Is Being Gay Genetic? Unbiased Answers & Studies | PhysicsForums

[2AlphaMales?!]

What we don't know is why the hypothalamus was different in straight and gay sheep - was it genetic, or was it something that happened in the womb?

Many brain structures develop in later life. The medium strength correlation in similarities between cerebral hemisphere "gay brains" of humans and those of stright people of the opposite sex seeming the most pertinent example:

...noted in the article is that even though the scans suggest
similarities in how the brain functions between the aforementioned
groups, it doesn’t really tease out whether these findings are the
result of biology, environment, hormones, etc.

http://www.wellsphere.com/mental-health-article/certain-brain-scans-show-similarity-between-gay-men-heterosexual-women/413862

There also seems some significant confusion (no pun intended) as to whether the "gay rams" were always in fact bisexual. I've yet to read a study reporting non-human animals as being exclusively homosexual in anything approaching 8% of the population. Perhaps the full text of the study is online somewhere...

Overpopulation as a cause of some homosexuality has also received some support in the past, and sheep often live in artificially dense/engineered communities which could perhaps be causing this possible "neurological predisposition toward homosexuality" to become evident behaviourally.

Given the number of studies involving human brains and sexual orientation I'm not sure looking to sheep for answeres is entirely expedient in any case.

 

[CCWilson]

I have always had the strongest feeling that the certainty that some people seem to have that homosexuality is genetically programmed and thus has an evolutionary explanation is wrong. But your dismissal of the possibility of an evolutionary explanation for homosexuality is naïve.

What you are saying is that probably there's no evolutionary explanation for homosexuality, but that I dismiss the possibility is naive. Odd logic. I guess what you're saying is that we shouldn't be too sure of our opinions on this subject, which is true, but there's nothing wrong with drawing tentative conclusions from existing evidence. Without opinions, we wouldn't have anything to talk about.

Of course homosexual men are capable of mating with women. But there's an obvious evolutionary reason that heterosexual men have an intense desire for women - because it promotes sexual congress that will send his genes forward. There can't be many characteristics more important than that in terms of evolutionary success. Evolution works by making more prominent those features which give slight survival or reproductive advantages, and eliminating those features which are even slightly disadvantageous. What trait would be worse in terms of genetic spread than a lack of interest in heterosexual copulation? Granted, it's possible that there's some complicated combination of factors that are at play that we haven't considered, but in terms of basic evolutionary theory, what characteristic would be more disadvantageous than homosexuality? Sometimes we overthink things and overlook the obvious. Again, to me the most likely explanation is that evolution wasn't able to devise a totally reliable scheme for ensuring heterosexuality, and sometimes errors (from an evolutionary standpoint) are made. Nobody's perfect.

 

[Ken Natton]

What you are saying is that probably there's no evolutionary explanation for homosexuality, but that I dismiss the possibility is naive. Odd logic.

I doubt that homosexulaity is genetically programmed because my observation of real human behaviour does not seem to fit that idea. But dismissing the possibility on the basis that you did so is what I see as naive. When scientists first started to observe the behaviour of hymenoptra, it seemed absolutely impossible to imagine that there could be any evolutionary advantage to sacrificing your own life for the good of others. But then they looked a bit closer and found that there really is such an evolutionary advantage, and that behaviour is indeed, genetically programmed.

 

[CCWilson]

With bees and ants, the DNA is so close in all members of a colony that it isn't difficult at all to see how sacrificing oneself can send copies of your own genes forward.

Look, without further evidence we have to make intelligent, informed guesses as to how evolution ended up with the present system of sexual preference determination. But logic tells me that a suicide gene would be weeded out, and a gene for infanticide would be weeded out, and a sexual disinterest gene would be weeded out, and a gay gene would be weeded out. What genetic tendencies would be more important to eliminate than those, from the standpoint of one's ability to send his genes into the next generation? I'll be happy to reevaluate my position if a good argument against it comes along.

 

[Ryan_m_b]

But logic tells me that a suicide gene would be weeded out, and a gene for infanticide would be weeded out, and a sexual disinterest gene would be weeded out, and a gay gene would be weeded out. What genetic tendencies would be more important to eliminate than those, from the standpoint of one's ability to send his genes into the next generation? I'll be happy to reevaluate my position if a good argument against it comes along.

You're looking at this too simplistically. Individual genes rarely map directly to physical traits, let alone behavioural ones. But leaving that aside and assuming that there existed a genotype that causes suicide before sexual maturity there are plausible reasons for its existence. Remember evolution favours species, not just indivudual organisms and a genotype that confers a tendency for suicide may also confer a tendency in others that causes them to proliferate more. In the case of homosexuality from an evolutionary perspective if we ignore kin selection, assume no homosexual individual reproduces and assume that there is no sociological advantage to having a percentage of the population as homosexual then it could still persist if the mechanism that gives rise to homosexuality some of the time has a greater effect on the rest of the population than without it. What I'm getting at here is that if a species were "programmed" for sexuality then they would have a greater advantage even if a fraction of them are rendered "infertile" by it.

 

[Monique]

Remember evolution favours species, not just indivudual organisms and a genotype that confers a tendency for suicide may also confer a tendency in others that causes them to proliferate more.

Indeed, seemingly detrimental traits can have positive influences as well. For example sickle cell anemia in malaria-infested regions.

The idea that natural selection weeds out detrimental traits is flawed as well, just look at all the illnesses that humans suffer from. For a trait to disappear there needs to be a strong negative selection, otherwise it will persist in a population (Hardy-Weinberg equilibrium). Just since how long has it been acceptable to lead a homosexual lifestyle? Before that people with homosexual feelings were in heterosexual relationships and had children. Today people in homosexual relationships can have children as well.

 

[CCWilson]

Indeed, seemingly detrimental traits can have positive influences as well. For example sickle cell anemia in malaria-infested regions.

Actually sickle cell anemia is not a positive for survival; those people die early. It's sickle cell trait - the heterozygous version - that offers some protection against malaria, probably because of alterations of the red cells. There are some American football players with sickle cell trait, which is usually asymptomatic but is associated with a slight risk, especially with prolonged extreme physical activity without time for recovery. Off subject.

 

[Evo]

With bees and ants, the DNA is so close in all members of a colony that it isn't difficult at all to see how sacrificing oneself can send copies of your own genes forward.

Look, without further evidence we have to make intelligent, informed guesses as to how evolution ended up with the present system of sexual preference determination. But logic tells me that a suicide gene would be weeded out, and a gene for infanticide would be weeded out, and a sexual disinterest gene would be weeded out, and a gay gene would be weeded out. What genetic tendencies would be more important to eliminate than those, from the standpoint of one's ability to send his genes into the next generation? I'll be happy to reevaluate my position if a good argument against it comes along.

First, you need to provide peer reviewed studies that back you up. Personal opinions don't suffice in the science forums.

 

[CCWilson]

Evolution is different from most of the sciences. You can't do experiments to test hypotheses, except maybe to a very limited degree with bacteria and such. Let's be honest. Evolutionary science is mostly a matter of logic. Darwin's theory was based on a lot of observations, not of evolution in action, but of its end results. It was really a brilliant, logical conclusion with no peer reviewed studies to back it up. We know more and more about some of the details of how inheritance works, and how some genes accomplish their tasks, but when it comes to personality traits and how genes affect those changes, we don't know very much. As far as I know, we don't have a clue about how genetics and particular alleles affect the structure of the brains and its chemicals and connections to cause aggressiveness, sexual preference, empathy, selfishness, altruism, and so on. Let's be a little modest in our certainty that peer reviewers have anything useful to tell us about all that.

Bottom line, we don't have the tools yet to investigate the functioning of the brain at the neuronal level. The discovery of mirror neurons was extremely cool but I don't think it's been the major breakthrough we hoped it would be, except as an suggestion of what the mechanism might be for empathy and learning - maybe. Until we get a lot further in our ability to understand how genetics affects the structure and functioning of the brain, we're stuck with using logic, based on what we do know. All this discussion that we're reading now about whether morality and religion are determined by what's in our genes - fascinating subjects - do you think that's come about because of scientific breakthroughs? No, it's the result of people trying to put two and two together based on what we do know.

The best peer reviewers can do when an evolution hypothesis is presented to them is pass judgment on whether it logically fits the facts.

What happened when somebody suggested that maybe homosexuality was selected for, or at least not eliminated, because an extra caregiver might increase survival of kin? Were experiments conducted to demonstrate its truth? If they were, we should have the results in 2000 years or so. No, presumably peer reviewers, if there were any for that idea, would have tried to figure out if that made more sense than other theories, would have tried to fit the possibilities into Hamilton's equation or some such, and they would have published their opinion, which might be right. Or not.

 

[atyy]

But logic tells me that a suicide gene would be weeded out, and a gene for infanticide would be weeded out, and a sexual disinterest gene would be weeded out, and a gay gene would be weeded out.

Here's some discussion in the literature which may be helpful for this thread.

http://www.ncbi.nlm.nih.gov/pubmed/20141266
http://homepage.psy.utexas.edu/homepage/group/busslab/pdffiles/evolutionary_psychology_AP_2010.pdf
"Exclusive homosexual orientation seems to defy evolutionary logic since it presumably fails to increase an individual’s reproductive success. Although evolutionary hypotheses have been proposed for homosexuality, as discussed earlier, none have received empirical support thus far (e.g., Bobrow & Bailey, 2001)."

http://www.ncbi.nlm.nih.gov/pubmed/19539396
http://www.faculty.ucr.edu/~mzuk/Bailey and Zuk 2009 Same sex behaviour.pdf
"Previous work has emphasized the apparent paradox of selection acting on non-reproductive individuals [1,2],"

"Over the past two decades, Drosophila researchers have examined a multitude of candidate genes implicated in the genetic and neurological control of sexual behavior. These studies have provided insights into sexual behavior in general, and as a byproduct have illustrated different mechanisms that can independently produce same-sex sexual behavior. What are these mechanisms, and what can we learn from them? Mutations in a Drosophila gene called fruitless have been known for nearly half a century to cause males to court other males [4]."

 

[EagleNebula]

Speaking from my own knowledge (which is quite limited on this subject) I think so. If we are talking about male sexuality that is, because that's where the most research has been made. For example, let's take a family. There is one father, one mother and four boys. The fifth is on it's way (and he'll be gay in this scenario). Is there any rational explanation to why the chances of getting a gay son would increase? Yes. But wait, so what is happening biologically to the baby boy?

When a mother is pregnant with a boy, the female body sees the male fetus as a foreign object. For protection, the female body sends out antibodies to the boy in the womb. The more antibodies the mother have, the more adapt her body becomes at feminizing the male fetus. Thus, giving birth to gay son.

Of course this not enough to say with 100% confidence that there is something as a gay gene, but I see result like these to be of value. At least we can see that it's not totally a decision for everyone. Maybe I'm talking total rubbish, but that's my opinion right now.

 

[Ryan_m_b]

Speaking from my own knowledge (which is quite limited on this subject) I think so. If we are talking about male sexuality that is, because that's where the most research has been made. For example, let's take a family. There is one father, one mother and four boys. The fifth is on it's way (and he'll be gay in this scenario). Is there any rational explanation to why the chances of getting a gay son would increase? Yes. But wait, so what is happening biologically to the baby boy?

When a mother is pregnant with a boy, the female body sees the male fetus as a foreign object. For protection, the female body sends out antibodies to the boy in the womb. The more antibodies the mother have, the more adapt her body becomes at feminizing the male fetus. Thus, giving birth to gay son.

Of course this not enough to say with 100% confidence that there is something as a gay gene, but I see result like these to be of value. At least we can see that it's not totally a decision for everyone. Maybe I'm talking total rubbish, but that's my opinion right now.

Welcome to the forums. I'm afraid to say that this is mostly total rubbish. Genes are sequences of DNA that are transcribed into RNA, they are not created or altered on the basis of antibodies in the womb. In fact if the mother rejected the embryo in the manner you describe it would not result in a viable pregnancy.

Whilst it has been reported that birth order and sibling sex and number have a statistically significant effect on a child's sexuality the mechanism is likely to be far more subtle and complex than simply a case of genetics.

 

[Antiphon]

Not rubbish at all. The probability of a homosexual male offspring increases dramatically with the number of offspring.

It seems to me the evolutionary function is obvious- it prevents anyone set of parents from monopolizing the gene pool by creating too many boys.

Suppose a couple decided to have 20 children and they all turned out boys. In a small clan this would greatly reduce the diversity if they all took wives. By ensuring that only the first 3-4 of the males are interested in reproducing the diversity is maintained.

 

[Ryan_m_b]

Not rubbish at all.

Aside from the mention that number of siblings has an effect on sexuality I don't see how you can say this.

 

[Borek]

From The Red Queen: Sex and the Evolution of Human Nature by Matt Ridley:

How could such a gene survive, given that gay men generally do not have children? There are two possible answers. One is that the gene is good for female fertility when in woman, to the same extent that it is bad for male fertility when in men. The second possibility is more intriguing. Laurence Hurst and David Haig of Oxford University believe that the gene might not be on the X chromosome after all. X genes are are not the only genes inherited through the female line¹. So are the genes of mitochondria, described in chapter 4, and the evidence linking the gene to a region of the X chromosome is still very shaky statistically. If the gay gene is in the mitochondria, then a conspiracu theory springs to the devious minds of Hurst and Haig, Perhaps the gay gene is like those "male killer" genes found in many insects, It effectively sterilizes males, causing the diversion of inherited wealth to female relatives. That would (until recently at least) have enhanced the breeding success of the descendants of these female relatives, which would have caused the gay gene to spread.

¹Earlier he wrote that there is some kind of evidence that the gay gene is inherited from the mother and not from the father.

 

[Ryan_m_b]

From The Red Queen: Sex and the Evolution of Human Nature by Matt Ridley:

¹Earlier he wrote that there is some kind of evidence that the gay gene is inherited from the mother and not from the father.

Perhaps there is more not stated here but this seems to ignore the wealth of sexual diversity seen in human societies as well as ignore female sexuality.

IMO discussions around sexuality from a biological standpoint only are insufficient. If we are to determine what it is about biology that leads to sexuality we need to look at the social sciences (and especially queer studies) to parameterize the discussion. For starters we'd do well to get rid of the notion of discrete sexuality i.e homo/bi/hetero in favour of something more representative of human experience.

 

[Borek]

This is just one paragraph from the whole chapter. And I have posted it to show possible explanations do exist. Whether they are correct and/or final is another question.

 

[CCWilson]

It's not impossible that a gene by chance had two effects - one for homosexuality which was negative with regard to reproductive success and one with some other positive effect on survival or reproductive success. But it seems unlikely that a random mutation which caused both a tendency toward homosexuality and some survival advantage would be of benefit in terms of Darwinian success. Also I doubt that there's any parallel to insect inheritance, since insects operate almost exclusively by instinct and humans don't. And the idea that an increasing rate of homosexuality in women who've had sons before is evidence of a gay gene is almost surely incorrect; that finding is much more compatible with the effects of changing hormones in the mother.

Here's what I've said before. Think of how sexual attraction works. We all have the plumbing to engage in heterosexual or homosexual activity. We all find members of both sexes attractive or unattractive in a non-sexual way. What determines sexual orientation is which body parts and general appearance characteristics are sexually stimulating. That has to do with connections of neurons in the brain somehow, and how that's wired to accomplish sexual preference we really don't have a clue. And we know that the hormones produced by both mother and embryo affect the development of our sex parts. If the hormones go wrong, there may be errors in the formation of penis, uterus, and so on - regardless of whether there are one or two X chromosomes. So if physical sexual characteristics are dramatically affected by hormones, it's pretty likely that sexual preference, the setting of which in the brain is much more subtle, I suspect, is likewise affect by hormones and maybe some other conditions in the womb.

So my conclusion, which is strongly supported by the increased rate of homosexuality in later sons, is that homosexuality is a developmental error (in Darwinian terms) caused by hormonal or other influences and not by one or more specific genes. It goes without saying that homosexuality is an error only in evolutionary terms and has nothing to do with the worth of those individuals; some errors are beneficial.

 

[Antiphon]

Aside from the mention that number of siblings has an effect on sexuality I don't see how you can say this.

I can say it because its a well-established phenomenon.

http://en.m.wikipedia.org/wiki/Fraternal_birth_order_and_male_sexual_orientation

http://userwww.service.emory.edu/~afranc5/Family%20and%20Sexual%20Orientation.pdf

 

[CCWilson]

Antiphon, you're not thinking clearly on this. Assuming the same father, the first born and the last born have the same chances of having certain genes. To say that the difference in frequency of homosexuality due to birth order is genetically related is not logical. Let's say, for the sake of argument, that a woman has four boys, each of which has the same genes, at least with respect to sexual orientation. If it were entirely genetic, all four would either be homosexual or heterosexual. Since boy #4 has a greater chance of being gay, according to those studies, it's likely that factors in the womb, not the DNA, are the reason for that. We know that the woman's ability to produce hormones changes with time and pregnancies, so the obvious conclusion is that bathing the fetus in slightly different hormones results in different outcomes.

 

[Antiphon]

It's not the genes inherited that predispose the offspring to homosexuality. It's the hormonal cocktail in the womb.

This argues against a gay inherited gene but still militates in favor of a genetic motivation for the presence of homosexuality. It's enforced by the mother's biochemistry as programmed by *her* genes.

Again, the conferred advantage is twofold; no one couple will dominate the male transmission to the next generation, but the excess males, as uncles, can still aid in survival of the original line.

Edit: it follows from this that homosexuality is not an error at all- it's a genetic diversity strategy that works for medium sized clans which is how the species grew up. Does this help in a city of 8 million? No, but neither does the tonsil anymore.

 

[marty1]

No. There is not.

 

[Antiphon]

No. There is not.

Apparently there is, but it's not carried exclusively by or expressed in gays; it's carried (presumably) by all women and perhaps men, but expressed exclusively in childbearing women and progressively with each pregnancy.

 

[CCWilson]

Antiphon, I thought you were arguing in favor of a gay gene in the conventional sense. But apparently you believe, as I do, that it's most likely the change in hormones in the uterus from first born to last born that results in higher rates of homosexuality. I agree that it's possible that this is something that evolution has found to be useful in every mother's genetics, but that's far from a settled question. Evolution doesn't do everything perfectly; it makes plenty of design mistakes; and it seems more likely to me that this is one of them than it is a group selection effect.

 

[Pythagorean]

When I hear about a "this" gene or a "that" gene, I tend to think of it as meaning 'genes associated with a disposition towards this or that'; not as a deterministic, life damning, molecule of human puppeteering, but as a shorter path to some behaviors or diseases (stress-diathesis).

 

[Antiphon]

Antiphon, I thought you were arguing in favor of a gay gene in the conventional sense. But apparently you believe, as I do, that it's most likely the change in hormones in the uterus from first born to last born that results in higher rates of homosexuality. I agree that it's possible that this is something that evolution has found to be useful in every mother's genetics, but that's far from a settled question. Evolution doesn't do everything perfectly; it makes plenty of design mistakes; and it seems more likely to me that this is one of them than it is a group selection effect.

Yes, many mistakes and dead ends get tried out. But they should get rejected when there's no advantage and a heavy cost. If homosexuality had no evolutionary advantage then I'd think the reasoning about it getting weeded out would apply. It would be tried and rejected, or at least not be expressed so much. It's an expensive thing to have a sizable portion if your population not participate in genetic propagation. It would be as if 2-5% of the population were born sterile. It's too expensive to do unless there's a survival advantage that offsets the cost.

 

[CCWilson]

Whether the survival advantages of adding non-reproductive participators outweigh the cost - that's what I question. Less food for everybody else in exchange for additional warriors and caregivers - I'm not sure how that equation works out.

Just as we peak in our athletic abilities in our early twenties, even though it would be better for personal and group survival to maintain our athleticism, it may be that a woman's reproductive capabilities naturally decline as well, just from wear and tear.

 

[Ken Natton]

These forums discourage mere assertion of opinion and prefer reference to evidence based conclusions. As has been discussed further up this thread, the reality is that the answer to this question remains unknown to science and it is not particularly certain if or when science will know more. But, as seems to be usual with this topic, there are some strikingly obvious false assumptions running through the posts made on this thread since its revival, centrally that homosexuality has anything whatever to do with non-reproduction. Lots of homosexuals have children. The desire to have children and all of the emotions associated with children are actually quite separate from the sexual urge. Again, the overwhelming majority of human sexual behaviour has nothing whatever to do with reproduction.

 

[CCWilson]

Ken, homosexuality would, I'm pretty sure, reduce the number of offspring. I suspect that statistics would bear that out today, as would logic. I'm sure that nobody here believes that homosexuals don't have kids, just fewer. It may not have expressed exactly that way, but we aren't stupid.

Darwin didn't observe evolution occurring. He saw the results after the fact. He didn't do experiments. His theory was a triumph of logic, based on available evidence. The time scale required for evolution of human behavior makes it impossible to observe or to conduct experiments. The best we can do is apply our powers of logic - what were the survival or reproductive advantages that would promote a "gay gene"? What does the available evidence suggest? Evolutionary science is different from other disciplines. If you read the literature, you'll find that most new proposals are speculative and are judged on how well they fit whatever evidence there is.

 

[Pythagorean]

So... why don't you provide the evidence, rather than attaching the word "logic" to it and "stupid" to the opposing view. I don't find that to be very sound reasoning.

 

[Ryan_m_b]

Ken, homosexuality would, I'm pretty sure, reduce the number of offspring. I suspect that statistics would bear that out today, as would logic. I'm sure that nobody here believes that homosexuals don't have kids, just fewer. It may not have expressed exactly that way, but we aren't stupid.

Rather than assume it would be best to look for some evidence bearing in mind the cultural attitudes towards homosexuality throughout human history.

Darwin didn't observe evolution occurring. He saw the results after the fact. He didn't do experiments. His theory was a triumph of logic, based on available evidence. The time scale required for evolution of human behavior makes it impossible to observe or to conduct experiments. The best we can do is apply our powers of logic - what were the survival or reproductive advantages that would promote a "gay gene"? What does the available evidence suggest? Evolutionary science is different from other disciplines. If you read the literature, you'll find that most new proposals are speculative and are judged on how well they fit whatever evidence there is.

This is misleading, of course evolutionary hypothesis can be tested experimentally. That's why we have model organisms.

 

[CCWilson]

Geez, guys. I said that WE weren't stupid, not that anybody else was. And remind me again how we can test human behavioral genetics experimentally. We can do evolutionary experiments on organisms that reproduce thousands of times as fast as we do, but not on humans, especially regarding things as subtle as personality traits and sexual preference. And any conclusions I draw are based on logic, trying to find the best explanation based on the evidence we have - in this case, the increased rate of homosexuality as the number of children increases - with absolutely no value judgments considered or implied.

 

[Pythagorean]

"Logic" or intuition? Our intuition in science is to not trust our intuition without evidence.

To answer your question about measuring behavioral genetics: twin studies

 

[CCWilson]

Darwin's theory was based strictly on logic - and intuition. He came up with an explanation that best suited the evidence. He didn't observe evolution in action. He didn't "prove" anything. Should we throw out his theory?

How do you propose that we prove why certain people are homosexual and others are heterosexual and how genetics are involved?

I think there's a fundamental misunderstanding of how advances in evolutionary theory come about. It all starts with speculation. A premise is proposed, and people examine it to see if it fits the existing evidence. If it makes more sense than previous theories, it becomes accepted. For example, there's a controversy about whether group selection (also called multi-level selection) occurs in evolution - whether certain traits are selected for because they benefit the group rather than the individual. Most experts say no, E.O. Wilson says yes. There are heated discussions about which side best fits the evidence. It's all based on logic, because there's damn little hard evidence.

Should we tell them to all shut up, because nobody can prove it definitively one way or the other? After all, they're just using - horrors - logic?

Oh, and twin studies are part of the evidence, as is the increased rate of homosexuality with additional sons. Still nothing is proved. Should we therefore stop thinking and discussing this issue?

 

[Pythagorean]

That's all well and good to practice in your own institution when writing grants and publishing papers, but at physicsforums, we try to stick to mainstream, accepted, theory.

Anyone can sit around and speculate hundreds of different "logical" explanations, but can you do the initial research and provide evidence? Any number of experts are likely already working on some manifestation of the answer to the question and may have already cleared the logical pitfalls that you might be falling into.

So maybe you provide the right answer, maybe you don't. Who knows? You're just speculating, not providing the actual evidence. It's not about "proving" in the rigid, mathematical sense, it's about providing evidence (time and time again) that supports your assertions. That is where evolution is now, that is why we accept it; not just because it sounded logical to Darwin, but because we've been able to reject a century of null hypotheses through quantitative analysis of the evidence.