hi bren,
here in the uk we have had several years of worry as to whether it is 'safe' to give kids a combined mumps, measles and rubella vacination.
andrew wakefield (i believe) did the research and drew the conclusions about a causal link between mmr and autism but he has been roundly discredited and evidence has been uncovered (so i believe) that this individual was financially connected with the company that produced the individual vaccines (rather than the combined mmr jab).
Alternative to MMR jab 'not safe' | Health | The Guardian
i worked for 4 years with 4 young men with asperger's syndrome and autism. i believe you are right with the continuum train of thought as my experience showed me that every behaviour could be seen as an exaggerated or minimised expression of 'normal' behaviour.
case in point: one guy (lets call him kieth) had a fascination with maps and walking. he would sit for hours hand drawing ordinance survey maps that looked astounding. so far so ordinary. but this behaviour linked in his love of exploring. he would walk for hours just to explore.
this ties in a bit with what crash said about the hunter/farmer hypothesis.
keith would get depressed sometimes because he felt he had very little to contribute to society. however, when he saw that his cartographic skills would have been very valuable if he had been born 500 years he would always perk up.
so the 'increase' in percieved 'cases' could be from the fact that the skills that 'touch' of asperger's syndrome can give an individual are in less demand in the modern world and this leaves some folk with fewer roles to occupy in society.
had kieth been born in 1507 he could have been kings' cartographer by now.
as for whats causing it? i was always led to believe they was an association (not causal) with fragile X syndrome so you could take a look at this:
Fragile X Syndrome and Autism | Understanding Fragile X
i don'y buy the anxiety angle one bit however.
the developing human brain is really fragile. there are so many thing that can damage it's development and cause developmental issues in latter life {as i know personally only too well
}.
Heavy metals have been implicated as had viral infections in the stomach or bowel (i forget which).
this is a good place to start looking around (although some of the links seem to crash my computer!)
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hope it is of some use.