Well, the diagnosis are definitely expanding. It's a spectrum diagnosis, it accounts for so many more people. Though now asperger's is being removed from the dsm-iv so . . . not sure what additional impact that will bring. But to give an example: my friend's son, had a brain tumor that once removed caused traumatic brain injury. He presents behaviors currently with asperger's and conduct disorder. He lacks the sophisticated language and social skills. Since he presents symptoms similar to that and the treatment course necessary is applied behavior analysis - the ASD diagnosis gets tacked on by his psychologist so the family can apply for those services.
Personally, I think, like many other diagnosis I think it will be part genetic/part environmental. Do you have a predisposition to it? Are you more affected by environmental factors? Whoever breaks this code will be a genius. Ranks up there with a cure for alzheimers.