There is no theory of the cause of autism which everyone has found convincing. There may be multiple causes. Thus we will review some of the proposed causes.
Most researchers are absolutely convinced that the cause is biological rather than psychological. Bernard Rimland in his book Infantile Autism (1965) cited the following evidence for a biological genesis and against the idea that parents cause their children to be autistic:
Points 4 and 9 are not generally accepted now, perhaps because of the broadening of the condition's definition over time, and perhaps because of additional observation & data collection.
There is still controversy over neurological differences in the brains of autistic people and the rest of the population. However, it does appear from evidence obtained through autopsies, MRI and PET scans that there are subtle cellular changes in the autistic brain. The increased incidence of seizures (20-30% develop seizures in adolescence) also points to neurological differences.
Some specific theories as to the cause of autistic symptoms:
I've also heard "Leaky Gut Syndrome" described as lack of the beneficial bacteria that aids digestion, and that the resulting matter in the bloodstream invokes an unnecessary immune reaction.
A phrase you will sometimes hear is "theory of mind" or "the theory of mind hypothesis". This is not so much a supposed cause of autism as an assertion as to its nature. The basic idea of the hypothesis is that autistic people lack an awareness of other peoples' minds that typical people start developing at a relatively young age, i.e. the autistic person doesn't so readily develop theories about what is going on in other people's minds. A corollary is that an autistic person's awareness of other people's minds is something that is developed intellectually through their own efforts. Furthermore, adherents of this theory suppose that some or all the other typical characteristics of autism stem from this one main deficit. The hypothesis is explained in some books (some have "Theory of Mind" in the title, also Uta Frith has written on it) and simple tests have been devised to test a person's awareness of other minds.