Homosexuality as an evolutionary advantage

[J Snow]

Okay, so I'm taking Evolutionary Psychology this year, and for the most part its been kind of uncomfortable (while interesting) because so much of the class has to do with passing on one's genes, sexual attraction, etc. and up until today the topic of homosexuality was largely ignored. Well today he proposed theories as to how homosexuality could be passed down as an advantage in evolution. Many of these theories I felt were actually somewhat offensive to the homosexual community, but I'm going to share them with you guys, get your impressions, and hopefully get some intellectual conversation going.

Also, he stated that it is well accepted within the field of evolutionary psychology that homosexuality HAS to have a genetic component and/or advantage or else there is no possible way that it could make up such a large percentage of our global society in virtually every culture in existence.

Disclaimer: This lecture was 2 and half hours long and I can probably not do the theories credit in one post, but I will summarize them as best I can. Also these theories are really only aimed at male homosexuality and not lesbianism.

The first theory (proposed in 1993) states that it could be an advantage to have a homosexual children, because it would provide an extra caretaker and food finder for their siblings children. The professor admitted this theory has been largely disproved by statisticians, but he shared it anyway because it is a common belief of lay people who attempt to come up with a logical explanation on their own.

The theory does not make much sense because the average family only had 2.1 for the majority of human evolution. To have half of your children provide you no offspring would be too large of hit for this to compensate for. Also, if this were the case we should expect to see about 50% of the population being gay, which is not the case.

The second theory (proposed in 1996) states that the vast majority of people who engage in homosexual behavior are actually bisexual (about 5x as much as exclusive gays, moving into some controversial discussion I feel). Personally, I don't think this is the case. He also argued that because a person is labeled as gay, it would be easier for them to be secretly have an unsuspected affair with a married woman and pass down genes that way. Basically, according to the theory, homosexual acts act as a means of practicing sexual before the individual finds a mate of the opposite sex.

I feel this theory not only grossly underestimates the amount of exclusive homosexuals (it stated that out of the 6% of people who have had homosexual experiences 5% admit to also having heterosexual sexual experiences, which to me is probably just trying to figure themselves out early in life for many) and further leads to negative bisexual stereotypes of promiscuity.

Also, my professor stated that their could be a gene that promotes dislike of bisexuals as a way of weeding out this sexual advantage they had over heterosexuals.

The third theory (proposed in 2008) states that their is basically a gene that promotes intense sexual attraction to males in females. This same gene in males would also cause them to be attracted to males and make them thereby homosexual. The theory proposes that even though the males in the family do not reproduce the females reproduce so much that it makes up for their brothers and thus the gene continues to be passed on.

I guess this third theory, if true, is very statistically accurate. We would expect to see approximately about as many gay people as there are today. What doesn't make sense to me about this theory is this, if this is the cause of homosexuality, you should almost NEVER have a family where there is a homosexual male and female (which doesn't seem all that uncommon to me) AND none of these theories explain the phenomena that for each male a woman gives birth to, the subsequent male child's odds of being homosexual dramatically increase.

Also, this theory also states no explanation for female same sex attractions and claims they are non-genetic.

Now, all that out of the way, I should also say the professor's personal beliefs were that there are almost certainly genetic AND non-genetic factors that play a role in one's sexuality (i.e. A child can have a gene to make them more predisposed to be attracted to the same sex, but the hormones the fetus is exposed to in the womb could play just as large or even larger factor in whether those attractions develop later on in life.)

So, I'm curious to see what other people think about all this.

[Enaithor]

The predisposition does make sense (like a diathesis-stress type arrangement) as the more older brothers a man has, the more likely he is to be gay. And not an environmental thing either (I think they did adoption study for this?), the mother's uterus seems to have some sort of memory of how many boys have occupied it.
The third theory does make sense, but I don't see why there can't be a lesbian equivalent? It would also make sense for there to be a woman-liking gene surely? But to make it statistically accurate, maybe without the presence of the Y chromosome you need to have 2 woman liking genes? Hence how a woman could have a lesbian daughter despite being straight herself.
Also, if these genes are recessive in the sex for which it would make them homosexual (which I think is most likely otherwise we'd have way more gay people most likely) then there is no problem with having a gay man and a lesbian as siblings. Also, it probably wouldn't be two alleles of the same gene that do this - this would explain bisexuality, as, for example, a woman could have 1 man-liking gene and 2 woman-liking genes, making her bisexual.

[J Snow]

I think you made some good points Enaithor. Here are some things that come to mind as a read them.

I believe the most accepted theory for why a second son is more likely to be gay then the first son and etc. is that the mother's body interprets the testosterone as a foreign invader, and as the mother has more children her body becomes more adapted to fend it off. However, as I think about it now, this could sort of fit in with the first theory. It could make sense that after you've already had 2 children, there likely wasn't going to be enough food to support like three grandchildren, so it could be advantageous for the third child to be gay and support the first two children's offspring.

Your idea about there being a female attraction gene occurred to me too. However, if I try to think about how the developers of the theory would respond, I think it would be something along these lines. Men are by their nature more sexually driven then women. They are less selective, more impulsive, and generally already don't turn down sex as often as women. I know its stereotypical but its statistically true. So, the thought that a female attracting gene that also caused women to be lesbian would lead to more child births instead of less seems unlikely to me.

Again, your point about how if the trait is recessive you could still have a homosexual brother and sister occurred to me. Still you should expect to see a strong negative correlation. From just personal experience it seems to me that the odds of a female with a gay brother being homosexual (or at least bisexual) are more likely positively correlated, or at least that of standard chance. That being said, that is my guy reaction. I have no evidence to back that up.

[Enaithor]

Well for a brother and sister I would expect there to be no correlation or a weak negative correlation if the genes are unrelated (my made up theory woo)
And it may well be that the female attraction gene could be not as strong - also, as testosterone is involved a lot in the male sexual response, the lack of presence of this in women would explain that. I would think that if the woman actually wants to have sex there would be more childbirth otherwise it's like, rape

[maverick]

I want a sociologist to study the link between homosexuality and artistic ability. Because I know so many queermo poets, painters, designers, stylists, and writers, it ain't even funny.

[Chandra]

Interesting. I don't know enough about this topic to comment much, but it occurs to me that if there is some type of gay gene, then societal pressure to marry the opposite sex and bear children has possibly helped to propagate it more than it would have if same-sex unions had been historically accepted. (In other words, the quickest way for those bible-thumpin' phobos to get rid of us is to let us hook up, already. )

[Steve712]

... if there is some type of gay gene ...

There is no singular gay gene, per se, but there are multiple genes which influence sexuality. It's called a polygenetic trait.

[Enaithor]

Originally Posted by Steve712

There is no singular gay gene, per se, but there are multiple genes which influence sexuality. It's called a polygenetic trait.

That is unconfirmed though. It's easy to assume that as the relationship between genetics and sexual orientation is complex that it must be polygenetic, but there are other things that complicate genes such as diathesis-stress and similar ideas.
(Obviously, diathesis-stress should only really be applied to actual disorders, as it implies a positive and negative, but you know what i mean)

[Steve712]

It's at least very likely, and there is supporting evidence for the idea that there are alleles rathers than a singular gene which influences sexuality. There is are two papers which I can remember off the top of my head which supports the idea of sexuality being a polygenetic trait.

The first used to be availalable publicly, but unfortunately I don't think that's so anymore. Here is the abstract, anyhow:

SpringerLink - Archives of Sexual Behavior, Volume 29, Number 1

The second is perhaps more relevant and, as it happens, is still publicly available:

http://courses.washington.edu/evpsyc...%20EHB2009.pdf

[crazyhead]

the phenomena that for each male a woman gives birth to, the subsequent male child's odds of being homosexual dramatically increase.

Population control. The more men a family has, the less the family actually needs to make any more children to thrive. If each subsequent male child still turns out straight, they'll just end up making more kids than they need. And eventually more kids than they can feed. x mouths to feed combined with not enough food leads to dying out.

[lowdownweather]

I've taken a few classes that ended up discussing not only genetics (inheritance, alleles, dominant/recessive, etc.) but also the molecular and cell biology aspects of gene expression. Just because a gene is there doesn't mean it's going to have any effect. Humans have several genes that code for enzymes that cows and other ruminants have that let them digest cellulose. Why can't humans? Because our copies of the gene are not expressed.

WARNING: this is gonna go pretty deep into the biology weeds. I'm the BIGGEST nerd about this kind of stuff. Some of it is my scientific (and unprofessional) opinion. I'm still a microbiology-major undergrad.

While the science on how much of a role genetics plays in complex traits is still quite active, I personally don't think something as complex as one's sexuality can be boiled down completely to genetics.

When they talk about traits involving multiple genes, they're usually talking about things like eye color, or what types of colors a flower will have, etc. Those sorts of traits are easy to link to multiple genes because each one codes for a different enzyme or protein that influences the outcome. While those are "complex," they are not anywhere near as complex as something like sexuality.

Everything sexual pretty much happens in your brain. Your genitals merely do what the brain tells them to. More on this in a (few) minute(s).

Before the hate mail starts flowing, I'm NOT equating homosexuality to depression/sickle-cell anemia/diabetes or calling it a disease. I'm merely using those conditions to illustrate the scientific point I am trying to make about nature vs. nurture, the things that influence the expression of ge

If you want to compare genetic vs. physiological bases (is there a plural for "basis"?) for a given trait, depression, sickle-cell anemia, and diabetes type 1 and 2 are all useful as comparisons.

Some traits, disorders and diseases have a very clear, definite genetic basis and others (as far as we can currently tell) don't. (Though this can definitely change as work continues).

One such disease would be sickle-cell anemia. A single point mutation in the gene that codes for the hemoglobin protein causes a different amino acid to be placed in the protein chain that eventually folds into hemoglobin. Then, the hemoglobin subunit can't fold properly. Some hemoglobins with this substitution can still fold properly and some can't. If enough hemoglobins in a red blood cell don't fold properly, the entire cell becomes sickle-shaped.

There's no real environmental influence or cause for it apart from those that would affect whether the mutated hemoglobin sickle-cell gene gets passed on (like malaria; malaria does not directly cause sickle-cell anemia but it indirectly helps the sickle-cell mutation get passed on as sickle-shaped red blood cells aren't as nutritious for the malaria parasite to eat; in this way, sickle-cell anemia actually becomes a survival advantage in areas where malaria is endemic.)

Type 1 diabetes is when you don't make enough insulin but you are still responsive to it if you are given it via injection or another method. As a type 1 diabetic, you don't make enough insulin because the cells in your pancreas that make it are attacked by your immune system, making type 1 diabetes an autoimmune disease. There are several genes that code for MHC proteins (MHC proteins are little tags that identify your cells as yours and foreign cells as foreign; it's one method your immune system uses to ID foreign cells) that they have ID'ed as contributing to type 1 diabetes. If I remember correctly, it's only the beta cells in the pancreas that express mutated versions of these MHC genes, thus making the cells appear foreign to your immune system.

Now, (again, if I am remembering this all correctly from my classes) variability in whether someone will get type 1 diabetes is not completely dependent on having mutated versions of the genes. Just because the beta cells have bad MHC protein tags on their surface doesn't automatically mean that they will get destroyed by the immune system. Something else might have to trigger an immune response to the beta cells if it isn't already underway. Now, this isn't as strictly based on genetics as sickle-cell is, but is still dependent on it for the most part. An autoimmune response would never start if you never had mutated MHC genes. Type 1 diabetes is a little closer to the "nurture" end of the spectrum than sickle-cell is, but it is still definitely on the "nature" (read: genetic) side.

Type 2 is much like type 1 in terms of a genetic basis: mutated versions of genes that involve insulin. In type 2, though, it's the receptors that are different, not the insulin-making cells. Mutated versions of the membrane proteins that respond to insulin contribute to type 2, but a genetic predisposition to type 2 is less likely to get you type 2 diabetes than a genetic predisposition to type 1 will be to get you type 1 diabetes. Lifestyle choices and physiological conditions (like being obese) well after birth can trigger it. It's closer to the "nurture" side of the spectrum than both type 1 diabetes and sickle-cell anemia.

Finally, in my little diatribe here, depression would be at the farthest end away from sickle-cell, closest to the "nurture" side of the spectrum. Some defects in the genes that code for receptors that take in serotonin can contribute to it, as can having too few alleles for one of the serotonin transporter proteins. Despite all of these things, environmental factors and life events (emotional trauma) so far seem to be more important in determining whether you have depression.

What about homesexuality? Personally, I think it's somewhere between type 2 diabetes and depression on the spectrum. Not completely genetic and more dependent on physiological factors in the brain outside of genetics. The uterus/mother's immune system reaction to successive male fetuses is one idea, but I think we'll get more possible explanations as neurophysiology makes more advances and molecular biology figures out more about gene expression.

Again, I'm not calling homosexuality a disease and am not trying to use biological explanations for its basis as justification for any sort of treatment/therapy for it. So, don't hate, please!

[Hitchhiker]

I also think this is fascinating and I enjoy reading others' responses, but I'm just curious; why is all the research done on gay men? There doesn't seem to be a large body of research on lesbians.

[Enaithor]

Originally Posted by Hitchhiker

I also think this is fascinating and I enjoy reading others' responses, but I'm just curious; why is all the research done on gay men? There doesn't seem to be a large body of research on lesbians.

I've always wondered that too...
Just random guessing, but I think it's because male homosexuality is in the media more? And in a significant minority of countries, male homosexuality is illegal but female homosexuality is legal
Also, I think for genetic purposes, if they think something is on a sex chromosome, when testing males, you know they got their X from their mother and their Y from their father, whereas with females you don't know which X came from where, without having to get at least one parent involved - experimental laziness, in a sense

[Steve712]

Female sexuality is underesearched in general, let alone female homosexuality. I wouldn't be able to say conclusively why, but I'm sure societal taboos and sexism have played their respective parts.