALL over the place. My mom - depressive. My dad - whacko. Yeah - it's around.
I think some parents (and, NOT saying anyone here at this site, or picking on any person) have issues with disciplining their children. I run a support site and could tell ya stories, but won't go there. And so, they go to psychiatrist and say "my child is out of control" when the fact of the matter is, you shoulda told him/her no and stuck to it.
I do believe some may be environmental. I have read/heard really interesting stories on the diets (like Jenny's miracle gluten-free, casein-free cure! LMAO) and seen some with exceptional success stories. I mean, I suppose if you live on McDonald's and Burger King, all that processed, fatty garbage, it can't be good for you, right?
And, I believe there's alot more awareness. Autism awareness is huge. Bipolar is controversial still, but hey, it's out there for kids, and it wasn't before. I've still talked to a psychiatrist or two that says ABSOLUTELY NOT until their 20's. Whatever. People are hearing about it, reading about it and seeing it. If I knew when my own son was 3 that his not talking, his obsessive spinning of everything and his lack of eye contact said AUTISM, I'd have been to a neuro way before I ever was. I didn't know. Now people know.
And, finally - if I'd have told my dad to shut up when I was 10, my teeth woulda been scrambled in my mouth, or on the ground, whichever. Can't do that now. Can't even raise a hand. My mom used to make me go get a switch. And, if she didn't like THAT switch I picked, she'd whack me with it, then go get a good one and whack me with that after LMAO! My dad loved the belt. I've been chased with the wooden spoon. Haha! Was it right? I dunno. But, I never, never EVER disrespected or embarrassed my parents. NEVERRRRRRRRRRRRRRRRRRRRRRRRRRRRR!
And now, these kids can do whatever the he$$ they wanna. And, they know it.
Good topic.