We seem to be closing in on the underlying etiology of homsexuality (excerpt below via Marginal Revolution) as a consequence of polygenic kin-selection.
"Why are there homosexuals? According to Darwinian thinking, a genetic trait that reduces the reproductive success cannot endure in the long run.Interesting that the phenotype of an individual with a reasonable smattering of 'gay genes' seems a lot more prosocial than the highly hetero/macho type. Perhaps the process of 'self-domestication' hypothesised to have coevolved - ie to have been selected for - during the rise of civilization has inadvertently promoted homosexuality as a side-effect?
"The answer sounds crazy: blood relatives of gays and lesbians have more offspring. ...
"That homosexuality has a genetic basis is evidenced by comparative studies. A study by US-researchers J. Michael Bailey and Richard Pillard reached a clear conclusion: With identical twin brothers of homosexuals, the probability that they are gay too amounts to 52 percent, with fraternal twins it is 22, with adopted siblings 11 percent.
"So can we estimate the proportion of genetic influence on homosexuality? Only to a degree. The estimates range from 31-74 percent heritability in men and 27-76 percent heritability in women. The interpretation of these statistics is made difficult by the fact that no precise figures on the incidence of homosexuality exist, because it is difficult to define who actually is homosexual. ...
"Miller has proposed an alternative explanation quite a few years ago. Genes always exists as doubles on chromosomes, in the form of two alleles. Genetic factors that promote homosexuality can survive in the gene pool if they mostly occur in a heterozygous (coupled with other alleles) form and increase the reproductive success of their carriers in this combination. Only in the rare cases where the inheritance is homozygous – both alleles are identical - homosexuality emerges and reduces fitness.
"A man who carries a small dose of gay genes in his genome would, according to the theory, improve his success in the heterosexual mating game. That “certain something” that heightens sex appeal probably consist exactly of those essentials which make homosexuals different from heterosexuals in the first place.
"According to his theory, the alleged "gay genes" equip men who carry the heterozygous disposition with an above-average degree of feminine traits such as sensitivity, gentleness and friendliness. Gay genes therefore form a natural antidote against "hypermasculine" genes that turn men into rough machos. They would promote properties that appeal to women and indicate a good suitability as a father and significant other.
"A lesbian disposition lends women reversed traits that helps their reproductive success. Surveys have already shown that psychologically "masculine" women have more sex contacts.
"Imagine, for example, there were five genes, each of which occurs in duplicate and increases the probability of homosexuality, Miller speculates. Only if a man had all five alleles in duplicate, he would be gay. "That would be an event that occurs with a probability of 1 to 32, meaning in 3 percent of all men." Such a system would already be evolutionary stable if a hint of homosexual disposition would increase the genetic fitness of heterosexuals by only 2 percent.
"What hitherto was pure academic speculation, a team led by epidemiologists Brendan Zietsch from Brisbane in Australia has empirically underpinned with a study of 5000 twin siblings. Metrosexuals, who in their appearance and lifestyle mix male and female characteristics, are the genetic proxies of homosexuals.
"The male and female subjects provided information about their personality traits, their sexual orientation and their total number of sexual partners. 2.2 percent of men and 0.6 percent of women admitted to having a purely gay or lesbian gender identity. There were also 13 percent male and 11 percent female "nonheterosexuals” who reported dating with both sexes.
"Crucial point: Both the siblings of homosexuals and those of nonheterosexuals possessed remarkably many personality traits of the opposite sex. And they also had a greater number of sexual partners than the siblings of heterosexuals. In the evolutionary past, before the invention of the pill and family planning, they should have had a particularly big reproductive success.
"The androgynous personality traits and above-average rates of sexual contacts which characterized these men and women were, according to the researchers' calculations, primarily due to genetic factors and not to environmental influences.
"The genetic vacancy which is caused by the reduced reproductive success of homosexuals is probably offset by the increased rate of reproduction achieved by their blood relatives. This, incidentally, also explains a puzzling fact which scientists previously could not figure out: Homosexuals have a larger than average number of relatives. This was first demonstrated for the maternal side, but is also true, according to the latest data, for the paternal side, perhaps even stronger.
"According to the results of psychologist Andrea Camperio Ciani from the University of Padua, not only the siblings, but also the mothers and aunts of homosexuals are offsetting their "reproductive shortcomings". They not only had a larger than average number of births, but had also been affected particularly rarely by miscarriages and infections. Maybe they are blessed with genes that produce a particularly strong "love of men".
"This would be conducive to their genetic fitness, as it would encourage them to have more children. With their sons, this aptitude could trigger an outright homosexual orientation. But even if those were to become reproductive “underachievers”, that could still be evolutionarily adaptive for mothers: If the same genes would procure them - and their daughters - a larger swarm of children.
"The fact that there are gene variants that provide a fitness benefit if they are "heterozygous" and therefore occur only in one edition of the genetic double set has long since been known in biology. Homozygous carriers on the other hand, who inherit the gene from both parents, are exposed to the fitness reducing effect of this dual system. The best-known example of the so-called heterozygous advantage is the gene for sickle cell anemia. ... "
Greg Cochran has noted (in the context of his rival 'gay germ hypothesis') that "It doesn’t exist in most hunter-gatherers: you have to explain what it is you’re even talking about when you ask them."
The 52% concordance on homosexuality for identical twins (rather than c. 100%) indicates that some kind of environmental effect is also present. A study reported in The Times today suggests this might be due to variant hormonal influences within the womb.
"It seemed like the sexuality differences were asserting themselves long before puberty. Dr Rieger said that this gave useful information about the development of sexual identity.The study, Gender Nonconformity of Identical Twins With Discordant Sexual Orientations: Evidence From Childhood Photographs, is unfortunately gated. There's a summary (Daily Mail) here.
“What we can do is rule out a few things now. A lot of people jump to the conclusion it must be genetics.” Past research has indeed shown there is a genetic component to sexuality but also that that is not the whole story. Given these twins shared the same genetics, it can’t be that in this case. “This shows there is something early on, in the early environment, that has nothing to do with genes but can still have a tremendous effect on sexual orientation.”
Insofar as it is possible to rule out parenting, this research did so — all the twins shared the same home. Dr Rieger thinks the most likely explanation then is something happening before birth.
“Prenatal hormones are the number one candidate,” he said. “Our theory is that even though twins are identical, what happens in the womb can be quite different. They can have different nutrition, different levels of hormones.”