The evidence for a genetic basis for male homosexuality seems to suggest a gene inhereited from the mother. This fits in with the IMHO best current model, where increased amounts of stress hormones from the mother during pregnancy causes some of the sex-linked neural differentiation to change. The gene might simply code for a slightly deficient placental screen of stress hormones. (Still, even in monozygotic twins reared apart, if one is homosexual the probability of the other being it too is just 50% - environmental interactions are clearly important).
If this holds true, then there might be an evolutionary advantage for the *mother's* genes in having homosexual male children. They are perhaps more likely to help offspring of their sisters, hence perpetuating a female-linked gene. This might also explain why female homosexuality seems rarer.
BTW, while I'm still at this subject, I read a fun paper yesterday:
"Sex Differences in Cognition: The Role of Testosterone and Sexual
Orientation" by Nick Neave, Meyrav Menaged and David R. Weightman, Brain and Cognition 41 245-262 (1999).
They tested hetero- and homo-sexual males and females on four
cognitive tasks where there has been evidence for sexual dimorphisms
(males are on average somewhat better at visuo-spatial tasks, while
women at verbal tasks). They found that in the mental rotation task
and verbal association task, homosexual males were closer to female
levels (in verbal association, they were better than females in fact),
while they did as well as males on verbal fluency and the
"water-level" task (draw correct water levels in tilted
glasses). Interestingly, female homosexuals did not differ much from heterosexuals in any of the tasks. There was also a link between the testosterone levels and performance on the visual tasks but not on the verbal ones. The authors suggest that much of the differences on the visual tasks might be due to different levels of activating testosterone. In short, it looks like homosexual males and females are not just of different "brain gender", but that the differences are more complex.
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