We could also conclude, using this method, that the Amish do not have autistic kids because they don't shave their beards. The correlation is equally valid and the conclusion equally invalid. I can use this logic to prove that wearing socks causes head trauma (every person surveyed in the trauma ward was wearing socks at the time of event that caused the injury).

Maybe there is something more to the Amish story. Would the Amish label a kid autistic? The diagnostic definition of autistic is so broad today that one has to ask if the growth in autism is largely due to this over-broad definition and the actions taken after the diagnosis. Like giving autistic kids drugs to dull their senses and keep them calm. Or behavioral factors like teaching them to be "disabled" and dependent (I've seen this). A parent of an autistic kid told me that the "special school" his kid was attending was teaching him to be autistic. Taking the kid out of that school and putting him in "normal" school mitigated many of the behavioral symptoms. Perhaps the Amish don't have autistic kids because they do not acknowledge autism, and are instead prone to address the needs of each child individually? Perhaps instead of declaring the kid disabled they instead focus on learning to function and thrive with the gifts given. Perhaps the difference is culture, not chemicals.