There are several reasons. I've read and you can google it, that one reason of increased autism rates is because people are waiting to get married, and usually find a mate in their career or field. So a lot of these people are marrying other intelligent people that probably have low level aspergers or other autistic genes which leads to more autistic kids. I have also recently read a study linking low iron to autism in babies, so older people having kids are absorbing less vitamins in general and probably eating poorly (just working people problems) and not getting enough nutrients. Combine smart working older people that have no time to cook and eat healthily and you get a boost in autism. There's also the fact that autism is ridiculously broad nowadays to add in more people now being diagnosed as well.