Hi Beti....

You ask an interesting question and it's one that many scientists are still looking for answers to.  One study in England had linked autism to the the MMR vaccination.  It's was the combination of MMR vaccination that caused autism the researcher said.  And when MMR was administered as 3 individual doses at some time spaced apart, there was no case of autism reported in the years the research did the study.

There was another study in one of the northern European countries (can't remember if it's Finland or Iceland) that linked drinking cow milk to ADHD.

Many other studies and observations have looked at the increased vaccination that are given to children, increased consumption of processed food and contamination food, pregnant woman or young children exposing to asbestos paint, and genetics as possible causes of autism and Asperger's syndrome.  

I don't know if an actual cause can be determined for why there is an increase cases of autism and Asperger's syndrome - it probably can only be said that it's biological.

Personally, I think that the increased diagnosis of autism and Asperger's syndrome has alot to do with "convenient label" for everyone.  When you have an "odd" child whose behavior can be embarrassing in the public, it's so convenient to be able to say "oh he's autistic" or "he's has AHDH" or "he has Asperger's syndrome".  I used to do therapy with autistic children and many times when I see a normal preschool misbehaving child in the mall and the parent turn around and tells me that his child is autistic, I would think to myself, you don't know what "autism" is.  My opinion, increased of diagnosis of autism and Asperger's syndrome - a convenient label.

Hope this helps!

Vicki