I clicked "purely genetic", but it may be more like, "The presence or absence of autism is genetic, but its expression can be affected by (probably prenatal) environmental factors." For example, autism is more common with: Older mothers and fathers, difficult pregnancies, poor prenatal care and nutrition, and illness during pregnancy. Environment can "cause" autism only in the sense that it can tip what would have been an NT with autistic traits into the range of mild PDD-NOS or Asperger's... The only exception I know of is in the case where there is brain injury or some other complicating factor that would cause any child to have overall developmental delay--epilepsy is a common one for autism, but the regular old bump to the head, a case of meningitis, or a short period of anoxia during birth might do it too. In those cases, it could be that the other illness or injury makes it much harder to use an autistic brain, to the point that what might have been a very mildly impaired child ends up with a severe disability. Autism tends to make one sensitive to all sorts of physical, mental, and emotional stressors. Disclaimer: This is my personal theory. I don't have the research to back up anything more than the first two sentences.