Here's another pointer towards autism being genetic. A recent study found that if one sibling had autism, there is as high as a 19% chance that the other sibling will have autism.
This flies in the face of conventional wisdom and earlier studies that autism in siblings floated under the 10% mark.
Even more striking: a family with two or more autistic children had a one in three chance of the next baby having autism.
This is a striking statistic, especially because autism is a spectrum condition, and it's not completely clear cut whether or not someone is autistic. That implies that there are also a large number of children at the tail end of the spectrum that were not diagnosed with ASD, but might have been on a different day, under different circumstances.
There wasn't a correlation between either IQ or severity of autism. That means that the severity of the first child's autism isn't a predictor for the severity of autism in the second child.\