August 01, 2011

The decline of the homosexual population (?)

Jesse Bering argues in Scientific American that the extension of marriage rights to homosexuals may lead to a decline of the homosexual population. This conclusion stems from three assumptions, which I enumerate below:
  1. There are heritable factors predisposing one to homosexuality
  2. These factors when expressed in heterosexual individuals increase their fitness
  3. Homosexual marriage will greatly reduce the number of offspring born to homosexual parents
#3 is reasonable: greater societal tolerance for homosexuals will indeed lead to a reduction of closeted homosexuals in fake heterosexual marriages, and to a reduction of offspring born to such individuals. This will lead to a reduction of any potential heritable factors predisposing to homosexuality in the population.

#2 seems to be based on a study that showed that heterosexual twins with a homosexual twin had more sex partners than those without. I find that very weak evidence, since sexual incontinence is not what produces children, and the most reproductively fertile people are usually those that marry young, not the ones who spend their youth with multiple sex partners. In any case, it is a very indirect measure of reproductive success.

#1 is the main reason for the seeming "paradox" of homosexuality: the (partial) genetic basis of homosexuality, together with the reduced fitness of homosexuals would result in negative selection that would penalize the trait: how could such genetic factors evolve (hence the need to create mechanisms such as in #2).

The problem, however, is that the analysis is based on a common-allele-common variant model. Homosexuality is relatively common, hence, it is mysterious how causative alleles for it would have grown in a population to levels necessary to account for the observed occurrence.

A real problem for that theory is that there is absolutely no evidence for such alleles. As far as I know not a single locus has been identified as being associated with homosexuality.

How can this be reconciled with the evidence for homosexuality being heritable?

The short answer is that homosexuality occurs in each generation de novo, perhaps due to an accretion of deleterious mutations impeding normal sexual behavior, or the chance recombination of commodity alleles that -in themselves- are neutral, but that -in combination- impede normal sexual reproductive behavior.

Rather than imagining the existence hypothetical and completely unattested alleles for homosexuality that persist in populations for many generations, we only have to accept the much more reasonable proposition that mutation load or simply an unfortunate draw from the recombination lottery produces a genome that predisposes its bearer to homosexual behavior, and hence to reduced genetic fitness.

So, there really is no great mystery: homosexuality is not a trait that persists due to elaborate balancing selection mechanisms, but rather the side effect of mundane negative selection: deleterious mutations that impede heterosexual attraction routinely occur in a small number of individuals de novo and these are culled from the gene pool in a few generations.

Hence, I predict that the homosexual population will not decline as a result of the legalization of homosexual marriage. Rather, a few percent of the population will continue to express the trait as they always have. It's also quite possible that the more tolerant social atmosphere may boost the non-genetic contributing factors to the trait, so an increase is not out of the question.


Fanty said...

I recall to have read a "news" somewhen 5-10 years ago that claimed that the genetic trigger of homosexuality was found. (and have never since read about that again).

But I recall that they claimed that its inheritated from the mothers side and said: Homosexual males NEVER have homosexual sons. But over average often, homosexuals have a homosexual uncle.

Oh yeah... and they also claimed the same gene mutation that makes males gay, does make females nymphoman.

But since this did make it to common knowlegde after, I say 8 years, it must have proven to be BS.

matt said...

I have read theories of genetic "causes" of homosexuality that would be passed predominantly from father to daughter, or mother to son, with sexual attraction to women or men respectively.
So these "alleles" if they exist have little reduction due to the elimination of "false" marriages.

Unknown said...

To my knowledge no genetic cause for homosexuality has ever been isolated.

And in any case those homosexual couples who want children can, and are, having children. I have at least two couples of my acquaintance who have. Nice kids from what I know of them

So its all moot.

Onur Dincer said...

It's also quite possible that the more tolerant social atmosphere may boost the non-genetic contributing factors to the trait, so an increase is not out of the question.

I think that is the biggest problem in this matter.

Belenos said...

It's got to be said that there is no conclusive evidence even for a genetic component predisposing people to homosexual behaviour (though personally I wouldn't be surprised if there was a small one).

The differing ways that various cultures manifest and codify same-sex sexual desire suggest to me that almost all researchers working on the topic today are mistaking cultural conventions (the gay/straight neo-gender paradigm dominant in western societies) for biological realities.

apostateimpressions said...

Ironically, if homosexuality is at all hereditary (whatever the mechanisms), then religions that are intolerant of homosexuality and that apply pressure for them to marry the opposite sex actually increase the humber of homosexuals by reducing the gradual elimination of traits that predispose to said indisposition.

Then again, possibly the celebate religious orders are Nature's way of compensating. Those who dont feel "called" to live a normal family life can join an order and live their life with others of the same sex. That way, homosexuality is eliminated and its more obvious expression is curtailed. This theory is consistent with the alleged preponderance of homosexuals in religious orders.

Judith Weingarten said...

Or maybe, we've been looking at the wrong genetic factors:

mathilda said...

A few years ago I saw identical male twins on Oprah. One gay, one straight. And you could tell which was which even before they spoke. They were dressed the same and sitting the same.

As far as I can tell homosexuality is due to a slightly skewed in utero exposure to sex hormonesas well as genes, as two genetically identical individuals with different orientations show that it isn't just genes.

You'd probably get a reduced fequency of male homosexuality due to smaller family sizes, as the chances of being gay increase the more older brothers you have.

Also; bit bemused as to why so many people seem to think homosexuality is a problem. It's a pretty minor defect, in the grand scheme of things. There are some nasty non consensual human sexual behaviours we should be looking for a root cause of instead of obsessing over what two (or more) guys like to do.

apostateimpressions said...

Mathilda's theory would suggest that we may be able to decrease frequency of male homosexuality by adjusting the hormonal balance in the womb. It might be possible for women to use some simple capsule, some herb or other, during pregnancy.

But would we as a society want to allow this, even if most parents would prefer to have straight kids? It might prevent the elimination of contributive and undesirable genetic traits if those who are genetically predisposed to parental abstention lack the hormonal factor and so go ahead and have kids of their own.

Perhaps maternal hormonal variation is one of Nature's methods to eliminate undesirable genetic traits to maintain the fitness of the race.

Fanty said...

Things are usualy never "Genes only".
Most "inheritated" things do need an envoiremental factor. Its just that the genes make the "risc" (or "chance") higher.

The most articles I readed about Personality aswell as Intelligence, claimed both these things are rougly 50%/50% DNA and envoiremental caused.

In that backgound it apears to be very unlikely that anything is purely envoiremental however.

But, there are a lot other sexual disorders beside homosexuality.

Some numbers for comparation:

German numbers:
Men who say, they are sexualy attracted by other men: 9%
Men who actually perform sex with another men: 1.5%

Women who say, they are sexualy attracted by other women: 19.5%
Women who actually perform sex with other women: 0.5%

People (no gender named) who are sexualy aroused by Sado_Masochist Sex techniques: 25%
People who actually perform SM Sex for real: 2%

Then there are humans who are aroused by body parts that, normaly, are not connected to sex.
Most often: feet

Here I know that the numbers strongly differ from country to country.

Its quiet high in Russia, Finland, Sweden and Norway with around 20%

And at about 10% in the mediteranean regions of Europe.

(scientists already tried the explanation: So more you hide a thing, because of cold weather for example, so more interesting it might be...)

Anonymous said...

The human trait or bundle of traits known as honosexuality is not a defect nor is it a disease. It is a naturally-occurring biological variation in human populations. It may have advantages in a population depending on the environment in which the population lives.

Georg said...

I wil recommend everyone interested in this topic to read the book Gay, Straight and the Reasons why by Simon LeVay. The follow link may be useful.

Roy said...

Annie, the levels of abuse suffered by foster children are vastly higher than those of natural children.
A gay couple's 'offspring' could - at best- have half the genetic heritability of a hetero couple.
For you to say they are "Nice kids" glosses over important issues of genetic investment, let alone the cognitive dissonance
of social dilemmas. Who goes to Father's day at school? This one, that one, both ?
To imply gay parenting is normal and not the exception is disingenuous of you.

Unknown said...


Personally would prefer that a child was raised by a straight couple than a gay couple. By two parents rather than one. Why go looking for extra trouble.

But the truth of the matter is the nature of the parents is by far the greatest factor in good parenting. The people are the the skyscraper. The gay/straight isue is the decorative tiles in the foyer.

All the studies show children of gay parents do better than the average child. IMO most of the damage done to them is done by tittilated parents of other children, and the inflexibility of authorities. Not by the fact that the childs parents are the same sex.

Fanty said...

"It is a naturally-occurring biological variation in human populations. It may have advantages in a population depending on the environment in which the population lives."

It may be a "natural occurring biological variation" (a random mutation in other words) but I dont see what advantages do individuals give to a species, that dont reproduce.

Except maybe if the stereotype would be correct, that gays are more creative than straight humans. No art without gays? ;)

apostateimpressions said...

<< [Homosexuality] may have advantages in a population depending on the environment in which the population lives. >>

I can understand that homosexuality may help maintain the fitness of a population through the gradual removal of undesirable traits from the gene pool -- because when homosexuals dont breed, those persons are removed from the gene pool.

Homosexuality adds nothing to the gene pool -- so its use must lie in what it removes from the gene pool. That seems to be common sense to me even if it isnt "politically correct" these days.

To use standard British slang -- after all even Socrates took his initial definitions from the common man -- homosexuality removes poofy blokes from the gene pool.

Some men have undesirable genetic traits that predispose them to homosexuality. When the final factor is added -- which could be maternal hormonal variation -- actual full-blown homosexuality removes them and their undesirable traits.

In other words, Nature maintains the essentially male/ female character of human populations, with men as "real men" and women as "real women". That seems obvious with a survey of the gender character of human populations. Homosexuality is one of the ways that Nature maintains gender -- by removing those who are predisposed to homosexuality from the gene pool.

Dienekes said...

As for Nature's way of removing those so defective from the gene pool, Ms Nature hasn't done a very good job over the millennia;

It's a dynamic balance. Nature creates individuals who are have non-reproductive sexual urges (such as homosexuals), and nature culls the genetic basis of such behaviors.

Nature ain't perfect: it makes mistakes and corrects them. Of course "mistake", in terms of evolution, is an organism that can't survive, or has no inclination to reproduce itself; this does not mean that said organism is not otherwise worthy.

Onur Dincer said...

I think homosexuality should be open; that way they don't marry with people of the opposite sex and thus don't reproduce (at least by natural means). But homosexuals should never be presented as role models when it comes to sexual matters. Because that may divert the otherwise heterosexual people to homosexuality. In short, homosexuals should reproduce as rare as possible (the ideal is their complete non-reproduction).

Grendal said...

Does homosexuality "run in families"? There doesn't seem to be much evidence for this. So is there any specific genetic groups of factors which lead to an individual being homosexual? If there are they must be extremely complex and obscure or, in spite of the fact that homosexuals have historically married and had children to cover-up their socially unacceotable trait, we would still have seen "breeding out" effects over the millenia.

Apostateimpressions says that some men have undesirable genetic traits that predispose them to homosexuality. But what human traits can be identified as having been lost to the species by this filtering process? Or is he saying that Nature creates homosexuals solely so she can remove them from the gene pool?

Unknown said...

I recalled reading what Greg Cochran said recently on Razib's blog concerning male homosex and de novo mutations so I offer it here:

"If homosexuality were caused in large part by de novo mutations, a noticeable fraction would be syndromic – that is, cause other noticeable phenotypic effects. Like Waardenburg syndrome, deafness and a white streak in the hair. Or fragile X: big ears and macroorchidism. More generally, retardation and funny-looking kids. I don’t think has ever noticed anything like that in homosexuals.

"Moreover, sexual interest is surely not as complicated as, say, hearing or vision, and thus should be a smaller mutational target – which implies that homosexuality should be rarer than blindness or deafness. But it’s much more common than genetic congenital deafness, which hits about 1 in 1500 kids.

That said, it is entirely possible than some small, maybe tiny fraction of homosexuality _is_ due to de novo mutations, and if anyone found such as a case, we could learn a lot from it. As we did with narcolepsy, in dogs."

Unknown said...

Why do we assume homosexuality is disadvantageous. In a society controlled by men (some of which will be homosexual) it could be quite advantageous to have a few homosexual men in the family. Particularly in societies based on patronage.

The vast majority of ants dont reproduce. Does not mean that they are not important to the ant society. It could be argued that having a couple of surplus homosexual uncles or aunts could be very advantageous for a child, in that they have more adults supplying resources for their growth and development.

Why do we assume homosexuality is a defect?

Unknown said...

Annie Mouse,

You are suggesting the so-called "gay uncle" hypothesis--that having an occasional gay male around helps a mother raise kids to maturity.

I can think of only one study published last year, I think, of an island population in which the author argued that gay males (they may have been transgendered, don't recall)helped in this way.

Other studies don't find that to be the case. Homosexual uncles are no more invested in the child-rearing than are straight uncles. In fact, once sexual maturation kicks in, males in general have other activities on their minds--child-rearing is not high on the list of importance to any male whose sex drive has kicked in.

It can't be successfully argued that selection would have produced homosexual males because they make better mothers than mothers themselves.

Unknown said...

Until now (for years), I've been only a reader, but since I'm shocked by the foregoing commentary, I have to speak up.

Firstly, why is homosexuality a "defect"? Speaking as a gay-ish man myself, if you only knew how many of your "straight" acquaintances weren't entirely "straight" all the time, you might have to readjust your perceptions of reality, or else the majority may be "defective".

Secondly, the "gay-uncle" phenomenon — at least in Hawai'i and the rest of Polynesia (as well as Scandinavia, my other home) — is very much alive and well, and I know it to be true, both from first- and second-hand experience. What's more, I can imagine "gay uncles" providing the same kind of support to their extended families and communities from the very first day of humanity.

Thirdly, and I'll make this brief, I'm surprised that people interested in the *science* of *life*, which is the point of this blog, as I've understood it for years, can dismiss peoples' entire lives as "defects" or embarrassments to society. Do you really consider it alright to speak of humans in such an in-human way? Really?

Dienekes said...

the "gay-uncle"

Uncles are half-related to their nephews/nieces compared to parents.

In a non-growing population, parents need to produce 2 healthy offspring in order for their genes not to decrease in frequency (on average); uncles need 4 nephews/nieces to do the same.

In fact, since people occasionally don't have siblings (so gay uncles can't invest on non-existent nephews), and siblings occasionally don't have offspring, gay uncles who are brothers to reproductive siblings must help produce >4 nephews so that hypothetical common "gay genes" will not decrease in frequency.

In short, the "gay uncle" phenomenon is quite unlikely to be responsible for the persistence of common "gay genes" in a population.

apostateimpressions said...

Thirdly, and I'll make this brief, I'm surprised that people interested in the *science* of *life*, which is the point of this blog, as I've understood it for years, can dismiss peoples' entire lives as "defects" or embarrassments to society. Do you really consider it alright to speak of humans in such an in-human way? Really?

Frankly yes, I do think that it is correct to speak of humans as "defective". We know that very many people are genetically defective, with all sorts of hereditable illnesses for instance. We know that humans are defective in many respects, physically, psychologically, IQ etc. We are biological machines with a genetic basis not gods and people have limitations, weaknesses and defects. Evolution works through trial and error, hit or miss.

Society is frank about that many people have genetic defects and that future generations would be better off if we removed genetic defects from the gene pool. Hospitals routinely screen for some conditions prenatal and almost all parents choose to avoid having defective kids when given the option.

Anyway, I dont think that anyone is particularly arguing that society has to breed out defective homosexuals. Homosexuals qua homosexuals dont breed anyway. Indeed I have argued that homosexuality is possibly one of Nature's ways that other, if predisposing, traits are removed from the gene pool.