A Possible Hereditary Model Explaining Homosexuality in Men

Timothy Kincaid

June 18th, 2008

“I say it’s a choice ‘cuz if homoseeeexshality was heriditary then why din’t they all die out? They ain’t got no kids.”

Some people believe that if homosexuality had some genetic contributor then the reduction in average number of children born to gay males would over time cause this gene to become extinct. However a new model challenges this assumption.

From Fox News

In 2004 the researchers studied about 200 Italian families and found that the mothers, maternal aunts and maternal grandmothers of gay men are more fecund, or fruitful, than average.

Recently, they tried to explain their findings with a number of genetic models, and found one that fit the bill.

“This is the first time that a model fits all our empirical data,” said Andrea Camperio-Ciani, an evolutionary psychologist at the University of Padova in Italy who led the study. “These genes work in a sexually antagonistic way — that means that when they’re represented in a female, they increase fecundity, and when they’re represented in a male, they decrease fecundity. It’s a trait that benefits one sex at the cost of the other.”

If the same genes create both homosexuality in men and increased fertility in women, then any losses in offspring that come about from the males would be made up for by the females of the family.

The research results can be found on PLoS ONE

We show that only the two-locus genetic model with at least one locus on the X chromosome, and in which gene expression is sexually antagonistic (increasing female fitness but decreasing male fitness), accounts for all known empirical data. Our results help clarify the basic evolutionary dynamics of male homosexuality, establishing this as a clearly ascertained sexually antagonistic human trait.


June 18th, 2008

I read the whole thing and it was interesting.

Of all the gay gene theories this is the only one that isn’t completely silly. Even so I doubt it will pan out.

Anyway it’s one thing to prove that a gene could theoretically survive through algebra. It’s another to find this gene and prove it exists. We need more meat on this one.

Side Note:
If male homosexuality is the trade off for extra female fertility it DEPENDS on this extra fertility for it’s survival. Family size is now based on choice, not fecundity. With most women having 2 children that gene wouldn’t last many more generations.

Liz Ditz

June 19th, 2008

Gene transmission doesn’t depend only on a parent-child relationship, as primates live in social groups. A productive question would be: does having a second degree relative who is homosexual, and therefore non-reproductive, increase an individual’s reproductive success? And if so, how?

Consider this scenario for early human societies: having a non-reproductive uncle or aunt, who continues to live with, and contribute economically to, his or her sibling’s family, means that there are more resources (food and child care) for the sibling’s children, thus giving the sibling’s children a reproductive advantage.


June 19th, 2008

Liz Ditz

I understand the principal of kinship but I don’t buy it for a lot of reasons. I should note that Simon Le Vay doesn’t buy this one either.

Consider this.

In every society there at least as many young, infertile women as there are gay men. Large numbers of these women devote their energy towards caring for the small children of their relatives. So it’s a gene? Mathematically speaking you could prove that this trait could be spread through genetics. But it’s not. Nearly all infertility in young women is the result of illness, and most of this is caused by STDs. Scientists would laugh at anyone who suggested otherwise. This version of the gay gene theory is just as silly.

Richard W. Fitch

June 19th, 2008

Even if this model and its data provide a clue for understanding male homosexuality, where do lesbian women fit into the equation?


June 19th, 2008

I’ve always found the, “if it’s genetic, they’ll all die out” explanation against genetic origins of homosexuality particularly elementary. A basic understanding of recessive genes answers this pretty easily.


June 19th, 2008

This theory is merely a biological version of anthropological models that have been a while. All these models say “evolution favors more than individuals who have lots of offspring, it also favors groups that produce many offspring and the healthier the better.” Lesbians and gays can certainly offer advantages to other children in their social group which tends to also be a similar gene pool.

Besides the fact that plenty of gays and lesbians do have children. The norm used to be, and still is in most of the world, that sex is mandatory for producing children or satisfying your husband and you don’t have to enjoy it or have a physical attraction to your partner. Alan Chambers and all the ex-gays who still appear gay are able to have kids.


June 19th, 2008

You have to remember that the people saying if it were a gene they’d all die off – are the same people (usually) that believe in a talking snake. Science isn’t their best subject.


June 19th, 2008

You guys have to keep one thing in mind. You are right, due to reasons we can’t yet understand SSA might be largely or entirely due to a gene(s).

But so far the evidence doesn’t support that. The latest Swedish twin study suggests that genes play a minor role.

“Overall, the environment shared by twins (including familial and societal attitudes) explained 0-17% of the choice of sexual partner, genetic factors 18-39% and the unique environment 61-66%. The individual’s unique environment includes, for example, circumstances during pregnancy and childbirth, physical and psychological trauma (e.g., accidents, violence, and disease), peer groups, and sexual experiences.”

To the best of my knowledge every twin study since 1991 suggests the same thing. Even Bailey’s 1991 twin study put concordence at just 50% and that was the high water mark. His 2000 study which he felt was much better dropped the concordence rate to 20%.

That doesn’t mean there aren’t dozens of genes that increase or decrease the odds someone will be gay. Those are certainly out there and scientists will eventually find them.

There could even be a rare mutation that makes someone gay all by itself. But rare is the key word.

Emily K

June 19th, 2008

“Environmental factors” doesn’t mean necessarily that psychological trauma is what causes gayness 80% of the time. I never say “Never”, as in psychological trauma “never” causes gays to happen – but I find it to be as rare as genes playing a factor 100%. More likely that factors beyond anyones control, such as hormonal exposure in the womb, play a large part. There are actually fewer genes that play a part in left-handedness than sexual orientation.


June 19th, 2008

Emily K

I never say “Never”, as in psychological trauma “never” causes gays to happen

I highly doubt that significant numbers of men turn gay because of traumatic events. For instance a dad dies in a car wreck or a child goes through a messy divorce. But I wouldn’t count out some sort of general physical trauma at a very early age or in the womb. I don’t mean a konk to the head but possibly something as lame as a flu virus exposure or something like that.

Exposure to meningitis before the age of about 3 can switch kids from right to left handed. The earlier it happens and the worse the infection the more likely the brain will switch hand orientation from right to left. It sounds totally crazy but it happens.

Chapter 2 in this paper talks about that phenomenon.


June 20th, 2008

Even if this model and its data provide a clue for understanding male homosexuality, where do lesbian women fit into the equation?

Well, of course we don’t study lesbians because they’re women and don’t count [end snark]

Seriously, I know I saw research some years ago that argued bisexual female primates were more favored by primate males because their children were more likely to survive into adulthood (because the female lovers would share food with one another’s children during times of stress/famine). Certainly we have seen the same attraction among straight men, and anecdotally it appears that women are far more likely to be bisexual than men. Thus, it is possible that lesbianism, while fulfilling many of the same supportive roles for human society, may be generated by a different mechanism, perhaps a gene for bisexuality that is expressed very strongly and elimates the attraction to men a bisexual woman would typically feel.

In general, though, I think the entire topic of the biological basis of human sexuality should be settled by a cursory examination of fetal development. We know that 1) all humans are 50% male and 50% female; 2) all embryos are basically the same; 3) a delicate dance of genes and hormones must occur in utero for a human baby to form. We also know that the physical process of fetal development can create intersexed people – who are physically, and often genetically, more of a mixture of the two genders than a typical human. Why then would it be shocking that some humans do not follow the strict male/female stereotypes?


June 20th, 2008

I read a study a while back that said that 2nd, 3rd, and 4th sons are much more likely to be gay. The authors of the study concluded that in-utero chemistry changed with multiple male births, effecting subsequent fetuses (although they didn’t have a mechanism).

Just seems that this recent finding could be confounded with that earlier finding – women with more male births could have this “younger-gay-sons” effect AND are more fecund (‘cuz they’ve HAD 2nd, 3rd, & 4th sons who are more likely to be gay).


January 10th, 2009

>> A basic understanding of recessive >>genes answers this pretty easily.
Huh? Recessive genes for blonde hair, etc., are not manifest in every generation, but are transmitted in the usual way. A gene that keeps you from reproducing can’t even make it to the next generation let alone skip to the one after that.


January 22nd, 2009

The fact that this study was conducted by an “evolutionary psychologist” show that the conclusions are biased. “Evolutionary Psychologists” also refute any cognitive basis for moral turpitude beyond social influences. Further, there is no presence of a control group in this study, which negates it’s validity. This study has not been replicated by other scientists, and therefore, is questionable at best.

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