One of the favorite arguments against homosexuality is that it couldn't survive evolutionary selection, since only male-female couples can reproduce naturally.
It has previously been shown (2004), by one of the same researchers involved with a follow-on study, that females in the maternal line of male homosexuals were more fertile than average. This suggests that such females have some genetic characteristics that at the same time help them have more offspring and also to have more male offspring with homosexual inclinations than the overall average. This should be enough to allow homosexual males to stay in the population at some level, even if they never actually have children of their own.
Of course, questions still remained. What evolutionary model of this situation would actually show statistically that homosexual males would continue to persist in the population? And what characteristics of certain females promote both higher fertility and higher proportion of homosexual male offspring?
There are now some answers:
Male Homosexuality Can Be Explained Through A Specific Model Of Darwinian Evolution, Study Shows (6/17/08)
An Italian research team, consisting of Andrea Camperio Ciani and Giovanni Zanzotto at the University of Padova and Paolo Cermelli at the University of Torino, found that the evolutionary origin and maintenance of male homosexuality in human populations could be explained by a model based around the idea of sexually antagonistic selection, in which genetic factors spread in the population by giving a reproductive advantage to one sex while disadvantaging the other.
Male homosexuality is thought to be influenced by psycho-social factors, as well as having a genetic component. This is suggested by the high concordance of sexual orientation in identical twins and the fact that homosexuality is more common in males belonging to the maternal line of male homosexuals. These effects have not been shown for female homosexuality, indicating that these two phenomena may have very different origins and dynamics.
Male homosexuality is difficult to explain under Darwinian evolutionary models, because carriers of genes predisposing towards male homosexuality would be likely to reproduce less than average, suggesting that alleles influencing homosexuality should progressively disappear from a population. This changed when previous work by Camperio Ciani and collaborators, published in 2004, showed that females in the maternal line of male homosexuals were more fertile than average.
It was necessary to consider specific models of traits and genetic inheritance in order to eliminate any (and possibly all) that were inconsistent with existing data:
Challenged by all these empirical data, the authors of the new study considered a range of different hypotheses for the genetic diffusion of male homosexuality. These included: the genetic maternal effects on sons, the heterozygote advantage (as is found in malaria resistance), and "sexually antagonistic selection." The latter is a particular aspect of Darwinian evolution, in which genetic factors spread in the population by giving a reproductive advantage to one sex while disadvantaging the other. ...
To discover and clarify the dynamics of the genetic factors for homosexuality, the researchers had to screen a large set of models and exclude them one by one. They concluded that the only possible model was that of sexually antagonistic selection. The other models did not fit the empirical data, either implying that the alleles would become extinct too easily or invade the population, or failing to describe the distribution patterns of male homosexuality and female fecundity observed in the families of homosexuals. Only the model of sexually antagonistic selection involving at least two genes -- at least one of which must be on the X chromosome (inherited in males only through their mother) -- accounted for all the known data.
The results of this model show the interaction of male homosexuality with increased female fecundity within human populations, in a complex dynamic, resulting in the maintenance of male homosexuality at stable and relatively low frequencies, and highlighting the effects of heredity through the maternal line.
It makes a lot of sense, when you stop to think about it for even a moment, that a genetic factor favoring male homosexuality should be on a chromosome (X) that females have one more copy of than males. In that way, the factor can potentially be inherited, regardless of what her mate's genetics are. It also helps if the genes, whatever they are, tend to make female offspring more fertile, even if the male offspring are less fertile (because of homosexuality).
So what sort of characteristic might it be that favors females by making females more likely to reproduce but males less likely? Another report on this research spells it out:
"Gay Genes" May Be Good for Women (6/18/08)
Camperio Ciani's team suggests that these gay genes may actually increase how attracted both men and women are to men rather than making gay men more "feminine," as some researchers had earlier proposed. Although this is bad for male fertility, it is good for female fertility and allows such genes to survive at low but stable rates in a population, the authors say.
Dean Hamer, a behavioral geneticist at the National Cancer Institute in Bethesda, Maryland, who pioneered the search for gay genes, calls the study "an elegant mathematical analysis." He adds that the team has come up with a "simple solution" to the Darwinian paradox posed by homosexuality: "What is a 'gay gene' in a man is a 'superstraight gene' in a woman," he says.
With the evolutionary questions about male homosexuality out of the way, it's interesting to note that there is also recent evidence about the physiological nature of it. That is, there are physical differences between the brains of homosexual and non-homosexual individuals, both male and females. And further, the brains of homosexual individuals are different in ways that make them more like the brains of non-homosexuals of the opposite sex.
Symmetry Of Homosexual Brain Resembles That Of Opposite Sex (6/17/08)
Swedish researchers have found that some physical attributes of the homosexual brain resemble those found in the opposite sex. ...
Some psychological tests have shown differences between men and women in the extent to which they employ the brain’s hemispheres in verbal tasks. Other research has hinted that homosexuals may exhibit the tendencies of the opposite sex in brain behavior unrelated to sexual activity.
Ivanka Savic and Per Lindström, of the Department of Clinical Neuroscience at the Karolinska Institute in Stockholm, Sweden, now report that the brains of heterosexual men and homosexual women are slightly asymmetric — the right hemisphere is larger than the left — and the brains of gay men and straight women are not.
Positron emission tomography (PET) scans taken by the researchers also show that in connectivity of the amygdala (which is important for emotional learning), lesbians resemble straight men, and gay men resemble straight women.
A couple of remarks about this. First, these results don't have much to do with those of the research discussed earlier, in that they don't indicate how females who tend to have male homosexual children might be more attracted to men. But they aren't inconsistent, either. Second, although there are physiological differences, these could be due to exposure of the fetus to sex hormones in the womb, rather than to genetic factors.
In fact, it is possible that genetic factors aren't involved in male homosexuality per se. It could be that the genetic factors carried by females who tend to have male homosexual children are responsible for the hormonal environment in their wombs that pruduces both homosexuality and the brain differences just noted. But this may be unlikely, as the effect would have to be different, depending on the sex of the child – female children would still get brain characteristics atypical of females, yet the same attraction to males as their mothers have.
Sexuality is complicated.
More news reports about this:
- Gay brains structured like those of the opposite sex (6/16/08)
- Gay men, straight women share brain detail: report (6/16/08)
- Scans see 'gay brain differences' (6/16/08)
- Study Says Brains of Gay Men and Women Are Similar (6/16/08)
- Gay men and straight women have similar brains, study says (6/17/08)
An interesting question that still remains is: what does the evolutionary and physiological evidence say about female homosexuality? Not being lesbian, or even female, I don't have any particularly good insight into this. Just speculating, I would guess that the physiological factors tipping a female towards preferring another female rather than a male as a partner are complicated.
However, it could be as simple as that, as in the research discussed above, homosexual females have, like straight males, neural wiring that results in attraction to females. So that, consequently, there are few natural inhibitions, and some rewards, for females to seek other females for pair bonding.
Furthermore, since females can easily get pregnant with only brief (and usually ready, willing, and able) assistance from males, it would not be difficult for paired females to raise children together. Perhaps even less stress than trying to do the same with a male partner who has a roving eye. The males, for their part, might be just as happy not to assume the burdens of fatherhood, so they can go off hunting (for game, or more females) with their pals.
The evolutionary position of males and females who pair with their own sex just isn't symmetrical. Male couples cannot have children of their own, without the substantial assistance of a female for an extended period of time (especially if you count, as you must, a couple years for breast feeding.) But female couples just don't have such a problem. Life's not fair. This lack of similar evolutionary obstacles suggests a larger tendency towards bisexuality, at least, in females, which seems to be the case.
Update, 7/12/08:
A recent study of twins concluded that homosexuality results from a mixture of genetic and environmental factors: Homosexual Behavior Largely Shaped By Genetics And Random Environmental Factors.
A somewhat earlier study by one of the authors of the twin research found that male homosexuals navigate in virtual reality in a way similar to (straight) females: Gay Men Navigate In A Similar Way To Women, Virtual Reality Researchers Find
Update, 8/20/08:
Here's more recent research from Camperio Ciani and colleagues:
Bisexuality passed on by 'hyper-heterosexuals' (8/15/08)
Tags: homosexuality, evolution
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