There’s something interesting about the paternal age effect in homosexuality – apparently there isn’t one. It certainly exists for autism, schizophrenia, manic-depression, and mental retardation, which suggests that de novo mutations play an important role in all of them. But homosexuality, although it is obviously a Darwinian mental illness – despite the opinions of the loons who wrote the DSM – does not seem to be mutation-driven. At least not much. It’s a different kind of crazy.
There were already reasons to think this. First, homosexuality is A. common and B. involves disruption of a relatively simple adaptation, one that ought to present a smallish mutational target. It would not be easy for something driven by mutation to become that common (> 1%), when the number of key genes involved in sexual targeting probably is not especially large. Certainly not large compared to the number that influence general intelligence.
Besides that, we (I have a tapeworm) don’t see syndromic forms of homosexuality, ones in which particular mutations cause the phenotype of interest (deafness, for example) but also cause other characteristic changes, such as wide-set eyes and a white streak of hair (Waardenburg syndrome). About 15% of the mutations causing deafness are syndromic. I don’t know the fraction of mutations causing mental retardation that are syndromic, but it must be substantial: that’s why people working with mentally retarded youth talk about FLKs – funny-looking kids. There might be genetic/syndromic kinds of homosexuality, but they must be rare. They might be very informative, however.
By the way, I’m supposed to say ‘intellectual disability”, instead of retardation, according to various morons.