I guess it could turn out that defining genetic differences between NT and autistic will be a complicated matter that will undermine the idea of neat categories. But isn't it already true that informed people do not believe in a neat, clean line of demarcation between autistic and NT, hence the idea of the broader autistic phenotype? I myself believe being an aspie is a matter of what one identifies onself as, in addition to having the autisitc neurotype. I know people who are SO aspie in abilties and neurology, but who are totally NT in their aspirations and values. Of course, they have problems with depression and low self-esteem, but who's fault is that?

I think it could also turn out that the gentic differences between NT and autistic could be surprisingly simple. Just consider the gentic switch that sets human foetuses on the path to becoming either male or female in body plan. Just one little gene (SRY), on one very undersized chromosome (the Y chromosome) triggers huge and complex differences in body form in members of the same biological species, and it does this very reliably (hermaphrodites are rare, and a result of chromosomal duplication rather than a genetic glitch). Sexual dimporphism is a marvel, when you think about it. And as autism is probably biologically similar to a sex difference (the extreme male brain etc), it seems quite possible to me that autism could be the result of a number of different versions of one simple genetic switch.