What pressures keep homosexuality in populations?

September 21, 2005 12:04 PM

From what I understand, homosexuality among humans throughout history (to the best of our knowledge) remains at 10%. Why is this? Are enough homosexuals propogating through heterosexual means to keep the genetic causes for homosexuality in the population?

I realize that no one really knows what causes homosexuality but being that its not a voluntary choice we can assume that there is a genetic factor at play, right? Most things that prevent us to reproduce don't show up until past the child-rearing years. What's the current thought as to why homosexuality remains constant within a given population (assuming that this is correct)?

I read on Wikipedia about the idea that non-aggressiveness amongst males in paticular promotes homosexuality, which seems flimsy to me. The only thing I can think of, is that the societal pressures to be straight would be so great that enough homosexuals at least try out being straight enough to keep everything in the gene pool.

Or perhaps am I misunderstanding evolutionary pressures all together?

posted by geoff. to science & nature (34 comments total)

While I'm not going to comment on homosexuality, there is no possible way anyone can claim that anything has been at "10% throughout history".

Tell me what percentage of the US population, today, is homosexual. You should get stuck right about where you try to define the term "homosexual". Plus the absolute lack of any numerical data on anything more than 100 years ago.
posted by GuyZero at 12:10 PM on September 21, 2005

I think the problem with this question is that it assumes that homosexuality has remained at 10% which I doubt to be true, especially when you examine certain cultures that embraced homosexual and bisexual tendencies (Grecian, Roman, Early Japanese cultures leap to mind). You can't ask a question about society and accept a good answer while accepting a false assumption.

Also, I'm not even going to get into genetic causes of homosexuality, but I'm sure some of our more DNA-minded brethren will.

(On preview, basically what Guy Zero said)
posted by cyphill at 12:16 PM on September 21, 2005

Or perhaps you're misunderstanding sexual preference all together, by thinking WAY too simplistically about human nature. "Gay" people sleep with the opposite sex for myriad reasons, "straight" people sleep with the same sex for myriad reasons, and lots of people don't consider themselves gay or straight.

Aside from that issue, you're assuming that the "gay gene" if there is one, would only be passed by parents who express that same genetic trait. Haven't you ever met a kid with blond hair and two brunette parents? My uncle is gay, my great grandmother had a child then lived with another woman for the next sixty years of her life, but both my parents express their sexuality with opposite-sex partners.

I personally don't ascribe my happygayness to anything other than good luck.
posted by pomegranate at 12:16 PM on September 21, 2005

IANAB, but I would say that it is genetic- however, I don't think it is as simple as homosexuals having a "gay gene" that can be or not be passed on to their offspring.

It could be a recessive set of genes that everyone has that are only activated under the right conditions. What these conditions are, who knows? I haven't heard any plausible explanations for the mechanism that makes people homosexual

I have heard theories about how homosexuals would be beneficial to a population, making it more likely that the population would survive and multiply, thus giving a competitive advantage to those populations. And arguably, too many homosexuals in a population would be bad for reproductive success, with a balance point being around 10%.
posted by gus at 12:18 PM on September 21, 2005

Homosexual representation is actually not 10%. Recent studies give a more reasonable figure of 1-3%. The 10% figure was based on the Kinsey studies, which are full of problems.

Also, recent studies have shown that the gay gene in men expresses itself in women as higher fertility traits. So, that advantage alone would ensure the preservation of male homosexuality.
posted by letterneversent at 12:21 PM on September 21, 2005

In what ways would homosexuality tendencies make a population more likely to survive and multiply?
posted by meta87 at 12:22 PM on September 21, 2005

I realize that no one really knows what causes homosexuality but being that its not a voluntary choice we can assume that there is a genetic factor at play, right?

Um no, why? You seem to confuse genetic with human and non-voluntary with determined. There are plenty of things that are not voluntary that are not expressions of genetic traits. If you love cheesecake is that voluntary? Is it the expression of a genetic trait? If you hate tequila because you once drank it far in excess of good sense is your have voluntary? Is it the expression of a genetic trait?
posted by OmieWise at 12:25 PM on September 21, 2005

That 10% number is an old canard, probably stemming from the original Kinsey report. It had some methodological problems (including a sampling population drawn primarily from big cities).

If you consider only those whose primary sexual attraction, and activity, is with the same sex, the number is more like 4-5%.

All that aside, here's an interesting article that addresses your question.
posted by curtm at 12:27 PM on September 21, 2005

Scary, curtm's link takes you to Microsoft (huh). This is what you want.

The first comment is pointed, underlying this debate is .. the presumption, at root for everything else said in relation to it, that sexuality is merely a tool for the propogation of a species via reproduction. Which is debatable, as is the other hidden premise that Gay people don't have children.
posted by grahamwell at 12:36 PM on September 21, 2005

I heard wrong from a professor apparently wrongly, about the 10% debacle. OmieWise, I used to not like red wine, then I stuck to it and started to drink it and now I enjoy it. Would the same be said for a straight person, if they have gay sex enough they would eventually enjoy it? Also people's taste preferences wax and wane over a lifetime (among foods, drinks, preferences in cars, etc.) but I don't know of anyone who does the same with sexual gender prefence. That obviously doesn't mean it doesn't happen, but are there any studies I can read that indicates that? It'd be interesting.

The 1-3% figure would be more understandable as far as evolution is concerned for something that relies perhaps on a multitude of factors to express itself (and thus can appear recessive in a population).
posted by geoff. at 12:38 PM on September 21, 2005

I'm a fan of the theory that that letterneversent presented, although I'm pretty sure sexuality is at least partly the result of more complex genetic interactions. Still, if the factors that make Neve Campbell hot are the same ones that make her brother Christian gay, then there may be a large enough boost to Neve's reproductive fitness to confer an advantage.

meta87: You could easily make an argument that having some percentage of a population who contribute to the resources of that population, but does not consume the resources needed to create and raise additional offspring could be quite benefical. Humans, like a lot of large mammals, tend to focus on having fewer but higher investment offspring, and having a few related adults around who can watch the kids or gather/raise food would help make sure that the kids survive to adulthood.

But indeed, we don't have much in the way of good historical records regarding the prevalance of homosexuality, much less good records in a evolutionarily significant timespan. I think wikipedia's full of it in this case.
posted by Mercaptan at 12:41 PM on September 21, 2005

There is a recent theory (Mefi thread) that male homosexuality is contributed to by a gene that's inherited from the mother's side, and that it also improves the mother's fertility. So the reason it doesn't die out is because it's a net advantage for the species. Also, it's in dispute whether it's 10% (that's just the Kinsey study number, which used a disproportionate amount of prison inmates for instance); a recent major sex survey has 5% of men self-identifying as gay.

Lesbianism, on the other hand, I'm thinking may merely be because females may exist on more of a spectrum sexually (most females respond equally to porn featuring either gender) and those at the far end identify as lesbians, unlike male sexuality which is more binary. One reason this wouldn't get selected out is that with the male's aggressive sexual role there'd be no shortage of heterosexual activity regardless of females' broader tastes.
posted by abcde at 12:46 PM on September 21, 2005

Assuming homosexuality is genetic, it's not hard to identify reasons why a seemingly anti-reproductive strategy wouldn't be selected out of existence:

1) Obviously, homosexuals sometimes marry and mate with someone of the opposite sex (but, almost certainly, this happens more often with Kinsey 4's and 5's than 6's, so we quickly come up against problems in definition, as others have cited.)

2) If a gay person didn't have kids, but participated in supporting other family members' kids, that extra support could end up making a sufficient difference in evolutionary fitness for those kids (i.e. surviving long enough to reproduce, and doing so) to be selected for. Remember, in terms of perpetuating one's genes, on average, the survival of two nephews/nieces = the survival of one child of one's own.

Mind you, while #2 is plausible, I wouldn't accept any assertion that it has made and does make a difference until I'd seen a very careful (and very difficult to create) study. And that would have to follow a more complete understanding of the heredity (or lack thereof) of homosexuality than we have now.
posted by Zed_Lopez at 12:47 PM on September 21, 2005

I realize that no one really knows what causes homosexuality but being that its not a voluntary choice we can assume that there is a genetic factor at play, right?

Not if there is a set of environmental or prenatal developmental factors determining it, no, which hasn't been ruled out.
posted by Rothko at 1:02 PM on September 21, 2005

Oh dear:

I realize that no one really knows what causes homosexuality but being that its not a voluntary choice we can assume that there is a genetic factor at play, right?

Not really. A large number of preferences are cultural and firmly established long before we are able to tell stories about ourselves. Two big examples are language and some diet preferences.

I think the bottom line is that nobody knows yet, and all of the theories proposed here are just speculation going way beyond the available evidence, much less the scant evidence in this thread. We don't even have robust tools to measure sexual orientation, much less come up with any hypotheses about the effects of natural selection on the distributions of sexual orientation over time.
posted by KirkJobSluder at 1:11 PM on September 21, 2005

Also people's taste preferences wax and wane over a lifetime (among foods, drinks, preferences in cars, etc.) but I don't know of anyone who does the same with sexual gender prefence.

You don't? You've never known anyone who experimented with bisexuality--or even exclusive homosexuality--in college and then settled down to an exclusively heterosexual life thereafter? That's a cliche by now, isn't it?
posted by wheat at 1:14 PM on September 21, 2005

Twin studies indicate that genetics does seem to play a role: "Bailey and Pillard (1991) found that [identical] twins are concordant for homosexuality 52% of the time, and [fraternal] twins only 22% of time time, a statistically significant difference that points to a large role for genes." That link addresses a lot of your question I think.

Much of what I would say has been said. The genetic basis for homosexuality is well established but that only means that some genes include, among their effects, a greater or lesser propensity for homosexuality.

To really evaluate the worth of those genetic factors you would have to know what they were, you would have to know what all their effects are, and you would have to be able to weigh those effects in your theorized evolutionary environment (not just individual survival of one organism). Fully untangling this sort of thing is (forever?) beyond the reach of genetics, sociology, and history, I believe.

So right now I think people are hoping for a simpler solution. Not as simple as "this gene => you're gay" anymore, but it could be something like the "fertile sister / gayish brother" gene mentioned above. Or that the gene makes some males better fathers to make up for the gay ones. Or it could be that the "gay gene" creates outcast males who contribute to the success of the gene pool in ways that don't involve popping out babies. You can theorize all day long!

From what I've read (which is damned little) I'd say that the people interested in the genetic basis for homosexuality are more on the "panning for gold" side of the room, rather than the "hot on the trail" side. Just to mix metaphors a little.
posted by fleacircus at 1:17 PM on September 21, 2005

geoff.-
I'm not sure how far anecdote goes here, but are you really suggesting that there are no preferences that are not genetically encoded? And remember, tolerating something is a far cry from choosing it, even if simply drinking the wine changed your preference.
posted by OmieWise at 1:40 PM on September 21, 2005

I have strawberry blonde hair. Neither of my parents do. Inheritance doesn't require that parents exhibit dominant genes for a particular trait.

Or am I misunderstanding the question?
posted by glenwood at 1:41 PM on September 21, 2005

Yes you're right, if all our choices were genetically encoded they would be no free will. I think I was on a different point, that you can choose your wine preferences but I was seeing being gay as being blue-eyed. I see your point.
posted by geoff. at 1:47 PM on September 21, 2005

It can be "genetic" but not as simple as most people view genetics; that is, like Mendel's smooth and wrinkled peas. Even eye color is currently understood to be 2 genes. Other reactions are even more complex.

There was a brief discussion of this in Nature via Nurture by Matt Ridley, basically analyzing Ray Blanchard's studies into male birth order and birth weight. He found that males who were born after older brothers were lighter at birth, potentially due to an increasingly strong maternal immune response in the womb. And those younger brothers had an increasingly strong "chance" to be homosexual. In a sense too, the "gay gene" (or genes, or gene complex...) maybe be on the mother. As an amusing sidenote, finger length may correlate with birth order and homosexuality too.
posted by artifarce at 2:03 PM on September 21, 2005

The Boston Globe ran an interesting article about a research project on what makes people gay. It did not address what would be the evolutionary pressures for homosexuality. If it is partly genetic, one would expect it to quickly disappear due to natural selection unless it conferred some hidden advantage to the genetic lines which carried it.

My pet theory is that its a form of population control. If certain environmental cues suggest overpopulation is imminent then the trait is activated at some point during the child's or fetus's development. Just as some salamander species produce cannibal mutants in overcrowding situations, we produce useful hunter-gatherers.

Of course, if this theory were correct it would predict sparsely populated regions would have noticeably lower gay populations. I'm not aware of any studies to support that.
posted by justkevin at 2:05 PM on September 21, 2005

1) I would check your facts on the "10% throughout history" statement. Bisexuality and homosexuality were prevalent pre-Christianity.
2) Even assuming homosexuality is purely genetic, I think you're grossly underestimating the amount of time it would take to make any significant change in the proportion of gay people to straight people.
3) It is interesting to note that studies have found gay animals.
posted by Count Ziggurat at 2:11 PM on September 21, 2005

In what ways would homosexuality tendencies make a population more likely to survive and multiply?

Having a steady supply of people who are not burdened by children and spouses could be a good thing for a society. Singles (homosexual or not) can devote more time to creative activities than can people who are yes-dearing their evenings away and scraping shit off the ceiling. Singles can take risks that family people might be unwilling to take.

Singles (and particularly singles within the extended family) can also be available to help their siblings and other relatives raise children. In this way, homosexuals could promote their own genetic lines without having children themselves.
posted by pracowity at 2:12 PM on September 21, 2005

abcde writes "Lesbianism, on the other hand, I'm thinking may merely be because females may exist on more of a spectrum sexually (most females respond equally to porn featuring either gender) and those at the far end identify as lesbians, unlike male sexuality which is more binary."

Ok, it's not the thread for this so I'll bite my tongue - hard! - and not speak at length about this, but for someone who has Spivak in their user profile, this is a ridiculously erroneous statement. It's an old, old fallacy that women are nearly bisexual, that most women respond to either gender in pr0n and that men are much more binary and either/or in their preferences. This just feeds into the same old stereotypes of women *and* men, and neither part of it is true. There are many crackpot and unproven theories about gender in particular (and many others that we're still revising and re-theorizing), and many blatantly false statements about sexuality specifically. Many of those theories are reproduced rather faithfully here, but this was one that I couldn't let slip through the cracks.
posted by fionab at 2:18 PM on September 21, 2005

fionab: How is it a fallacy to say that women respond to either gender in porn? Studies have shown this by mesuring neural response, etc.
posted by delmoi at 2:21 PM on September 21, 2005

Female bonobos, who are the leaders in their communities, are very sexually open and use lesbian activities to resolve conflict with other females, and hence remain peaceful and focused in maintaining power.
posted by artifarce at 2:26 PM on September 21, 2005

fleacircus: Much of what I would say has been said. The genetic basis for homosexuality is well established but that only means that some genes include, among their effects, a greater or lesser propensity for homosexuality.

True, my beef is that you can't get there from the subjective experience of "not a choice."

There are a bunch of other possible hypotheses as well. My hunch is that the answer has to do with multiple factors and statistical ranges. Natural selection doesn't act directly on genes, but on phenotypes. In general it favors "Goldilocks" phenotypes: not too much, not too little, but just right. For example human height is influenced by five genes. Most combinations give you something within a standard deviation of average. A handful of combinations will give you basketball players or olympic gymnasts. It is quite possible that the "Goldilocks" phenotype involved in homosexuality has more to do with MOTSS sociability than sexuality.

In other words, it may have nothing to do with producing altruistic aunts/uncles or population control, and everything to do with MOTSS hunting/foraging groups, pre-natal hormone balance, or a quirk of brain development.

But again, I don't think that psychometrics in regards to sexual orientation is really there yet.

delmoi: How is it a fallacy to say that women respond to either gender in porn? Studies have shown this by mesuring neural response, etc.

This is assuming quite a bit of construct validity in regards to the "looking at porn" studies.
posted by KirkJobSluder at 2:27 PM on September 21, 2005

I think the theory proposed by letterneversent makes loads of sense. In particular, it makes sense when you think (a la selfish gene) that evolution doesn't work in terms of a species, but rather in terms of the genes of individuals. As long as SOMEONE is helped to reproduce by these genes, then they will persist.

Are enough homosexuals propogating through heterosexual means to keep the genetic causes for homosexuality in the population? Accepting your other assumptions, the question answers itself.

However, presumably deeply homophobic cultures that force men to marry women for cover pretty much ensure perpetuation of gay genes. So if there is no advantage the the gay gene, once society becomes more accepting, it will paradoxically go extinct. (That was tongue in cheek, if you missed it).
posted by i_am_joe's_spleen at 2:35 PM on September 21, 2005

Which if I may rant a bit further. One of my big irritations with how we talk about natural selection with the "gay gene" is the assumption that, in spite of the fact that the available evidence points to multiple factors involved, that the extremes of the range must serve some adaptive purpose.
posted by KirkJobSluder at 2:41 PM on September 21, 2005

Several people have already noted here that evolution acts at the level of the gene, not the level of the organism. Thus, non-reproducing organisms can provide an evolutionary advantage if they help their genes to be passed on through relatives, even though they have no direct descendants.

I wanted to point out that this isn't just theoretical--it's demonstrated to an amazing extreme in honeybees. Roughly 99% of honeybees cannot reproduce, and they've done just fine.
posted by DevilsAdvocate at 2:59 PM on September 21, 2005

Excellent can of worms Geoff!

The most recent research I read of (in a UK broadsheet about two months ago, but I can't recall which) found evidence to support the theory that homosexuality in males occurs genetically, but it's not so simple as finding the right gene.

Forgive me if I get a few of the details wrong, because I'm going off a very flimsy memory, but I think the article proposed that homosexuality was caused in the womb when certain parts of the brain were exposed to more estrogen than testosterone. So the possible cause is not a gene per se, but a chemical imbalance which leads to a different development. It's much more complicated than flicking a switch, which could explain why there's a bit more variety amongst gay men than Republicans would have you think. Could that explain why we didn't all die out when we stopped getting married?

Interestingly, I seem to recall the explanation didn't apply equally to gay women. And I also remember seeing data on lesbian numbers which suggested there are far fewer of them than gay men. But I don't want to be flamed so am probably wrong.
posted by londonmark at 3:20 PM on September 21, 2005

artifarce, that finger length study is fascinating! Thanks for posting it.
posted by 6550 at 10:59 PM on September 21, 2005

There's also the possibility that gay behavior was socially helpful at some point in our evolutionary past (something that might be supported by bonobos) before culture changed to make it more taboo.
If you could assuage a rival by jerking him off, that would allow both of you to focus on more group-oriented goals, rather than conflict. That preclivity could have been genetically transfered like the proclivity towards enjoying sweet foods.
It's also important to realize that people were evolving for millions of years, and that geologically we're a pretty young species yet...
posted by klangklangston at 11:13 PM on September 21, 2005