The existence of mirror neurons is a theory. Also, you're confusing Autism with all Autism Spectrum Disorders as a whole; Asperger's Syndrome, for example.

Autism != Asperger's Syndrome

I am not confusing them so much as avoiding trying to write a textbook on autism when addressing the original issue. I was sloppy. I may not have succeeded at my goal of not writing too much.

The existence of mirror neurons is not just a theory. A mirror neuron is (by definition) a neuron that fires both when you do something and someone else has done something. Those are observed. It is a theory that it causes autism, but not a theory that they fire less in autistic children, or that these areas of the brain are almost precisely the same places where autistic spectrum people have thinner brains then controls.

than not then