From a brief report in New Scientist, 1 July:
IF YOU’RE male, having more older brothers makes it more likely you’ll be gay. .. Each older brother increases your chance of being homosexual by about 30 per cent…This statistic has been dogged by the suggestion that it’s due to social rather than biological factors. Sceptics propose that rough-and-tumble play between brothers may lead the younger boys to become gay.
Now Anthony Bogaert, at Brock University in St Catharines in Canada, has largely ruled that out. He looked at a total of 944 homosexual and heterosexual men, including one group raised with non-biological male siblings. Bogaert reasoned that if simply being brought up with a lot of older brothers produced the effect, it shouldn’t matter if they had the same mother or not. In fact, it did matter: only the number of biological older brothers was linked to sexual orientation. This was true even when the biological older brothers lived separately. “It’s pretty strong in suggesting a prenatal origin,” he says.
The older brother observation is supported by several references. Bogaert’s study needs to be replicated, and qualified comments on his methods would be welcome. Meanwhile let’s assume he’s right. The great thing about evolutionary psychology is that anyone can play, so let’s speculate why.
One odd feature of the culture wars is that on homosexuality the usual bias is reversed: homophobes think homosexuality is learnt, liberals that it’s innate. But innate can mean several things.
There are lots of problems with a straight genetic explanation. At first sight a gene for homosexuality looks as suicidal as one for haemophilia, and would be quickly selected out. Hereditary genetic diseases have mercifully low incidences: 1 in 10,000 of the US population suffer from cystic fibrosis, the commonest. (Down’s syndrome has a higher incidence, 1 in 800 births, but it comes from a copying mistake not inheritance). Even cystic fibrosis probably only survives at this incidence because one copy of the gene (carried by 5% of the US population) conveys what used to be a major benefit: protection against typhoid. Better known is the protection against malaria offered by one copy of the gene for sickle-cell anaemia, carried by 1 in 12 African-Americans,
Male homosexuality is certainly much more prevalent than these; surveys vary but 5% is a fair guess. This looks much too high for a deleterious gene. So perhaps it’s an advantageous gene, good for society by providing psychological and religious insights, bonding, and hairdressing? The problem here is that such benefits accrue to the group, and while group selection is theoretically possible, most biologists think it’s unlikely, as it would in practice always be crowded out by individual selection. The genetic theory also has problems dealing with the birth-order effect.
Bogaert himself is careful to assert a prenatal effect not a genetic one. In fact he thinks it’s epigenetic:
A theory of male homosexuality consistent with the present findings is a maternal immune response to succeeding male pregnancies….No direct support exists for a maternal immune response that underlies the fraternal birth-order effect, but various lines of evidence exist in this theory’s favor.
Let’s run with this. In whose genetic interest would be this response? Not the mother’s or the father’s – their Darwinian interest is to have breeding sons. But it could well pay an elder son to have a younger brother who would not be a sexual competitor for females but remain a natural ally in his own reproductive strategy. An ally in what, for example? Hunting, for personal survival and buying sexual favours from women with meat; fighting, to dominate and intimidate rival males; and rape. So Bogaert’s maternal immune response could be the result of a sort of vaccine produced in the womb by male fetuses, and not accidentally.
I have no qualifications on this stuff – don’t blame Bogaert for my armchair theory. But if you don’t think such manipulation is possible, think of the tiny parasite toxoplasma gondii, that makes a rat lose its fear of cats so that it can migrate to its preferred host. And think about this, from Frank Sulloway, the expert on the social effects of birth order:
In the Taiwanese aphid (Pseudoregma alexanderi), offspring exist in two forms, one of which is a soldier caste that defends the other caste from attack.
So much for the stereotype of homosexual as wimp.