I really think the biggest part of the rise is the increase in diagnosis. Every person on this thread has described autism which I would consider very mild. I don't mean to downplay any struggle these kids/young adults have in life, but it's an entirely different experience than the autistic young man I know well. He requires a caregiver 24/7, throws out a random word or two once every few months but does not communicate in any way verbal or otherwise. He is very violent at times. He requires a male caregiver because he has grown to large for women to handle when he is angered. He is mostly toilet trained. He does love cartoons and plays the piano by ear, but wouldn't be able to obsess or study a subject because he can't use a computer or read.
I know there are similar traits to those discussed in this thread, but this was the face of autism 20 some years ago. People "back then" didn't worry about diagnosing kids who were capable of learning and who were just slower or quirky. (And don't get me wrong, these kids need the services they are getting. Especially with the state of education today, It's become too large of a beast for any child to navigate with developmental delays.) But, now that we have expanded the spectrum to include people with similar characteristics who require assistance to learn is what I think that has made the number of diagnosed cases explode. I don't think there are more diagnosis like the young man I know... but there are a lot more diagnosis of the more "mild" types of cases. (Again, I mean no disrespect to any family or child struggling with autism, I just couldn't think of any other language to use to describe the difference from one end to the other of the broad spectrum that is autism.)
Back to the OP's original thread (sorry for going off-thread for a moment), I never really worried about plastics. My husband does. He especially doesn't like plastics used for re-warming in the microwave, etc. My kids are pretty typical kids (whatever that means

) and they've eaten plenty of foods stored, prepared, re-warmed in plastics. Maybe they just didn't have a predisposition? Or maybe it's not plastic? I would say that plastic (at a glance) makes more sense to me than vaccines and some other theories, just because the "timing" of increased diagnosis combined with increased use of plastics for food storage and cooking, etc, fits a bit better. But, I would have to see some serious science to convince me plastics caused autism.
I think it will take time, but I do think there will be breakthroughs in autism and the reasons behind it. It just will take time because the brain and hormones and genes are endlessly complex.