Causes of Autism

In a recent study of over 2 million children who were closely related researchers discovered that the risk of autism was about 50% related to genetics. For example identical twins had a 150 time increased risk, while cousins had a two times increase. Their conclusion was that 1/2 the cause is a genetic pre-disposition, while unknown environmental factors contribute to the other 50%

The question of course is whether these environmental factors are behavioral or related to exposures to chemicals, etc.