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Vaniver comments on [LINK] Loss of local knowledge affecting intellectual trends - Less Wrong
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Sure, it's a toy example without sex. Even so, if the mutation rate were one in a million instead of one in a thousand, you would still have a equilibrium ratio of As to Bs. When you add in sex and precursor genes (that is, you don't have schizophrenia unless you have two copies of an allele, or you need multiple different alleles, etc.) then the selective pressure depends on the prevalence- as the condition gets rare, the selection pressure on the precursors lowers because potential mates are unlikely to have the other half necessary to get the condition. (This gives you another equilibrium ratio of the number of people with the condition.)
The real incidence of schizophrenia is about one in two hundred. That suggests there's something going on beyond mutation- either some of the schizophrenia precursors are positive, or it's caused by a virus, or genes just determine susceptibility, or the heredity has to do with prenatal environments, or so on. (With inclusive 'or's.)
Sure, those are all possibilities. But there are other possibilities also. For example, it could be that the selection pressure was really only high fairly recently. Most people who get schzophrenia get it sometime between around 15 to 35 years of age, and for a large fraction the symptoms come and go. So in many classical societies they would have had time to reproduce. Moreover, in some societies people with symptoms became things like shamans. So the selection pressure would likely have been not nearly as negative in the past, possibly to the point where neutral drift could account for a decent fraction of the alleles.
All of that said, I agree that it is likely that some of the alleles which produce a likelyhood of schizophrenia at some point had positive selection pressures on them for other reasons. But humans are so far from our ancestral environment that alleles which once had positive selection pressure don't necessarily have much or any today.