I am sceptical to it being an evolutionary trait.
Firstly, evolution is not global over a species, the increase in diagnosed autism is present in both the US, Europe, and Asia. They can be considered three populations, and individuals usually breed within their population, limiting evolutionary traits to said population.
Secondly, I have a hard time seeing that autism would give you more offspring. Are people with autsim having more children? If not, they are not really more succesful in this environment, and thus there is no natural selection.
Third, how would an autistic species survive? If autism turns out to be a winnig trait, and speciation occurs, that is, the new species can no longer breed fertile with "old" humans, how is an autistic species going to survive? Old humans have no longer no incentive to help the autists, since they can no longer interbreed, and many with autism cannot function on their own.
Fourth, the rise is far too quick with regards to how long a generation is for humans. We suffer no environmental stress that would increase the evolutionary pace, and even then it would take many, many generations for even a small trait to turn up, let alone a different brain structure.
I would lean towards that the explanation for the rise is a wide definitions of what autism is, and sometimes over-zelous doctors putting diagnoses where they may not be needed.
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