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Thursday, June 26, 2008

Wait, Why Are There Gay Men?

posted by on June 26 at 10:40 AM

If being a gay man is an inborn, inherent trait with some genetic basis—as the massive, overwhelming, credible, sound, tenable, probable, corroborating, confirming, affirmative collection of scientific evidence states—why are there gay men at all? It’s a trait that strongly discourages procreative sex. Less sex with women means less babies and therefore less spreading of the gay genes.

These alleles should drop out of the population.


Well, what is known about gay men and their family members?

i. Gay men are everywhere, persisting in every culture and in every human population at more-or-less the same frequency—regardless of how much a culture loves gay men.

ii. The sisters, mothers and aunts of gay men have more babies than those without a gay brother, son or nephew—but only if the relation is through the gay man’s mother.

iii. A gay man’s male relatives are more likely to be gay—but only if related again through the gay man’s mother.

Well, we can come up with a few possible explanations, and see what best fits these observations.
1. Kin Selection.

The idea? A gay man in the family can only help make the heterosexual relatives pop out more kids and have the kids do better after birth. Babysitting, sexual counseling, consoling, food preparation, hunting…. it’s all gotta be good for making kids, right? Even if the gay uncle, brother or son doesn’t have babies himself, all those related babies are so much better off, the gay alleles survive to make future gay men!

Sadly, this appears to not be the explanation.

2. Overdominance.

This is the gay-is-like-sickle-cell-anemia argument. If having two gay alleles makes you gay, and therefore less prone to baby-making, perhaps having one gay allele makes you a better straight man. Therefore, straight men carrying one gay allele and one straight allele do better than their all straight allele counterparts—the gay alleles survive!

3. Maternal effects

In other words, the ever popular mom-made-you-gay theory. Genetically this time. For almost all genes, we get one copy from mom and one copy from dad. For a few of these genes, one of these copies is always turned off from the mom or dad, called genetic imprinting. For example, while dads tend to want the biggest babies possible, mothers tend to prefer surviving childbirth—genetically speaking here. So, the mother’s copies of the genes for growing big tend to be turned off in the baby. Perhaps the same thing is going on for genes that make boys straight.

4. Sexually antagonistic selection.

This is the general blame-women theory. Perhaps the gene for making a gay man (not so good for future reproductive prospects) is super good for straight women (making baby making more likely and easier).

Ok, well which is it? Andrea Camperio Ciani, Paolo Cermelli, Giovanni Zanzotto recently published a possible answer in PlosONE.

Running the available empiric data about gay men through a whole bunch of models of these possibilities, they discovered one combination that best fits reality and a few aren’t really possible at all.

Overdominance seems really unlikely. None of the models including this idea fit the data all that well. Nor did the models based on maternal effects. It appears that mom cannot make you gay. Sorry.

The best fits needed two genetic loci (two genes), with at least one of these loci on the X chromosome. Recall, while women get two X-chromosomes, men only get one. Additionally, at least one of these alleles must be sexually antagonistic—in favor of women reproducing if they have it, even if it makes you gay as a boy.

Or, as the authors of the study stated:

Our analysis allows us to draw several conclusions that clarify the basic evolutionary dynamics of the genetic factors influencing human male homosexuality and the related female fecundity increase, resolving a number of open questions. As a main point, we can exclude the GFMH propagation mechanisms based on overdominance (male heterozygote advantage), because none of the models (1b), (5a), (5b) account satisfactorily for the sexual-orientation asymmetries of requirement (B1). At this level of genetic analysis, we can also exclude maternal effects, including maternal genomic imprinting, as they lead too easily to GFMH extinction or fixation, against requirement (A). Only the hypothesis that the GFMH are characterized by sexually antagonistic selection (i.e. the GFMH favor one sex and disfavor the other) produces viable population genetic models (see the case (4) above) leading to the persistence of the trait at low frequencies and capable of accounting for the related pedigree asymmetries. For this reason, predictions of possible widespread diffusion of male homo- or bisexuality in human populations are not warranted, as stable low levels of this character are actually compatible with a broad range of parameters in population genetic models.

For what could this allele be? Well, an obvious choice is digging dudes. If a woman has an allele that really makes her like guys, she’s more likely to have babies than a woman who has a less guy-loving allele for this gene. If she passes on this dude loving allele to her son, via the X-chromosome, perhaps he’ll be gay. But since she’s having more babies, it’s a wash.


(Updated for clarity and some more details.)

RSS icon Comments


I would blame primogeniture. There was a study I read where the first born males are rarely ever gay. Obviously, taking them out of the breeding pool and the family pool would make them less taxing on the available resources.

Posted by OR Matt | June 26, 2008 10:51 AM

does anyone still believe that gender and sexuality are mainly, or at least partly, socially constructed? or is there only one way to be "gay" or "straight", and it is biologically pre-determined at birth?

Posted by wf | June 26, 2008 10:52 AM


Posted by Mr. Poe | June 26, 2008 10:55 AM

This is a first-born homosexual right here, pleased to meet you.

Actually, though, the more older brothers a man has, the more likely he is to be gay: one explanation why male same-sex attraction is selected is to reduce procreative competition when there are more and more males. That's a take on primogeniture, in a way.

Posted by Simac | June 26, 2008 11:03 AM

Is anyone doing comparable science to figure out why women are lesbians? It's interesting, because on one hand you hear studies like the recent one that equated lesbian/straight male brain structure and gay male/straight female brain structure, which might make one think things could be analogous. Then on the other hand, you have this whole "women's sexuality is fluid" argument, which is true for some women, but which is not true for many, many dykes I know. Finally, there's good old sexism, which will keep studies on those confusing ladies well behind those on standard-setting men anyway. So where does the science on lesbians stand now?

Posted by greendyke | June 26, 2008 11:04 AM


I think almost everyone believes environment has some impact on sexuality. With men, genes do seem to matter a whole bunch--perhaps more than initially expected.

For women, the sense is the complete opposite--genetic factors seem to be dwarfed by the environmental factors. The brain scan paper that you referenced nicely illustrated this. That isn't to say that female sexuality isn't an intrinsic trait. Just that factors other than genetics, like hormonal conditions in the womb, seem to be predominant.

Posted by Jonathan Golob | June 26, 2008 11:08 AM

I always assumed it was because without them we wouldn't have any decent clothes or television shows.

Posted by Fnarf | June 26, 2008 11:09 AM

This is great stuff Jonathon, but what about lesbians? Is this not the same for them as well? Or did they just study gay men?

Posted by Original Monique | June 26, 2008 11:11 AM

Nevermind! I didn't read through all the comments before posting


Posted by Original Monique | June 26, 2008 11:13 AM

There was an issue of Psychology Today recently that had an article with some of these studies. There was an interesting study in Italy studying women with lots of children and gay relatives and the correlation between the two.
It will be nice when lesbians get this kind of attention, but we're always at least a decade behind looking at womens issues.

Posted by Enigma | June 26, 2008 11:21 AM

Yay, and it's topical. Pride Weekend almost upon us!

Posted by Non | June 26, 2008 11:22 AM

Fnarf --

That's theory #1. I was so sad to find out this probably isn't true.

Posted by Jonathan Golob | June 26, 2008 11:37 AM

"A whole bunch"-- that's very scientific. And also again doesn't explain, if "environment" matters, why there's no discussion of history or culture in your speculative analysis.

Posted by wf | June 26, 2008 11:39 AM

Simac, I didn't mean to insult you, if that's how you interpret it ... yes there are OBVIOUSLY exceptions.

As for the scientific rational for why women are lexbians. The rational for womens sexuality is more fluid makes a lot of sense (not that there is anything wrong with it)

But maybe, JUST MAYBE, women are just more accustomed at putting up with bull shit and oppression throughout history than men. Perhaps men were more willing to come out of the closet with such destructive and intense consequences it intrigues the curiousity.

I mean scientists are humans too, what would you be more interested in studying? The men who are so sexually frustrated they come to forsake their family and fuck like rabbits, or the women who "discover their true feelings". Show me the female version of Larry Craig, then we'll talk.

Posted by OR Matt | June 26, 2008 11:42 AM

wf --

You want a detailed breakdown? Take a moment and write a question to Dear science and I'll take the time.

In the meantime, you want some data? Why not read one of the dozen peer-reviewed scientific articles to which I linked in the post.

Posted by Jonathan Golob | June 26, 2008 11:42 AM

@ 1 There really ought not to be any *blame* involved since there is nothing *wrong* with being Gay. That said, I am a gay 1st-born male and know several others. I read the report of that study and it seems that younger male siblings are simply slightly more likely to be gay if there is an older male sibling.

Posted by inkweary | June 26, 2008 11:45 AM

So what?

While I appreciate the advances that science makes, none of this should matter one iota relative to the social and political questions.

What if I don't have the gay gene, but want to experiment? What if I don't have the gay gene, but social factors have led me to choose homosexuality some or all of the time?

In the end, what we should care most about is the freedom to shape our lives, the freedom of self-determination.

If science is successful at identifying biological roots to homosexuality, there will certainly be those who will push for the vaccine or other methods of "curing" homosexuality. And, then, we'll be back at the gate of "freedom" arguing the point still.

So, let's just focus where the argument will take us anyway. :-)

Posted by Timothy | June 26, 2008 11:48 AM


You said that, for women "genetic factors seem to be dwarfed by the environmental factors" and then you said "that isn't to say that female sexuality isn't an intrinsic trait".

If lesbianism is primarily environmental, how can it be intrinsic or genetic?

Posted by blank12357 | June 26, 2008 11:50 AM

Where's random mutation? Where's "it's not a gene, it's hormones in the womb"?

Posted by Mr Fuzzy | June 26, 2008 11:54 AM

Lesbians: nobody wants to study you unless the subjects are naked and with each other.

Posted by Non | June 26, 2008 11:56 AM

@18 It's not a black and white issue...genetic v environment.
Lesbianism being primarily environmental...I think what the article and Jonathan is stating is:

at this point in scientific knowledge, there does not appear to be a strong link to a particular gene or genes with lesbianism. Furthermore, there doesn't not appear to be an obvious evolutionary mechanism (genetic based) as to why lesbianism occurs. The alternative hypothesis at this time, is environmental causes (development).

Posted by cw | June 26, 2008 12:03 PM

@18 and 19:

Mutations, at least productive mutations, are quite rare. Too rare to account for lesbians.

Not every intrinsic trait has to be genetic. Epigenetic changes, whether random or due to hormone exposure during pregnancy, can be lifelong and as unchangable as your genetics.

Epigenetics are changes in how the DNA is packaged, turning on and off genes. These changes can be copied from generation to generation of cells, persisting throughout life.

Posted by Jonathan Golob | June 26, 2008 12:07 PM

@18, read the rest of the Golob comment you quoted.  The answer to your question is right after the quote you truncated.

That isn't to say that female sexuality isn't an intrinsic trait. Just that factors other than genetics, like hormonal conditions in the womb, seem to be predominant.
Posted by lostboy | June 26, 2008 12:12 PM

...or the answer is @22, if I type too slowly.

Posted by lostboy | June 26, 2008 12:14 PM

Not sure where you got the idea that homophobes were also evolutionists.

Posted by K | June 26, 2008 1:25 PM

I still like #1, kin selection, so much that I'm not quite willing to give it up based on a survey of 120 men in England. I know it's merely anecdotal, but I haven't personally seen any evidence that female relatives of gay men have a greater affinity for straight men.

Posted by Erica T. | June 26, 2008 4:34 PM

It seems reasonable to assume that population control is partly the reason. Whether it's to increase or take the pressure off population growth. Humans have apparently had scrapes with population depletions so great as to be a threat to species survival. Likewise, if the environment was unable to support growth.

Posted by Vince | June 27, 2008 6:33 AM

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