Autism is a sociological phenomenon caused by a little bit of all the speculated reasons -- but it is more of a sociological problem than it is a personal psychological or neurophysical problem, as there is nothing truly dysfunctional in the mind of the autistic - rather it is the opposite: there is nothing wrong with them, or rather, their environment, and the emotions and behavior we deem "normal" have nothing to develop out of. This is because our emotions and behavior only exist dependent on external factors - as in, they somehow pertain to survival.
In our modern society, survival is truly not that large of a concern, and therefore any behaviors which we deem "normal" related to survival could only be perpetuated by causes we inflict upon ourselves.

Individuals who are autistic "missed out" on being exposed to those survival-rooted behaviors during a critical stage in childhood development; they weren't exposed to those "normal behaviors" as they'd develop naturally, and this cascaded into an inability to ever be able to learn them.