Thursday, February 26, 2009

More on the origins of male homosexuality

Are there other theories on the origins of male homosexuality? Yes, quite a few. These theories generally fall into two categories: those that explain the relatively small variance due to genetic factors (30-45%) and those that explain the larger variance due to some environmental factor. Conceivably, we could be looking at an interaction between the two, i.e., an environmental factor (a pathogen, prenatal trauma, or perhaps environmental estrogens) interacting with a genetic predisposition (partly feminized male brains, as postulated by Ed Miller).

This week, we will look at another theory. Like Miller, Zietsch et al. (2008) postulate a genetic predisposition maintained by a balanced polymorphism. Unlike Miller, they believe this predisposition somehow increases mating success, rather than offspring survival.

Using twin data, they found that heterosexuals tend to have more sexual partners over a lifetime if they have a homosexual twin than if they have a heterosexual twin. This increased mating success, however, was significant only when the heterosexual was female. The authors attribute these findings to a genetically influenced behavioral trait.

Our hypothesis is that a number of pleiotropic (more than one effect) genes predispose to homosexuality but also contribute to reproductive fitness in heterosexuals. In the case of males, there are a number of alleles that promote femininity: if only a few of these alleles are inherited, reproductive success is enhanced via increased levels of attractive but typically feminine traits such as kindness, sensitivity, empathy, and tenderness. However, if a large number of alleles are inherited, even the feminine characteristic of attraction to males is produced. In females, the converse explanation could be used—a low dose could lead to advantageous typically masculine characteristics such as sexual assertiveness or competitiveness, and a large dose could further lead to attraction to females. (Zietsch et al., 2008)

The authors attribute this hypothesis to Ed Miller, although Miller’s theory has more to do with increased offspring survival (due to increased paternal investment) than to increased success on the mate market.

Does this hypothesis explain the authors' findings? Does it predict that hetero women with gay or lesbian siblings have more lifetime sexual partners? No. First, it doesn’t explain why such an effect is confined to female heterosexuals. Nor is it clear why this hypothesis should account equally well for hetero women with gay brothers and for hetero women with lesbian sisters. The authors seem to be arguing that these women have more lifetime partners than do other women because they’re more feminine in some cases and more masculine in others.

Hmm … Could we be looking at a shared family environment effect? Parental attitudes toward homosexuality in offspring tend to correlate with parental attitudes toward promiscuity in offspring, especially in daughters. This correlation is especially high if we compare religious families with non-religious ones. If a mother and father accept expression of homosexuality in their children, they would probably also accept their daughters having more sexual partners. On these grounds alone, a gay or lesbian child is likelier to have a sister who has more sexual partners on average. But this correlation would not be a cause-and-effect one. It would flow from a common cause: parental permissiveness.

One could shoot back: “Homosexuals are not made by permissive parents. They’re born that way.” True, but the participants in this study were asked to self-identify as heterosexual or homosexual (on a scale of 1 to 6). Even if family environment doesn’t determine latent sexual orientation, this factor would probably influence one’s willingness to affirm it, both to oneself and to others.


Zietsch, B.P., Morley, K.I., Shekar, S.N., Verweij, J.H., Keller, M.C., Macgregor, S. Wright, M.J., Bailey, J.M., & Martin, N.G. (2008). Genetic factors predisposing to homosexuality may increase mating success in heterosexuals. Evolution & Human Behavior, 29, 424-433.


Tod said...

Zietsch et al.(2008)

Are they taking this into account? Bailey & Pillard, (1991) According to their data, 52% (29/56) of monozygotic cotwins, 22% (12/54) of dizygotic cotwins, and 11% (6/57) of adoptive brothers were homosexual. Heritabilities of homosexuality were calculated using these results under a wide range of assumptions of the population base rate and ascertainment bias. Under all conditions considered, heritabilities were substantial (h2 was between .31 and .74 in all cases). However, "the rate of homosexuality among nontwin biological siblings, as reported by probands, 9.2% (13/142), was significantly lower than would be predicted by a simple genetic hypothesis and other published reports."

Bailey and Pillard suggested that this anomaly between the present data and previous reports might be due to the fact that they were looking at nontwin biological siblings of twin probands, whereas other reports had reported the concordance rate between pairs of nontwin siblings where, on the whole, neither sibling was a member of a twin pair. A difference in concordance rates may then occur "if the causes of homosexuality in twins and singletons were different, i.e., if a special twin environment contributes to the development of sexual orientation.

GNXP comment from 'ben g'
From my psychology book: Even identical twins may receve not-so-identical prenatal nurture. Two-thirds of identical twins share the same placenta, and thus a more similar prenatal environment (although one might get a richer blood supply and weigh more at birth). Other identical twins have separate placentas. In this arrangement, one placenta sometimes has a more advantageous placement that provides better nourishment and a better placental barrier against viruses. Early indications are that, compared with same-placenta identical twins, those who develop with separate placentas are somewhat less similar in their psychological traits, such as self-control and social competence (Phelps & others, 1997; Sokol & others, 1995) -- pg. 114 of Psychology (8th Edition) by David C. Myers

And here are the full references:

*J.A. Phelps, J.O. Davis, and K.M. Schwartz, "Nature, Nurture, and Twin Research Strategies," Current Directions in Psychological Science, Vol. 6, No. 5 (October 1997), pp. 117–121.

*D.K. Sokol, C.A. Moore, R.J. Rose, C. J. Williams, T. Reed, and J.C. Christian, "Intrapair Differences in Personality and Cognitive Ability among Young Monozygotic Twins Distinguished by Chorion Type," Behavior Genetics, Vol. 25, No. 5 (September 1995), pp. 457–466.

So the higher rate of homosexuality in DZ twin men may due to them being less protected fron viruses; a bit like the very significantly higher rate of schizophrenia in DZ twins.
Moreover a twin brother's testosterone in the uterus can reduce his female twin´s chances of marrying and having children..

As I understand it in evolutionary time women increased their reproductive fitness by being choosy. How could having more sexual partners pay off for them, surely that would entail being less choosy? .

Multiple Sclerosis and Optic Neuritis About half to three quarters of the people who get idiopathic optic neuritis go on to develop MS.[...] who developed auto antibodies to their myelin as a direct result of inflammatory mediator damage. I don't think there is information available which allows us to come to any conclusion about this. It is even possible that the initial priming of the immune system by direct nerve damage from sinusitis can be maintained and promoted by chronic acinetobacter respiratory infection or vice versa...

The idea (somewhat abstract) of antibodies to damaged nerves is interesting as it has similarities to immune mediated damage to nerves seen in the various experimental autoimmune encephalomyelitis models used to study MS.

Immune system over-reaction (possibly caused by year round excess adipose tissue reducing vitamin D levels)is more likely than the alternatives.

Environmental estrogens,including birth control pills, are mainly modern (apart from the ones made in food by micro-orginisms).
If environmental estrogens are a cause of homosexuality then the proportion of men who are homosexual would be rapidly increasing.

Tod said...

Almost a fifth of men never father a child. Those with Schizoid personality disorder lack the desire for sexual experiences with another person but are less than 1% of the population.

Subtracting those who are infertile, schizophrenic, personality disorder sufferers or homosexuals still leaves about 10%. That is a lot of men whose failure to father children is just as puzzling as homosexuality.

Could these just be the men who are not competing for women. In a time and place where a surplus of women existed with monogamy they would not suffer any reduction to their reproductive fitness.

Peter Frost said...


A lot of homosexuality seems to be due to something happening in utero, so I would expect to see a difference between twins who share the same placenta and twins who don't.

There is one source of environmental estrogens that you overlooked. I'll discuss it in a forthcoming post.

I'm not surprised that 1 in 5 men fail to reproduce. The marriage market is increasingly biased against men, even in comparison to the 1980s. This is partly due to babyboomer males divorcing and remarrying younger females, but there are other factors. More and more single women are single mothers, and such women have less value in the marriage market, particularly for men who wish to have a family of their own.

Tod said...

High mortality among men is hypothesised to be balanced by an a subsequent excess of male children being born.

This would greatly reduce the chance of conditions for the sexual selection of women being sustained for many generations
Fathering sons or daughters may be in men's genes

"In an article published online in the journal Evolutionary Biology, the researcher suggests that men carry a gene that controls their ratio of X to Y sperm, and thus the likelihood of their fathering sons or daughters. Women carry the gene as well and pass it along, but do not express it [...] Gellatly's theory can also explain why an increase in boy births may be seen after a war. Families with more sons will be more likely to have surviving male children, who can pass along their genes, while families with fewer male offspring are less likely to have surviving sons"

Is this support for Milleresque explainations?
Homosexual men who do not reproduce are a high proportion of non reproducing men. Reproductively 'missing' men are compensated for by a subsequent bias for male children. Hence this mechanism could be increasing the number of men as a result of some of the men in the mainly-sons families being homosexual.

Homosexuals tend to have older brothers and therefor tend to be born in families with a lot of sons

Geoff said...

Dean Hammer's book, "The Science of Desire," describes his research in the early 1990's on the potentiality of a "gay gene." His findings strongly suggested that a supposed "gay gene" existed on the X chromosome, implying then that it is passed from the maternal side of an offspring's heredity. We see the same thing with hemophilia and anomolous color vision -- both are passed via the X chromosome, from mother to son. The reason that these characteristics are expressed in males is that males have an X and a Y chromosome. If a characteristic exists on the X chromosome, and not on the Y, it will express itself because there is no other possibility.

The location of this gay gene is at Xq28. Hammer's findings showed that this area of the chromosome predisposes for homosexuality, with a 1 in 100,000 possibility of this happening due to chance.

Furthermore, many studies have shown that instances in which a male offspring is homosexual often results in a higher percentage of homosexual males on the maternal side of the family than the societal statistic of 4%. Conversely, the paternal side of the family tends to be concurrent with societal statistics, further suggesting that this gene is passed from the mother.