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?
What causes your depression: serotonin or what you experience? The answer: the question is invalid! Both play a role, and are interconnected. You cannot say that one of these is the cause.
A lot of people think antidepressants are worthless. Is psychotherapy better than an antidepressant? There is no evidence for this. It seems to me that antidepressants suffer a disadvantage in our perception as compared to psychotherapy, in that their mechanism of action is quite invisible and not instinctively comprehensible.
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.