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Originally posted by SpiritoftheNightSky
The sudden appearance of a useless limb that may even be a disadvantage in survival would not be an advantage either.
Is Autism Genetic?
We know for sure that autism runs in families. Siblings of autistic people are more likely to be autistic, and twins are extremely likely to share autistic traits. This means there is almost certainly a genetic component to autism.
But that doesn't necessarily mean that a single gene is reponsible, or that genetics is the only risk factor for autism. In fact, most researchers believe that a combination of several genetic differences, PLUS some form of environmental "insult," may lead to autism.
Many Autisms, Many Genes?
Even more confusing, many researchers believe that different autistic people can trace their autism to different causes. That's because, as a "spectrum disorder," autism can present with such very different symptoms.
Some people with autism have co-existing conditions such as epilepsy, mental illness (bi-polar disorder is fairly common among autistic people), gastrointestinal issues and sleep disorders. Others have no such conditions, but do present with sensory sensitivities. Yet others are hyperactive.
Dr. Lisa Croen, a researcher at Kaiser Permanente in Oakland, California, has conducted dozens of autism studies. She suggests that people think about autism as "many concentric circles" of symptoms. It's possible that many different genes are implicated in autism -- and that different sets of genes may be implicated for different autistic people.
Originally posted by OnceReturned
The reason that autism is diagnosed more and more is because the autism spectrum is being expanded. This means that the criteria for diagnosis are being adapted to be applicable to more and more people.