Friday, July 07, 2006

Is homosexuality due to genetic conflict or cooperation in the family?

I have been thinking a bit about these observations that indicate a markedly higher rate of homosexuality in boys with older brothers. I wonder if this could be seen as part of a genetic strategy on the part of males to remove their younger brothers from the breeding pool and perhaps enhance the suppport they receive in their own breeding efforts.

If male foetuses were capable of influencing intrauterine conditions for subsequent male babies in a way that tended to cause homosexual orientation, via effects on hormones and male foetal development, this could be a good genetic strategy for them in a small society. If early humans lived in small bands of a couple of hundred people [as is often suggested], the advantage of removing one's brothers from the potential breeding pool, while quite possibly having them still in the band and able to contribute to the provisioning and care of one's own young, could have been considerable.

Analogies with queen bees and their infertile worker sisters come to mind.

ADDENDUM: On further reflection, I wonder if a more likely scenario is that of parental effects. If a pregnant woman's body has a tendency to reject what it perceives as immunologically foreign in a male foetus, namely the masculine elements, this will tend to put pressure on male foetuses. Those which already have a low tendency to masculinise, for genetic or environmental reasons, may be tipped over the balance into homosexual orientation. Since an individual that is going to be less masculine in any case will probably not compete as well in the struggle for desirable mates, it might be desirable from the mother's point of view for her "to cut her losses" genetically and allow or encourage the production of a homosexual son. As noted above, in the kind of small band in which humans probably lived in prehistoric times, it might be desirable for her more masculine sons to have less competition and more help from their homosexual brothers. Having later sons homosexual is especially likely to be acceptable, because the mother already has normal sons in the population.

The argument against homosexual orientation being simply a sporadic developmental anomaly is that it seems to occur at a rather high frequency. It seems possible that its occurrence is under some kind of control. Appropriate allocation of resources (including sexual orientation) to males is particularly important because their breeding results - and hence the mother's success in passing on her genes to the next generation via her sons - are going to be much more variable than for daughters. This is because an individual male can potentially father many more children than an individual female can bear.

FURTHER ADDENDUM: Mother birds vary the amount of the sex hormone precursor, androstenedione, that they provide to their young depending on the life history typical of the species, according to this report:

"In birds, mothers pass androgens [male hormones] to their eggs, and these hormones have been shown to influence the development and behavior of nestlings."

A more recent consideration is here.



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