# Evolution and the prevalence of homosexuality **URL:** \ **Category:** Factual Questions\ **Created:** [November 12, 2014, 4:05pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458 "2014-11-12T16:05:35Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 12, 2014, 9:30pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/21 "2014-11-12T21:30:13Z")
> [@Morgenstern](#): > > That study seems flawed for several reasons. 200 is a very small sample, and it’s a sample that’s limited to a Roman Catholic culture. That culture tends to have a more children due to religious practices. I’d be careful of putting too much faith in that finding. Good thing then that there has been a follow-up study with a larger and more diverse group that found the same result: > [@](#): > > Here, we suggest that the empirical support for kin selection and maternal effects cannot account for the low universal frequency and stability of the distribution of homosexuals. To identify the responsible compensatory mechanism, we analyzed fecundity in 2,100 European female relatives, i.e., aunts and grandmothers, of either homosexual or heterosexual probands who were matched in terms of age, culture and sampling strategy. Female relatives were chosen to avoid the sampling bias of the fraternal birth order effect, which occurs when indirectly sampling mothers though their homosexual sons. We observed that the maternal aunts and grandmothers of homosexual probands were significantly more fecund compared with the maternal aunts and maternal grandmothers of the heterosexual probands. No difference in fecundity was observed in the paternal female lines (grandmothers or aunts) from either of the two proband groups. Moreover, due to the selective increase in maternal female fecundity, the total female fecundity was significantly higher in homosexual than heterosexual probands, thus compensating for the reduced fecundity of homosexuals. > > SOURCE: [Fecundity of Paternal and Maternal Non-Parental Female Relatives of Homosexual and Heterosexual Men](http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0051088) ---
**Author:** ![clairobscur](https://avatars.discourse-cdn.com/v4/letter/c/839c29/32.png) [@clairobscur](https://boards.straightdope.com/u/clairobscur)\ **Post date:** [November 12, 2014, 9:35pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/22 "2014-11-12T21:35:04Z")
> [@Morgenstern](#): > > That study seems flawed for several reasons. 200 is a very small sample, and it’s a sample that’s limited to a Roman Catholic culture. That culture tends to have a more children due to religious practices. Presumably, they were comparing the families in the sample. Female relatives (most of them Roman catholic) of gay people were more fecund than females (also Roman catholic) who didn’t have a gay relative. If you study genes related to the production of melanin, it doesn’t matter whether you’re studying Congolese or Norwegians, as long as you don’t compare Congolese with Norwegians. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 12, 2014, 9:42pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/23 "2014-11-12T21:42:01Z")
> [@Urbanredneck](#): > > Modern San Fransisco, Paul Lynn-Ellen Degeneres homosexuality is different from the concept of plain sexual attraction to ones own gender. In the past people with those attractions just fell into life and societys norms and had families and children while just seeing their own gender on the side. > > Its only in modern times where they can live openly and all. Depends on the society. This may be true in recent Western societies, but not always in non-Western traditional societies. Some Native American societies were accepting or at least tolerant of homosexuals, sometimes as a “third sex” or as men who lived as women. ---
**Author:** ![mr.jp](https://avatars.discourse-cdn.com/v4/letter/m/48db29/32.png) [@mr.jp](https://boards.straightdope.com/u/mr.jp)\ **Post date:** [November 12, 2014, 9:53pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/24 "2014-11-12T21:53:16Z")
> [@Smeghead](#): > > First, and most glaringly, you’re making a HUGE underlying assumption: that homosexuality is genetic. It’s not. Or at least not entirely. According to wikipedia “genetic effects explained .34–.39 of the variance” Which is quite a substantial proportion, enough to say that it is genetic, although not as highly genetic as say, height. > [@](#): > > Second, you’re assuming that homosexuality reduces fitness. This may be true, particular in modern times, but I’d want to see hard evidence that this has been true for enough time to impact human evolution. Even today, there are plenty of homosexual people who have children for one reason or another. Back in the days when social pressures were higher to keep these urges secret, we would expect that even more homosexual people would have married and reproduced. I addressed this (shortly). Of course there is no hard evidence here, and there never will be. But it seems pretty clear to me that homosexuality will affect the number of offspring. I would expect the homosexual male to tend to have less sex with his wife, to tend to find her later, to tend to not care as much about her looks, and to tend to not be as committed to the relationship as a heterosexual male. And these things affect the number of offspring and the fitness of the offspring. > [@](#): > > Selective pressure needs to be applied for a long, long time in a species like humans, with high population numbers and long generation times, before it can have any effect. That depends on the fitness effect of the trait. A fitness effect of ±1% will be fixed in the population rather quickly. An example of this is the recent adult milk drinking mutation. > [@](#): > > Finally, I’d just say that as a biologist, I think it is generally a big mistake to attempt to understand very complex human behavior and society using just this sort of very very simplistic Darwinian point of view. There are just too many other factors acting, and there’s a HUGE risk of slipping into an evolutionary “just-so” story way of explaining things, without bothering with any sort of evidence or research. I agree to some degree. I guess what I am interested in is the most plausible just so story (even though it wouldn’t be backed up by hard evidence), and I don’t think any of the ones I am familiar with are plausible. Also, I don’t think it’s necessarily like this. Perhaps the right explanation will come along, and it will be obviously correct. Like, assume we didn’t know the evolutionary reason that elephants have big ears. Then I could ask for it, and people could say: “it is not fully genetic”, “selection takes a long time to work”, “perhaps it’s neutral to fitness”, “perhaps it’s to scare away predators”, “perhaps it’s too simplistic Darwinian to expect to make a story about it”, etc etc. But there is a more plausible explanation that I just wouldn’t have thought of, and perhaps that is the case here too. ---
**Author:** ![mr.jp](https://avatars.discourse-cdn.com/v4/letter/m/48db29/32.png) [@mr.jp](https://boards.straightdope.com/u/mr.jp)\ **Post date:** [November 12, 2014, 9:54pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/25 "2014-11-12T21:54:24Z")
> [@Whack-a-Mole](#): > > The OP missed another possibility: > > Women related to homosexual males are more fecund (have more children): That’s #2. ---
**Author:** ![mr.jp](https://avatars.discourse-cdn.com/v4/letter/m/48db29/32.png) [@mr.jp](https://boards.straightdope.com/u/mr.jp)\ **Post date:** [November 12, 2014, 10:17pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/26 "2014-11-12T22:17:16Z")
> [@Colibri](#): > > However, there are many other behavioral characteristics that probably reduce reproductive success, and we rarely ask if there is a genetic component maintaining them in the population. These may include simply being asexual, celibacy due to religious reasons, or a degree of shyness or social awkwardness that precludes ever getting a date. The percentage of the population going childless for these reasons may well exceed that of homosexuals. I think this is a great point, and I have thought about this as well. There are explanations for these that sound somewhat more plausible (to me) than the explanations for the prevalence of homosexuality. Eg. celibacy for religious reasons could be caused by an even more powerful evolutionary force causing children to believe intensely and unquestioningly in what their parents say. Social shyness could be a result of that the number of people you would meet in your lifetime was much lower in prehistoric times, and therefore the impression that you make on any individual actually is extremely important. Asexuality is equally puzzling to me though, and I considered including it in the OP. Also abnormal sexual attractions such as pedophilia or zoophilia. It seems to me that evolution is tripping up over something that should be simple and is hugely important. If I were to design a creature, the first thing I would do is make it desire to survive, and the second thing I would do is make it want to make babies or do things that would cause it to make babies. ---
**Author:** ![Essured](https://avatars.discourse-cdn.com/v4/letter/e/278dde/32.png) [@Essured](https://boards.straightdope.com/u/Essured)\ **Post date:** [November 12, 2014, 10:21pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/27 "2014-11-12T22:21:03Z")
> [@md2000](#): > > Also, based on human nature, female sexual urges probably didn’t figure much into their ultimate reproductive outcome, knowing what we do of typical male behaviour over the course of history. So there wouldn’t be any selection there anyway… Do you have any cite for this, or is it just a WAG? I find it _very_ strange to assume that female choice would have no evoutionary impact. Why would it be one-sided? It would be extremely interesting if that was the case. It’s not like forcible rape is the most common method of reproduction in humans, or most of the primates (AFAIK). Birds for example - don’t many species have the male competing via dance, plumage & nest-building with the female selecting her mate based on what she sees? I’m sure there are similar examples in the primate world, if only because the primate world covers vast differences. So yeah, just wondering if there are actual scientific / evolutionary theories that suggest female mate selection has no impact on reproductive outcome? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 12, 2014, 10:21pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/28 "2014-11-12T22:21:58Z")
> [@mr.jp](#): > > That’s #2. Ah, so you did. My bad. > [@](#): > > My thoughts: I think that evolution can find alternative ways to achieve this, such as encoding it on the X chromosome. Also, I don’t think the increased fecundity of the female children would be enough to counterweight the reduced fecundity of the homosexual male children. I will disagree with your thoughts on this. First, there is nothing that says evolution must take the most efficient or sensible route. Very often it doesn’t (e.g. the 15 foot long laryngeal nerve in a giraffe’s neck). There may be some explanation for why a trait for homosexuality is passed the way it is but for all I know it was just chance that it worked out the way it did and it turned out to be an overall beneficial trait for the species. Second, increased fecundity of females easily outweighs males. It is not a 1:1 relationship. A single male can father many dozens of children. Dropping 2% of males out of the pool seeking female partners doesn’t even make a dent. The other males will happily and easily fill the gap the gay males leave. Females on the other hand have relatively few children so a woman who has six children instead of four or two is not so easily counterbalanced. Mother Nature would much rather have the females more likely to churn out numerous babies. Whatever males are around will likely be happy to oblige them. ---
**Author:** ![mr.jp](https://avatars.discourse-cdn.com/v4/letter/m/48db29/32.png) [@mr.jp](https://boards.straightdope.com/u/mr.jp)\ **Post date:** [November 12, 2014, 10:41pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/29 "2014-11-12T22:41:21Z")
> [@md2000](#): > > My theory - there’s a series of “recognizers” our brains are preprogrammed to recognize things - healthy, young, old, cute (as in babies and puppies are “cute” attractive), repulsive, etc. This is how evolution has shaped our brains and the way they work. In order to construct he complex mechanism of our brains, things have to connect “just right”. If during gestation the program (for lack of a better word) for what turns someone on accidentally develops or gets connected to the program designed for the other sex, then that’s a price to be paid for having a flexible system that allows a strong sex drive in the first place. I really like this theory. I’m not convinced though. Couldn’t it work a little harder at getting those last 5ish percent connected right? One thing I find interesting is the strong finding that the odds of a man being homosexual increases by 33% by each older brother. This tells me that there is something that could be done to reduce the chance of offspring homosexuality by at least 33%. Actually it looks to me like a system actively trying to create a varied batch. First making some play-it-safe heterosexuals, and then increasing the chances of getting some homosexuals, which will confer some sort of advantage. ---
**Author:** ![DinoR](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dinor/32/2821_2.png) [@DinoR](https://boards.straightdope.com/u/DinoR)\ **Post date:** [November 12, 2014, 11:44pm UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/30 "2014-11-12T23:44:45Z")
> [@mr.jp](#): > > If I were to design a creature, the first thing I would do is make it desire to survive, and the second thing I would do is make it want to make babies or do things that would cause it to make babies. You designed a rabbit 😉 /tongue in cheek Those first two rules already have some areas of conflict that would need subrules to sort out. Babies take resources which affects survival of current, self sustaining, breeding age adults. Unless you can kick new babies out very quickly after birth and have good survival rates to breeding age, the balance between those two basic rules is touchy. ---
**Author:** ![slowlearner](https://avatars.discourse-cdn.com/v4/letter/s/a88e57/32.png) [@slowlearner](https://boards.straightdope.com/u/slowlearner)\ **Post date:** [November 13, 2014, 12:23am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/31 "2014-11-13T00:23:58Z")
Too easy for it all to sound like, homosex doesn’t create a fitness advantage, therefore there is something fundamentally wrong with it, or fundamentally wrong with the omniscient evolution that cannot seem to get rid of it. In fact, it is only fundamentalist cultures like the one that dominated Western Europe so long, or the fundamentalists over there who find homosex wrong, and messing around with the wrong sex always persists in the ignorant cultures that find it distasteful. Evolution has nothing to say about sexual preference, some are born gay, some choose to be gay, some just want to jump on anything that comes along on 2 legs. The science of evolution is as challenging and difficult to understand as quantum mechanics, don’t know why people are so quick to think they have some special insight into it, I sure don’t… ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 13, 2014, 12:40am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/32 "2014-11-13T00:40:08Z")
> [@Colibri](#): > > Depends on the society. This may be true in recent Western societies, but not always in non-Western traditional societies. Some Native American societies were accepting or at least tolerant of homosexuals, sometimes as a “third sex” or as men who lived as women. And we have virtually no idea how homosexuality was seen in ~99% of our evolution as a species. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [November 13, 2014, 2:11am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/33 "2014-11-13T02:11:50Z")
Another possibility is that while homosexuality may be congenital, it is not necessarily genetic. Quite some time ago (and I’m sorry I have no cite) I read about a study of pigs. It turns out that a male pig whose position in the womb was between two females was more likely to be homosexual than one whose position was between two males, with similar results for females. So it might be that the womb environment plays a big role. Of course with humans who are not often born as part of a set of triplets, it cannot be as in pigs. Nonetheless the environment in the womb might determine the state of the fetus. This pushes the problem back one generation. Why don’t women have more welcoming wombs? Well, maybe they have no way of determining sex and it is better for females to get plenty of estrogen and males to get more androgen. But I don’t know; at this point it is all speculation. But suggestive, no? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 13, 2014, 2:22am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/34 "2014-11-13T02:22:57Z")
> [@Hari\_Seldon](#): > > Another possibility is that while homosexuality may be congenital, it is not necessarily genetic. Quite some time ago (and I’m sorry I have no cite) I read about a study of pigs. It turns out that a male pig whose position in the womb was between two females was more likely to be homosexual than one whose position was between two males, with similar results for females. So it might be that the womb environment plays a big role. Of course with humans who are not often born as part of a set of triplets, it cannot be as in pigs. Nonetheless the environment in the womb might determine the state of the fetus. Actually, birth order of males has been implicated in homosexuality in some cases in humans. [Fraternal birth order and male sexual orientation.](http://en.wikipedia.org/wiki/Fraternal_birth_order_and_male_sexual_orientation) > [@](#): > > A correlation between fraternal birth order and male sexual orientation has been suggested by research. Ray Blanchard identified the association and referred to it as the fraternal birth order effect. In several studies, the observation is that the more older brothers a man has, the greater the probability is that he will have a homosexual orientation.[1] It has sometimes been called the older brother effect. It has been estimated that 15% of the homosexual demographic is associated with fraternal birth order.[2] > [@](#): > > An in-utero maternal immune response has been hypothesized for this effect.[6][16][20][21] The effect is present regardless of whether or not the older brothers are raised in the same family environment with the boy. There is no effect when the number of older brothers is increased by adopted brothers or stepbrothers. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 13, 2014, 3:39am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/35 "2014-11-13T03:39:33Z")
> [@](#): > > Quoth **mr. jp** : > > It seems to me that evolution is tripping up over something that should be simple and is hugely important. OK, important I’ll grant you. But what on Earth makes you think sexual behaviors are simple? ---
**Author:** ![sbunny8](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@sbunny8](https://boards.straightdope.com/u/sbunny8)\ **Post date:** [November 13, 2014, 5:01am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/36 "2014-11-13T05:01:36Z")
> [@Colibri](#): > > Actually, birth order of males has been implicated in homosexuality in some cases in humans. > > [Fraternal birth order and male sexual orientation.](http://en.wikipedia.org/wiki/Fraternal_birth_order_and_male_sexual_orientation) That’s what I said in post #13, but I didn’t have a cite. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 13, 2014, 5:13am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/37 "2014-11-13T05:13:57Z")
> [@sbunny8](#): > > That’s what I said in post #13, but I didn’t have a cite. I recalled that someone had mentioned this earlier, but didn’t go back to check what was said. ---
**Author:** ![UltraVires](https://avatars.discourse-cdn.com/v4/letter/u/ecccb3/32.png) [@UltraVires](https://boards.straightdope.com/u/UltraVires)\ **Post date:** [November 13, 2014, 7:49am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/38 "2014-11-13T07:49:43Z")
> [@dofe](#): > > Evolution occurs through populations, not individuals. Even in humans, we see traits that may be disadvantageous to individuals remain in certain populations because they increase the fitness of a population as a whole. The classic example is [sickle cell trait](http://en.wikipedia.org/wiki/Sickle-cell_disease#Genetics): here, an inidividual who receives defective copies of the gene from both parents will develop sickle cell anemia early and (in pre-modern times) likely die before he or she gets a chance to reproduce. However, an individual who receives one defective and one normal copy of the gene from the parents (and is thus heterozygous) will not only remain asymptomtic from sickle cell disease, but will experience stronger resistance to the parasite that causes malaria. Because this mutated gene confers advantages to the survival of the population as a whole, it was selected for in populations living in areas where malaria was prevalent, even if it is harmful to certain individuals. But it is still the advantage to individuals that cause the anomaly to prevail. In your example it is the individuals who pass on the sickle cell gene because it benefits them (or at least doesn’t kill them before they can reproduce). The “gay uncle” hypothesis doesn’t work the same way because any genetic anomaly, even if helpful to society, would die with him and not be passed on to the next generation. I vote for the in utereo suggestions stated upthread. That makes the most sense. ---
**Author:** ![SlackerInc](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slackerinc/32/275_2.png) [@SlackerInc](https://boards.straightdope.com/u/SlackerInc)\ **Post date:** [November 13, 2014, 9:17am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/39 "2014-11-13T09:17:10Z")
> [@marshmallow](#): > > I don’t think it’s been shown that it even has to be genetic. It could all be due to hormones in the womb. Right. I remember reading years ago that women with male fetuses who experienced severe stress at a very specific stage in pregnancy were more likely to bear boys that went on to be gay. An example was women who were in London during the Blitz. Googling this just now, I found a [cite](http://www.neoteny.org/neoteny/a/homosexuality.html), but I can certainly understand how this could be a radioactive area of scientific inquiry (similar to race and IQ): on the one hand, it disproves the notion that homosexuality is a “lifestyle choice”, but it also comes awfully close to sounding like a birth _defect_. I also like the “gay uncle” hypothesis, at least as a corollary that helps explain why a gene would not develop that would actively cancel out these congenital effects. There are many evolutionary survival strategies (ESS) that achieve relatively equal success despite radically different “methods”, and thus fall into a semi-stable stasis or cycle in a somewhat predictable way. For the most obvious instance: quantity over quality can work: go spread your seed as far and wide as you can. But not all men do this, and though women’s mate selection is a big part of it, it’s also because sticking around and really concentrating resources on a smaller number of offspring can pass on your genes as well as just leaving them for the mother to worry about. ---
**Author:** ![mr.jp](https://avatars.discourse-cdn.com/v4/letter/m/48db29/32.png) [@mr.jp](https://boards.straightdope.com/u/mr.jp)\ **Post date:** [November 13, 2014, 10:09am UTC](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458/40 "2014-11-13T10:09:18Z")
> [@Chronos](#): > > OK, important I’ll grant you. But what on Earth makes you think sexual behaviors are simple? Recognize members of the opposite sex, and desire to do sex with them. Seems way less complex than say, language capability, or the liver. [Previous page](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458.md?page=1) [Next page](https://boards.straightdope.com/t/evolution-and-the-prevalence-of-homosexuality/704458.md?page=3)