Now if the vaccine isn't responsible for the increase in autism, what is? I gather one theory is that the age of childbearing has gone up in recent decades. Another theory is that in California, where autism has experienced a particular increase, more engineers are marrying other engineers, and the left-brain engineering personality type is associated with autism.
One increase for the number of diagnosed cases of autism is just that: more cases are being diagnosed. That is, more children who once were simply labeled "slow" or "difficult" or some other adjective are now diagnosed with autism, even though very little is known about autism itself. My wife teaches special ed at an elementary school, so I hear about this all the time. "Autism" has become a sort of catch-all for kids who have strange issues that do not respond to conventional treatements. That's not to say that there are not some genuine cases of autism, but many things that once were not labeled autism now are. That is one big reason that we see a rise in cases--not because more kids have it than used to, but because we are more liable to diagnose something as "autism" today than we were 20 years ago.