The basis for most of the severe mental disorders is in your genes and the early development of the brain. But we will not be able to find THE cause of these disorders. Why not?
The probability of having autism is greater for an individual if it runs in their family. This suggests hereditary factors. But what do you actually inherit? What you inherit is a predisposition for a specific development of your brain. This, together with what you experience while growing up, can lead to the development of a mild to severe form of autism.