Epistemic status: speculating about things I'm not familiar with; hoping to be educated in the comments. This post is a question, not an answer.

ETA: this comment thread seems to be leading towards the best answer so far.

There's a question I've seen many times, most recently in Scott Alexander's recent links thread. This latest variant goes like this:

Old question “why does evolution allow homosexuality to exist when it decreases reproduction?” seems to have been solved, at least in fruit flies: the female relatives of gayer fruit flies have more children. Same thing appears to be true in humans. Unclear if lesbianism has a similar aetiology.

Obligate male homosexuality greatly harms reproductive fitness. And so, the argument goes, there must be some other selection pressure, one great enough to overcome the drastic effect of not having any children. The comments on that post list several other proposed answers, all of them suggesting a tradeoff vs. a benefit elsewhere: for instance, that it pays to have some proportion of gay men who invest their resources in their nieces and nephews instead of their own children.

But how do we know if this is a valid question - if the situation really needs to be explained at all?

For obvious political and social reasons, it's hard to be sure how many people are homosexual. Note that we are interested only in obligate homosexuality - bisexuals presumably don't have strongly reduced fitness. The Wikipedia article doesn't really distinguish obligate homosexuality from bi-, pan- and even trans-sexuals. The discussion in the SSC comments used an (unsourced?) range of 1%-3%, which seems at least consistent with other sources, so let's run with that.

The rate of major birth defects in the US, as reported by the CDC, is also about 3%. This counts both developmental and genetic problems, and includes everything from anencephaly (invariably fatal) through Down syndrome (severe but survivable) to cleft palates (minor). But most of these, at least 1.5% of births, were always fatal before modern medicine, and many of the others reduced fitness (via mate selection, if nothing else). Various other defects and diseases, which only manifest later in life, are also thought to be influenced or determined during early development. And so is sexual preference.

(Whether homosexuality is a developmental disorder is not the point; I'm comparing the effect of selection pressure on fatal teratology with its effect on reduced-fitness homosexuality.)

Embryological development is a complex and fragile process, and there are many ways for it to go wrong. We don't wonder how it is possible that selection pressure allows anencephaly to occur in 1 in 4859 births. There are certainly direct causes of anencephaly, explanations of why it happens when it does, but (I think) we don't a priori expect them to be due to tradeoffs yielding benefits elsewhere. It's just as plausible that the tradeoffs involved are against even worse (counterfactual) problems elsewhere - or that there are just no available mutations that don't have these or equally severe problems.

Could it be that linking sexual preference to the biological gender is, for some complex developmental reason, fragile enough that it goes wrong despite all selection pressure to the contrary, that it has no redeeming qualities from the viewpoint of evolution, and that is all there is to it?

When faced with any phenotype with reduced fitness, how can we judge if there is something to be explained - a beneficial tradeoff elsewhere to search for - or merely a hard problem evolution couldn't solve completely? And is there a way to quantify this question, relating it to the known mathematical models of genetics?


1. I'm posting this in the spirit of recent suggestions to post more and accept lower quality of (our own) posts to Discussion.

2. I'm going to sleep now and will start replying to comments about 10 hours from now; sorry for the inconvenience.

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As far as I can tell from a brief look at the paper, it makes no attempt to estimate whether the increased fecundity of the female relative is enough to compensate for the reduced fecundity of the homosexuals. In fact, I didn't see it give any estimate for the latter. They merely do a p-test and pronounce the differences "significant". This is a situation where we are primarily interested in the magnitude of the effect, not merely finding evidence that it is positive.

Right--as well, the standard complaint about extending this logic to humans is that the most credible increased fertility effect for female relatives is desire-based. But it's easy for women to get pregnant more frequently and have more children--the hard part is keeping those children fed until they're self-sufficient, and dividing your wealth among all your children, neither of which it seems gay uncles help with enough to explain the effect.

But how do we know if this is a valid question - if the situation really needs to be explained at all?

Percentage of the population with the condition. (RichardKennaway's comment explains how you would calculate the 'expected rate' from underlying conditions, but here I'll just use statistical comparisons.)

The rate of major birth defects in the US, as reported by the CDC, is also about 3%.

This comparison is a category error--the comparison isn't "homosexuality" vs. "all birth defects," but "homosexuality" vs. "any i... (read more)

This is true (noted also by Douglas_Knight here). It's a little weakened by the fact that homosexuality likely has several different possible causes, which should also be considered separately. The loss of fitness due to homosexuality is also much less than that due to severe birth defects. I think given these initial data, the question still deserves a quantitative analysis. But I can't really judge because I don't know the relevant math - that's what I hoped someone would give in response to my post. From the gene's eye view, an allele isn't guaranteed to be selected during recombination. It seems to me that after conception, natural selection should operate on the fitness of the actual child as well as the inclusive fitness of the parents affected by raising the child. But I'm just handwaving and guessing here.
It does?
Wikipedia on causes of homosexuality says there are "various biological causes", and quotes: We're interested specifically in the heritable component, which seems to be relatively small. There certainly seem to be some specific genetic loci involved, one on the X chromosome and another probably on chromosome 8. At least one proposed etiology says the mother's immune reaction to the fetus affects its sexual determination, so we should be looking at genetic correlations in mothers of homosexual children as well as in the children themselves.
I don't much care about quotes from very prestigious professional bodies; they are always mealy-mouthed and self-serving, where they do not endorse pleasing claims on radically insufficient evidence (eg diet). What's more relevant is the actual research, like the link to https://en.wikipedia.org/wiki/Biology_and_sexual_orientation ; I'm not too impressed, as it seems to be mostly a laundry list of fairly dubious corrrelations and downstream effects or irrelevant to humans animal research (does anyone think that deleting an entire gene in mice to turn them gay tells us anything relevant about causes in healthy humans?).
Fair enough. I should rely less on popularized professional consensus in this kind of thing. Being who you are, it's no surprise that you have this attitude (and correctly so).
Since when is anything in biology, neurology, or psychology simple?
Lots of things are simple. If the world is not simple, inference is impossible. Many things turn out to be straightforward; as complex and intricate a phenomenon as AIDS is, 'HIV causes AIDS' is much more accurate than 'AIDS is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences; in recent years, biologically-based theories have been favored by experts...' In statistical modeling, it's far from surprising to discover that a few variables have most of the predictive value and that it's only the last few percent which require extreme complexity to predict or model.
You have explained why inference is hard in biology :-) A technical term for the "problem" is pleiotropy. Many small scale biological features are re-used over and over, if they break, many things can break a bit. Primary ciliary dyskinesia is an example if this. In the meantime, many complex adaptive structures (like "the ability to hear") are caused by more than one subcomponent, so any of several different subcomponents breaking can produce a symptomatically similar disruption of the complex structure. Biological causes and biological outcomes are in a many-to-many relationship, with lots of "best effort" failover systems as backups. The amount of effort to put into fixing up a structure is itself something that most of the animal kingdom has optimized a bit, for example via the poorly named "heat shock proteins" that suppress mutational expression in good times but reveal the mutations in bad times. In the case of male homosexuality, one cause that I recall hearing debate about was that a male child causes a mother's body to change (current best guess is something immunological), such that later male fetuses appear to have their sexual development mildly disrupted. If I recall correctly, the process looks probabilistically cumulative, so that there's something like a 1/3 chance of homosexuality by the time you get to the fifth or sixth male child from the same mother. Again, if I recall correctly, with modern demography this effect might be enough to account for ~20% of gay men? This is somewhat controversial, but le wik has some of the debate.

Obligate male homosexuality greatly harms reproductive fitness.

I'd imagine female homosexuality should have a similar effect.

On the other hand, 'obligate' is a strong word to use for things as complicated as human behavior. Knowing several older male homsexuals with biological children via socially-imposed marriage customs, I don't think the effect is as large as many assume under many environments.

I'd imagine female homosexuality should have a similar effect.

Except that ... how to put this delicately? Historically, men who would prefer not to have sex with women have been more likely to get their way than women who would prefer not to have sex with men.

Perhaps in a matriarchal society having a man that belongs to no woman would provide some benefit for women who could not get a mate otherwise?
Additionally, a woman can choose to have sex just for procreation, whereas a man has to be attracted to the woman, or otherwise he won't be able to perform.
Specifically, there are gay men who are repulsed by the prospect of sex with women, and then gay men who are simply not interested (or not as interested) in sex with women.* It seems to me that 'obligate' is meant to refer to the first cluster, and should have a reproductive penalty roughly the same as male asexuality. (Figuring out what percentage of the population falls into that cluster is hard, since you can't really go off self-reports of preference; you want self-reports of anti-preference.) *Straight men can consider how they would feel about having sex with another man; for some, there's a visceral disgust reaction, and for some there's just a "but... why would I do that?" reaction or a "eh, any port in a storm" reaction. In farming societies where monogamy is the norm and marriages are economic arrangements, it seems to me that the reproductive cost of sexual interest in men is minor (or possibly positive, if men in power are willing to trade resources for sexual favors). But in societies where polygamy is the norm and men compete for women, it seems likely to me that any man who is less interested in winning is less likely to win, and the costs of sexual interest in men might grow significantly.
In our own society, monogamy is the norm, but marriages are not solely economic arrangements and men do compete for women (and vice versa). If even a small percentage of men does not marry, then it makes sense for men who are not attracted to women to be among them, possibly even reaping some rewards (e.g. two men living together, even if not married, may have a higher than average household income).
Is that really in need of an explanation, though? Some people are repulsed (by stuff), and others aren't. That is to say that whatever causes homosexuality probably doesn't also necessarily cause disgust for heterosexual activity. Homosexuality could serve in such an instance to reduce competition, though. If you take animals such as lions, the males are extraordinarily competitive, to the point where they will kill children of other lions and drive away all competing males. Humans can't sustain that kind of competition, though. Humans are vulnerable for longer when we are children and require more investment to mature than most other mammals, so if competition is too fierce and leads to infanticide, the species can't sustain itself. In addition, humans are geared for cooperation with other humans, since we're naturally social creatures and capable of learning and imitation from others. So I think humans don't really have much to gain from competition and more to gain from cooperation. And considering that, it might be the case that the "gay uncle" really does help the reproductive fitness of the group, or at least, doesn't hurt it so much that genes that create the potential for homosexuality are selected against. Not every individual in a species has to be geared for reproduction, particularly if the species is very social and organizes itself into groups. In bees, for example, the vast majority of bees are totally infertile with just a handful of fertile bees in a single hive. It doesn't matter that the bees individually have terrible reproductive fitness, because the hive as a whole has very high reproductive fitness. Monogamy isn't necessary; it works with polyandry and other arrangements such as levirate marriage too.
It seems to me that any mental feature that can be explained should be explained. It makes great sense to me why there are people who feel visceral disgust at the prospect of sexual activity with their siblings, and that there are men who feel visceral disgust at the prospect of sexual activity with men. (Interestingly enough, this sort of 'homophobia' seems more heritable than homosexuality is, which makes sense--this is actually selectively positive!) So what's going on with a man who feels visceral disgust at the prospect of sexual activity with women? Is it the same sort of homophobia reflex, but miswired? Is it something else? If you don't find explaining things interesting, well, I recommend a career in something other than science :P How does evolution work? There are two core pieces: variation, and selection. Variation is the uninteresting bit, and selection is the interesting bit: entities that reproduce themselves become more common in later generations, because they reproduced themselves. It's not a question of what "has" to happen. It's a question of "what happens." Similarly, explanations need to fit together with every other part of what we see. Suppose a model where gay uncles help people in their family raise children, are generous with their wealth to family members, and so on. In such a case, does it make sense for families to disown gay sons? (Remember, we're trying to explain why 3% of men are gay. We can't accept any explanations that would make, say, half of men gay, because that doesn't fit with the facts!)
The first is clear to me, but the second isn't. Why would homosexual male sex be a bad thing, as long as it didn't cause men not to seek out women as well? Of course some resources would be spent on it instead of on mating, but humans have a lot more sex in general than necessary for procreation, and many other 'unnecessary' social activities like games. The usual reasons given (e.g. bonding) also make sense between male pairs. Such sex could (counterfactually) also relieve some sexual tension without inviting jealousy, since another man might provide variety and quick simulation but not replace the long-term woman partner. This seems to apply even more strongly to women, whose fitness doesn't benefit from promiscuous heterosexual sex like male fitness does. We also need to understand why those explanations are in fact wrong, otherwise we're risking retrofitting explanations to the data by choosing explanations without fully understanding what makes them right.
Primarily, that; secondarily, disease risk. It seems to me that there are many men who put up with women only for the sex, and if they could get that satisfaction elsewhere, they would. It looks like a number of ancient societies had sanctioned male-male sexual relationships, often but not always of the 'old mentor / young protege' variety. But it's hard for us to tell how common those were (specifically, how many of those partnerships were actually sexual, instead of just knowing that some were). Interestingly, female bisexuality seem much more common than male bisexuality, and also considerably more fluid.
That's an excellent point I missed. If promiscuity with other men came at the expense of promiscuity with other women, it wouldn't be a problem. But male promiscuity is often limited only by the number of willing and attractive partners, so it would still increase the number of overall partners.
The explanation is the same as most other "repulsions", that you happened to develop a response of visceral disgust to a particular stimulus. Most people are repulsed by something; vegetarians can be repulsed by the concept of eating meat, men can be repulsed by reading excerpts from Twilight, and children can be repulsed by green vegetables. None of these need "explanations", beyond the obvious: because humans are malleable and can learn to hate things, particularly if they are taught to hate it. If people can learn to be repulsed by meat, or green vegetables, or squishy chick-lit, they can damn well learn to be repulsed by women. So I say that repulsion towards women doesn't need an explanation, since people develop (and overcome) visceral responses to all sorts of things all the time. I've seen nothing to show me that revulsion to women is somehow inherent to homosexuality, and while I'm not asking you to prove it, I hope you have a good reason for believing it. Do they? Or is that just another assumption of yours? Do you have a good reason for believing this one?
"Happened" is typically used when a result is undetermined or not strongly expected: "the die happened to come up a six." It would sound weird to use it when a result is determined in advance: "the cup happened to fall once I let it go." It's certainly not inherent to all homosexuality. Whenever this topic comes up, this part of American on Purpose comes to mind: (I don't seem to have this reaction, myself. But my experiences are also minimal.) Does who what? Do families disown gay sons? Yes; my first boyfriend was kicked out by his family, and it seems a disproportionately large fraction of homeless youth are LGBT, with some large fraction of those claiming their parents forced them out.
Not "undetermined", but the result of a process complex enough that it's very difficult to predict the outcome or identify the root cause. I should be more clear: "Does it make sense for families to disown gay sons assuming that homosexuality improves group fitness" is a loaded question because it carries with it an assumption that disowning gay sons has evolutionary roots, or is ingrained behavior in humans, or is common. And I'm not convinced of any of these things. After all, my family didn't disown me, and it seems like disowning gay sons is becoming increasingly uncommon. You asked whether it made sense for families to disown gay sons. Does it make sense for families to disown children for being the wrong religion? Does it make sense for families to disown children for being pregnant?
Difficult's a two-place word, and so I'm not sure it makes much sense to argue about whether or not something is 'objectively' difficult, instead of difficult at various states of knowledge. It's not quite that disowning gay sons has evolutionary roots, but that disowning gay sons is not so heavily disfavored as to be extincted. For example, cultures where childbirth is prohibited mostly die out, and so on. But even less obvious things that have an effect on reproductive success are strongly motivating; in cultures with prohibitions against masturbation, those prohibitions are mostly not followed; in cultures where doctors tell mothers to avoid touching their infants because of disease risk, those prohibitions are mostly not followed, and so on. (The impulse for mothers to touch their babies seems very strong, and also very healthy--it actually lowers disease risk by informing the mother what antibodies she needs to produce for her child, and seems critical for proper psychological development.) And traditional behavior gives us an imperfect window into the economics of the past, which is what's under discussion when we talk about historical selective fitness. If gay sons were helpful enough with nephews and nieces that it was as if they had had their own children, it seems to me they would be welcomed and lauded as examples of loving selflessness. But if gay sons were reproductively disadvantageous, and in particular if it was reasonable to expect that homosexuality is contagious, then there's little cost and some reproductive benefit to forcing them out of the home. (I should note that the hypothesis that one gene causes both female fecundity and male homosexuality is also consistent with disowning gay sons, but I think that one has other challenges.) Thankfully, people are much more motivated today by individual and relationship satisfaction, neither of which disowning is helpful with. (My family didn't disown me either.) It suspect it made sense for religio
Traditional behavior is so widely varied, though, that it's difficult to draw any conclusions. Some traditional societies practiced polyandry, others, polygamy, and still others, levirate marriage, and avunculism, and so forth. Some traditional societies were accepting of homosexuality and even transgenderism. You say that cultures that prohibit childbirth die out, but many diverse cultures have a thriving tradition of monasticism (which is even worse for reproductive fitness than homosexuality!) Would they? "Gay" is a recent category; traditional societies did not attempt to classify humans in that way and it only became popular when religious authorities attempted to criminalize it and early psychologists attempted to medicalize it. Men were not "gay" or "homosexual", they were more or less inclined towards other men.
I think we should keep in mind just how far back we're talking. I'm not saying we inherited homosexuality from our common ancestor with the modern fruit fly, but at least our common ancestor with other great apes. Framing the question as why would it be selected for in the context of human societies is probably wrong, when what we want to know is why it wasn't sufficiently selected against given it already existed (I doubt we'll ever figure what advantage it gave the proto-ape whose social structures we'll never know). Once a trait already manifests in 3% of the population, it takes work to get rid of it, and even within that 3%, it was doubtful the case that 0% of them reproduced while 100% of heterosexual men reproduced. I'm sure it wasn't exactly parity, but it's possible there is no explanation in terms of the organization of human societies except for we're really optimized to enjoy sex, sometimes that wire gets flipped, and it doesn't provide an advantage, but it also doesn't give enough of a disadvantage to completely disappear within 300,000 years. Don't forget also, that if some gene combo is necessary but not sufficient, and requires other developmental factors to manifest that don't manifest in your brothers and cousins (which seems to be the case if it's only 20% between twins), then when they reproduce, even if you don't, the gene still gets passed on. Take me, for example. I'm not gay, but I am sterile and don't want kids anyway. Nonetheless, I have 3 sisters and 13 cousins that have had kids so far. Without doing the exact math, off the top of my head I'm guessing at least 80-90% of whatever I'm carrying made it to the next generation. Edit: Also, one last thing is we don't know the prevalence in the ancestral population. Given it's roughly 100% bisexual in such a closely related other species, it could have been fairly high in the common ancestor, obviously not 100% obligate, but more than 3%, and it actually has been selected against, a lot, just
Are you explicitly suggesting group selection? It doesn't work outside of very special circumstances. (The linked wiki article comes across as rather unpleasant in tone. Your user account is new and if you're new to the site, I don't want you to make you think this is typical. In that case please just use the wiki page as a link index to other articles about the subject, and for a counterpoint on when group selection might work after all, see e.g. this post.) Eusocial insects make it work by making workers more related to their sisters than they would be to their own daughters. So workers prefer to raise their sisters than to give birth to their own sisters.
Bees are obviously a very extreme example of this but I think they're an apt one. The important point is that fertility is not selected for in bees, but instead, the activity of raising infertile workers. The incorrect assumption about homosexuality is that a person is homosexual because of their genes. It's more likely that homosexuality is caused by the genes of the parent. After all while an individual is only concerned with his own reproductive fitness to the exclusion of his siblings, the parent is equally concerned with the reproductive fitness of all their children. It's true that homosexuality cannot be selected for, but a propensity to have homosexual children CAN be selected for if it increases the reproductive fitness of older siblings.
This is true, but it's not the same as in bees: there, it is in the interest of each worker not to have children and raise its sisters instead.
No, it isn't. Workers do not have the ability to choose whether to be a queen or not. That choice is made for them when they are raised. A larva cannot feed itself royal jelly. And it should be obvious that given the choice, a worker would always choose to be a queen. After all, you're related to the existing queen (she's usually your mother or your sister), but you're even more related to yourself, so anything that increases your own reproduction at her expense is a good thing. Conversely, workers, given the choice, will almost always raise other bees as workers, because they are always more related to the existing queen than to her offspring.
Wikipedia says here that workers are 75% related to their sisters. They would only be 50% related to their own children after mating with an unrelated male. So mutations that cause larva to grow up as queens without royal jelly are actually deleterious. If workers were fertile, they would still evolve not to lay eggs while the queen is alive.
No, from the point of view of their genes they are given a signal, they get to choose whether to obey it, i.e., a hypothetical mutation that causes the larva to develop into a queen even if not fed royal jelly doesn't seem like the kind of thing that would be hard to spontaneously happen.
That's funny. I just realized that since homosexuality is mildly heritable, a lack of socially-imposed marriage customs, brought about by political empowerment of homosexuals, could actually (mildly) reduce the number of homosexuals. I wonder if the same is true for asexuals.
Homosexuals are still interested in bearing and raising children. In a permissive political and social climate, with same-sex marriage, they can use surrogate mothers. Lesbians can, of course, bear their own children.
Using sperm or egg donors is a 50% fitness cost. (adopting is 100% fitness cost)
That's true, although they could improve that by asking their parents or siblings to be donors.
But they don't. Anyhow, that's still a 25% fitness cost, which is huge.
From the perspective of the parents of the gay couple, it's a 0% fitness cost, though.
Irrelevant. We are discussing the dynamics of genes. If the gene has low fitness, it will be purged from the population and the rate of homosexuality will decline, even if the grandparents have full fitness. By using a donor, we are re-randomizing. Instead of a parent with the gene, we are replacing with a parent with a 50% chance of having the gene. Similarly, what if two people with achondroplasia have children, repeatedly, until 2 survive infancy. Then the parents have fitness 1, but 1/4 of their children were homozygous and died in infancy. Their children are uniformly drawn from the remaining 3 children, consisting of 2 heterozygous, 1 homozygous normal. So the gene frequency has gone down from 1/2 in the parents to 1/3 in children. The parents have full fitness, but the gene has fitness only 2/3.
I don't actually know if they do or not, but why don't more people do that? Maybe they're afraid the donors will expect or demand some rights or involvement in the child's life, which isn't a problem with anonymous or hired donors. Agreed regarding the fitness cost. That makes chaosmage's comment correct, although I didn't realize it at first; his comparison with asexuals led to me to think his proposed mechanism was simply homosexuals raising fewer children, whether their own or not.
You might as well ask why people adopt strangers as ask why they half-adopt by using strangers as donors. The answer is that people are adaptation-executors, not deliberate fitness maximizers. I hear about people asking for donations from friends more than family, which seems to me to be a bigger risk. Being on bad terms with family because of orientation probably is a reason, though.
People have always sometimes adopted the children of near relatives who couldn't care for them, at much higher rates than adopting strangers; that's fitness increasing. It's not a stretch to imagine that crossing over to sperm and egg donations. Both friends and family have the large advantage (at least I imagine it to be so) that they are known quantities: you know them well and can evaluate them as biological parents just as you would a potential mate. I wonder why sperm banks don't seem to offer the sperm of people with known qualities at higher prices: not just screened for diseases, but known to have strong positive traits like good looks and intelligence.
Incest is bad, and their children would be unhealthy and retarded.
No, you use one partner, and sperm or egg from a relative of the other partner.
Ok, well that works.
Surrogate mothers don't one of the people in a gay couple more of a father than the other person. That's not necessarily desirable to a gay couple. Adopting children might be a more straightforward solution if the society makes it easy for homosexuals to adopt.
It's not clear that making none of the two men a father is better (i.e. more desirable to them) than making just one a father. They could each father one of two children, for instance.
If one is a father there are concerns about that person having more rights about determining the fate of the child then the other partner.
If they are married, such concerns would only come up in a divorce. Child custody battles are weird and ugly enough in heterosexual marriages; I don't know what they would look like in homosexual ones. Also, even the man who is not a father may still consider it a better choice. Just as some heterosexual couples one of whom is infertile may prefer a surrogate mother or the sperm bank to adoption.
Legal rights aren't the only that matter. Raising children means that you have to make a lot of parenting decisions. If one partner feels that he has more right to influence those decisions that's an issue.
I'll take your word for it. I've never been in that situation and don't have any instincts for how it might feel. It might also be relevant that the other partner might want to invest fewer resources in the child because it's not biologically theirs.
This is true, especially in pre-modern societies, and further supports the argument that we shouldn't be surprised by the high incidence of homosexuality.

Homosexuality has a low heritability, IIRC something like 20% concordance for identical twins. If there was a gay gene you would imagine it would be higher.

Greg Cochrane:

Ulcers are far more heritable than homosexuality, and genetics matters: but you don’t get the ulcers without h. pylori.

Yes, that was one of the reasons I posted this. Wikipedia says even identical twins that share a placenta can differ in sexual orientation, and you don't get more shared-genes-and-environment than that.

Homosexuality is mildly heritable. You can't posit that it's hard for evolution to find a way around it, because it has ways. Perhaps not perfect ways, but the heritability should quickly fall it unmeasurably small values. That really requires explanation. Either there is some hidden benefit (an advantage when expressed in the other sex, avuncular investment, heterozygote advantage) or a recent environmental change (industrial pollution, red queen).

But even if it were not heritable, I think it would still be a big mystery. When you compare it to birth defe... (read more)

You seem to be making an extremely strong claim: that every heritable deleterious condition must be due to a hidden benefit or recent change. (And in a long-term stable evolutionary environment, would always be due to a hidden benefit.) What is the basis for this? Why can't we simply posit that some tasks are complex (embryological development), and some tasks are adversarial (countering parasites and diseases), and this explains a large part of heritable fitness-reducing traits? This is a good argument. Do we know or expect that all or most cases of homosexuality have the same underlying cause, or their heritable component does, so that a single localized mutation could eliminate them? Or could there be many different causes? Also, this feels in need of more quantitative argument. Anencephaly, or even all causes of stillbirths taken together, obviously cause lower fitness for the children than even obligate homosexuality. On the other hand, they don't make the parents spend resources raising low-fitness children whose homosexuality only becomes apparent at puberty. I don't really know what size difference to expect here and whether to be surprised by two orders of magnitude.
I don't think he's making a claim that strong. It depends on strength of the heritability and the size of the trait on selection pressure. It also depends on the extend to which you easily get mutations in genes that produce the trait. Random gene mutations lead to birth defects. It's not clear why you should get homosexuality in the same way through random mutations.
Part of my argument is that homosexuality (inasfar as it is determined in the womb) may not (always) be caused directly by a mutation, but by the fact that even in the absence of any mutation, the developmental process linking gender to sexual orientation is complex and fragile and has a significant failure rate.
Then where does the heritability come from?
You're right, this is a problem. If something is heritable but also uncommon, it should be possible for selection pressure to act on it. So why doesn't it? That clarifies the issue for me. What I wrote before (in this comment thread) now seems mistaken. Thank you for helping me to understand better. It seems my original question was partly due to a misunderstanding. The crucial fact about homosexuality isn't just the lowered fitness or relatively high incidence, but the (partial) heritability. Then the only relevant remaining question is: can we quantitatively estimate the reduced fitness due to homosexuality and so calculate the unlikelihood of its 3% rate being due to chance, drift, etc?
Your original post does not talk about heritability. So my answer was: heritability. But then I noticed that it was in the post title and I was confused and did not change my answer. ---------------------------------------- I posted a calculation on SSC. Let’s say that there is a mutation that is spontaneously created in 1 in 10,000, that it has a 10% chance of producing a homosexual phenotype and that the phenotype has 0.9 fitness, that is, yields an average of 1.8 children. Then the fitness of the gene in 0.99. So in equilibrium, the gene only reproduces itself 99%, so the remaining 1% must be made up by the spontaneous mutation. That is, the prevalence is 100x the spontaneous mutation rate. The prevalence is 1% of the population, of which 0.99% inherit the gene and 0.01% spontaneously acquire it. That’s a genotypic prevalence of 1%. The phenotypic prevalence is 0.1%. I think 1 in 10,000 is the standard rule of thumb mutation rate. For example, achondroplasia (dwarfism) has a spontaneous mutation rate of 3 in 100,000 and an inherited rate of 1 in 100,000. Apparently dwarfs have about 1/4 of replacement fertility. (The fatality of homozygous achondroplasia complicates the situation. The gene definitely has a fitness of 1/4, but if dwarfs only marry dwarfs, the fitness of the people would be 3/8, I think.) Also, the approach to equilibrium is exponential with base the fitness.
Its more like 10^-8. http://www.sciencemag.org/content/328/5978/636.abstract
Nope, that's the wrong rate. I gave you a worked example, with real empirical data: achondroplasia has a spontaneous rate of 3/100,000. Try to understand the example rather than pulling facts out of your head with no understanding.

That seems needlessly inflammatory.

The rate of spontaneous mutation in humans is on the order of 10^-8 per haploid base-pair per generation. A few references: one (a blog post summarizing multiple academic papers), two (I think this is one of the papers cited by the foregoing), three (older; rate is ~2x higher), four (intermediate in age and also in estimated rate).

A given gene is many base-pairs long. Accordingly, the "per locus" mutation rate -- which you can think of as how often a particular gene goes wrong, treating all its failure modes as equivalent -- is on the order of 1000x higher. (This, I take it, is where the ~3x10^-5 spontaneous rate for achondroplasia comes from.) The usual cited per-locus rates are, accordingly, on the order of 10^-5, typically a bit lower; see e.g. one (a textbook, which specifically mentions achondroplasia and gives some reasons not to take its rate of spontaneous occurrence too literally as a per-locus mutation rate), two (old paper by J B S Haldane, mostly of historical interest now), three (journal article).

Which figure is more relevant in a given case depends on whether it's one where messing up a single protein, no matter how, cause... (read more)

punnet square: Aa x Aa ..................A...........a . A...........AA.........Aa . a............Aa..........aa AA = death or infertile Aa = like the parents aa = normal looks like 3/4 to me. not sure where you got 3/8. (also where two dwarves can have a normal child). If you selectively look at the living (of which there are 3 options - Aa, Aa or aa) the gene has a 2/3 chance of being passed on. Assuming no other pressures apply.
First of all, you should distinguish between the fitness of the gene and the fitness of the people. Second, I am using as input the empirical observation that the fitness of the achondroplasia gene is 1/4. Third, and tangentially, you should distinguish between the fitness of the children and parents. (1 gene vs parents) Let us consider the 3 surviving children. Out of the 6 copies of the gene, 4 are wild type and 2 are achondroplasia. But in the parents, half of the genes are achondroplasia. Thus, regardless of how many children the parents have, the fitness of the gene is 2/3 the fitness of the parents. (2) Empirically, 1/4 of achondroplasia births are inherited and 3/4 are de novo. Assuming equilibrium, the gene is producing 1/4 of replacement fertility, so it has a fitness of 1/4. If dwarfs only reproduce with non-dwarfs, they, too, have a fitness of 1/4. But if they only reproduce with dwarfs, they have a fitness 3/2 of the gene, thus 3/8. ---------------------------------------- (3 parents vs children) The 3/4 you compute is the reduction in the proportion of pregnancies yield children. This is a kind of infertility, though more emotionally difficult. It is only relevant if the parents are trying to reproduce as fast as possible. In the modern world, parents usually target a small fixed number of children and infertility has little effect. In both farmer and forager societies, children were probably modulated to available food supply. Such a wasted pregnancy does not reduce the number of children by 1, but probably delays future children by a year. If the usual interval is 4, this might reduce fitness by 1/4. But the effect is probably significantly smaller. If people are reproducing at the optimal speed, taking into account risk of famine, a small perturbation probably has little effect.
sorry; point 2 again, (Aa x aa should product a 1/2 not a 1/4) acondroplasia X normal ............A.............a ...a.......Aa..........aa ...a......Aa...........aa 50%Aa acondroplasia 50%aa normal or am I confused somewhere? Is that not the punnet square?
Sure, that's the punnet square. You should stop drawing punnet squares and ask yourself why you are drawing them and ask what role they play. The number 1/4 is the empirical fitness. It is mainly about how many children dwarfs have. You cannot guess that number by looking at punnet squares.
Thanks for the calculation! I'll probably revise the post tonight when I have some leisure time to integrate all the new info.
Congratulation for good updating.

I wonder why there hasn't been more selection against women having a difficult time giving birth. It's risky, but some women give birth more easily than others, so there's enough variation for evolution to work on.

Part of the reason is probably because of a conflict between the mother, father, and baby. The baby gets all of the benefit of, say, having a bigger head at birth but some of the cost is born by the mother and little by the father so from the baby and father's viewpoint the head size will be bigger than it would be for the mom.
The real conflict is between the mother and the laws of nature that say that she can't have hips the size of barn doors and still be able to walk.
I'm not sure what you've got in mind there, but it would be worth looking at whether there are some tradeoffs between being able to give birth easily and how good a woman is at walking and/or running.
The larger the hip the easier it is to give birth but the less efficient the gait becomes for a bipedal animal, since at each step the projection of the center of mass on the ground is horizontally farther from the foot. That's the reason why women swing their hips much more than men as they walk. While it looks sexy it is not very evolutionary advantageous if a saber-toothed cat is chasing you. Birds and bipedal reptiles don't have that problem though, since their birth canal does not pass between the hip bones, thus they can get away with horizontally small hips and still be able to lay eggs which in some species can be as large as one fifth of the their body.
While in hunter gatherer times it would have been important to run fast, as civilisation arises you get chased by bears less often, and so I could guess that hips would start to become wider.
The hips already became a lot wider X-/ along with other body parts. And given contemporary medicine, a narrow pelvis isn't really a big deal nowadays.
That seems orthogonal to NancyLebovitz's point: some women have lower risk than other women when birthing infants with a given head size. Also, if a woman dies in childbirth, or even if she's injured enough to not lactate well in the days immediately following birth, that strongly impacts the baby. Evolution may not care about how painful birth is, or even how long it takes (now that we don't need to fear predators), but it certainly cares about risk of injury or death to the mother.
Some gene increases the size of the baby's head at birth raising the baby's eventual IQ by X, but also increases the chance that the mom will die during childbirth by Y. There will exist (X,Y) such that the baby having the gene will increase the expected number of great grandchildren that the dad and baby will have, but decrease the expected number of great grandchildren that the mom will have.
How big a selection pressure does higher IQ command? Wikipedia on Fertility and Intelligence: Many local times & places have had a negative correlation between IQ and fertility. Of course, fertility isn't precisely the same as inclusive fitness, but it's strongly correlated. On the other hand, many women die in childbirth or suffer significant complications in many parts of the world even today.
Enough that Homo Sapiens has a brain volume nearly twice as high as Homo Habilis. (~600 cc to ~1200 cc).
I'm asking about selection today, or in the last few millenia, not two million years ago in a different species.
Today, as in the past ~40 years it has been negative.
Exactly. Thanks. I was just going to make that point. Pelvic structure is in play as well as head size. Also note that there's more to raising children than lactating. Women typically needed to be in good enough shape to do hunting/gathering/food growing. See Sarah Hrdy's Mother Nature for a reminder that motherhood takes place in the world, not just between the mother and child.

For obvious political and social reasons, it's hard to be sure how many people are homosexual. Note that we are interested only in obligate homosexuality - bisexuals presumably don't have strongly reduced fitness. The Wikipedia article doesn't really distinguish obligate homosexuality from bi-, pan- and even trans-sexuals. The discussion in the SSC comments used an (unsourced?) range of 1%-3%, which seems at least consistent with other sources, so let's run with that.

Was obligate homosexuality common in the ancestral environment, anyway? If I understand... (read more)

Gay historian Rictor Norton vehemently disagrees with the notion that gay identities are recent. Here is his basic position: * Gay identities have existing for a long time, not just recognition of gay behaviors * Recent conceptions of homosexuality are politicized, but this does not mean that concepts of homosexuality are new * The politicization of modern gay politics, combined with poor record-keeping and past suppression, erases the history of gay identities and cultures. He takes a position against social constructionism: To see more, check out these excerpts from The Myth of the Modern Homosexual.
So there were slurs to refer to people who engaged in socially objectionable sexual behaviors. It doesn't mean that these people were obligate homosexuals and considered themselves as such.
That's Foucault's theory, but Rictor Norton's book I linked to convincingly debunks Foucault as ideological and ahistorical. Quoting an excerpt, here are historical cases of unmarried men going for each other instead of marriage and children: These guys sound like they are exclusive, obligate homosexuals. As for identity, just because the historical labels for queer people were negative, it does not mean that those terms were just externally-imposed slurs, and that homosexual identities did not exist: Rictor Norton is a widely published queer historian, his research goes back centuries, and seems very solid. I think we should go with his account and toss Foucault's social constructionism.
Makes sense. However all these examples are from Christian Western societies, I wonder about non-Western or pre-Christian societies.
With exceptions such as Catholic priests.
So, are you basically saying that the current Western concept of a being gay is mostly the result of identity politics?
If you want some more fun with the subject, check out Hanne Blank's Straight which argues that the identity of heterosexual is a fairly recent thing-- only about a century old, as I recall. Previously, people thought in terms of sexual behaviors, not identities.
Obviously, the heterosexual identity can only exist in contrast to the homosexual identity. If a group of squid people suddenly appeared on earth, you could bet that a vertebrate identity would develop pretty fast.
That may be obvious if you think about it, but I, at least, hadn't thought about it, and found it to be surprising. I'm willing to bet that I'm more typical on this point.
To some extent probably it is: the gay identity historically arose as a reaction against the previous negative view of homosexuals as people affected by a mental disease. Indeed the word "gay" was chosen specifically to avoid and reverse the negative connotations of "homosexual". To some other extent, it is probably be a result of Western societies becoming more wealthy, democratic and individualistic, therefore individuals feel more free to follow their preferences rather than social expectations of their family/clan/state.
I don't think that's necessarily implied. Obligate homosexuals probably always existed, we just can't be sure if it was at the same relatively high rates as today. But only recently have they organized socially and politically to demand equal rights. As part of this movement, homosexuality became an important part of their identity, and formed a group identity, and so the social and psychological character of how people express their own homosexuality changed. But that doesn't mean the core features of being attracted to people of the same gender, and not attracted and unwilling to have sex with members of the opposite gender, changed. I see this as similar to the historical emergence of nation-states. A medieval peasant didn't consider being French an important part of who they were, didn't have a French citizenship. But they still lived in France and spoke French; in that sense there were Frenchmen then just as today.
But if you go sufficiently back in time, there was no such thing as France or the French language.
Why does that matter? If you go sufficiently far back in time, there was no such thing as humans, either. Statements about humans, and about Frenchmen, are still valid within the right historical time frame.
This is actually really relevant to the point--it used to be that a person from Paris and a person from Marseille would have enough difficulty understanding each other that they are functionally speaking different languages. The government of France put a tremendous amount of effort into convincing everyone living in their borders that "being French" was a thing and that it described them, in large part by enforcing homogenization. In order to make the cluster of "Frenchmen" more distinct, outlying members had to be moved closer to the center (and foreign members moved further away from the center).
Yes, that is a very good point. It was a bad example.

This is a question that needs to be approached mathematically or not at all.

Given a phenotypical property P (supposed binary), currently present in a proportion X of the population, and having heritability H and selective disadvantage D, how will X vary over time, measured in units of a generation?

Solving this mathematical problem requires a mathematical definition of H and D, which I don't have, but this must be standard population genetics. Is there a population geneticist in the house?

One can also add various complications to the model, such as heterozy... (read more)

I've studied population genetics. Let x be the prevalence of the genes. N hetrozygotes=2x(1-x) N homozygotes=x^2 For stability: x(1-x) He+Ho x^2=x Here, He could stand for homozygote fitness or homosexual fitness, and He is hetro fitness: (hetrozygotes only have one gene to pass on, so this term is divided by two) Sanity check: He=Ho=1 => x=x i.e. neutrality implies stability at all levels of prevalence (exc. stochasisity) He=(1-x * Ho)/(1-x) So, if 5% of people are gay (supported by e.g. number of people signed up to ok cupid) x=0.22, and Ho=0 (oversimplfication) then He=1/0.78=1.28 If only 20% of gayness (to use the scientific term) is explained by genetics, then x=0.1 and He=1.01. I don't know enough to even guess whether He=1.28 is plausible, but He=1.01 certainly is a modest fitness increase. As far as being supported by random mutations, well, the mutation rate is around 10^-8 (I think there are more sublties to this, but its accurate to a first approximation), and since homozygotes have prevalence of x^2, this is enough to support a prevalence [EDIT: a prevalence of hetrozygote carriers] of 10^-4.
Surely that can't be right. If an allele is present with probability 10^-8, the probability of its being present in one of two places is 1-(1-10^-8)^2, which is not anything like 10^-4; it's almost exactly 2.10^-8. (This doesn't change the point I think you're making, namely that there is no possible way that every instance of non-heterosexuality is the result of an independent mutation at the same site.)
What I meant to say was that if 10^-4 of the population are hetrozygote carriers, then 10^-8 will be homozygotes and their genes will be lost to the next generation (assuming zero fitness), so if new mutations are created at 10^-8, then a prevalence of 10^-4 of the population being hetrozygote carriers is the steady state. In this case, the proportion of homosexuals (or heamophiliacs or whatever) would be 10^-8 times the number of nucleotides that will cause the condition if they mutate. If the gene in question is dominant, then it's still 10^-8. But yes, homosexuality cannot be primary caused by random mutations.
Oh, I'm very sorry: I completely misunderstood what you were doing, and failed to read your last paragraph as an application of the calculation you'd just done. Just to say it again in a different way for the benefit of anyone else who misunderstood in the same way as I did, the point is this: In equilibrium, the rate at which a mutation enters the population has to equal the rate at which it leaves. If it's rare and recessive, the main way it leaves will be by homozygotes being less fit. So, taking the simplest possible approximations everywhere: if the mutation rate is m and the prevalence of this thing in (the genes of) the population is f, then every new generation will gain a fraction m and lose a fraction f^2 from dead/infertile homozygotes, so we should have m=f^2. So, e.g., if m=10^-8 then we will eventually get f=10^-4. All of this needs adjusting if having the thing heterozygously makes a difference to fitness, or if having it homozygously doesn't reduce your fitness to zero, and that adjustment is what the more complicated formula in skeptical_lurker's earlier comment is for.
Not sure this model is adequate: if there is no advantage whatsoever, the proportion X can only go down -- there is nothing which can increase it. That begs the question of how did the original mutation spread to X percentage of the population. And if X is only going down, you're probably looking at some variety of exponential decay...
One possibility is spontaneous mutation. Again you would have to plug in a rate for that and see what the mathematics says. Another is that the genes involved aren't mutations, they're alleles that are losing ground to mutations that do better. Whether either of these or something else can fit the case of homosexuality I don't know.
I don't think the second case can fit homosexuality. If homosexuality-promoting alleles are losing ground to mutations that do better, why haven't they completely vanished yet? It's unlikely that going back, homosexuality rates were far higher; that would make prehistorical men very unusual among other mammals.
Spontaneous mutations and random genetic drift. (There's also the case where X is the original variant and has been decaying for a long time, but that's not relevant to homosexuality.)
A spontaneous mutation produces 1 (one) individual and pure genetic drift is unlikely to get to noticeable percentages of the population (with some exceptions, of course).
The random walk will either lead to fixation or extinguishing the spontaneous mutation, and the probability of fixation for a neutral mutation is meaningful: "the rate of fixation for a mutation not subject to selection is simply the rate of introduction of such mutations." Hence, genetic drift is powerful enough that it can separate isolated populations.
Well, the rate of fixation of any neutral mutation is the rate of neutral mutations in general and so is meaningful. The chances of fixation of a particular neutral mutation are the chances of this particular mutation happening and so are very very small. So if you look at a newborn baby and go "Hmm, this baby has a novel mutation X, it looks to be neutral, what are the chances that this specific mutation X will fix itself in the population?", the chances are very low. On the other hand, if you aggregate all novel mutations of all babies born, say, during this year, then the chances that one (and we don't know which one) of these mutations will survive and get fixed are more meaningful.
This is irrelevant to the original claims you were making, which I was responding to: http://lesswrong.com/r/discussion/lw/mbl/when_does_heritable_low_fitness_need_to_be/cgrk?context=1#cgrk You claimed: This is totally wrong. It can easily increase: genetic drift/random walk. Not only is it possible, all sorts of neutral mutations reach fixation all the time. It's true any particular instance of any mutation may not win the lottery, but it's quite different to argue that there is no such thing as a lottery and no one has ever won a lottery! There is no question to beg; the gene in question may simply be yet another lottery winner like what regularly happens. At any time, there are countless variants which are slowly working their way up to fixation or down to extinction. It is entirely meaningful to ask about the probability they will make it to either endpoint and how fast, and if you are looking at an existing variant with a particular prevalence, weird to object to it on the basis that each time the novel mutation appeared (and it could have appeared many times before succeeding in spreading as much as it did) it had little chance of spreading; perhaps, yet, it did.
Read the comment you linked more carefully. I'm not talking about reality -- I'm talking about the model which RichardKennaway proposed. Specifically, I find this model too simple because it has a single force acting on the spread of a gene -- the "selective disadvantage D". Note, by the way, that it's not about neutral mutations at all, presumably D is not zero and we are talking about mutations which are actually selected against. Given that, the expected value of X (I'll grant you that I should have been more clear that I'm talking about the expected value and not about what one instantiation of a random process could possibly be) must decrease. In the model proposed there is.
Yes, exactly this: I would like very much to know if a quantitative model predicts homosexuality should not persist at current rates without positive selection.
Alternatively it could mean something in the environment has recently changed that causes some genotypes that wouldn't previously manifest phenotype P to do so.
For instance, the gene could cause homosexuality iff you eat soy products.
Or being exposed a some virus as westhunter hypothesizes. Or being exposed to pro-homosexual memes as the conservatives suspect.
A virus which we would not have been exposed to in our evolutionary environment? If this hypothetical virus was confined to a specific geographical location before being transmitted during the age of sail, then there could be a population with ancestory from that location who would have evolved natural immunity. Do you know of any such population? If so, it could be a way to prove the theory.
That's would certainly prove the theory, as well as helping pinpoint the virus. But many viruses appeared in historical times, either making the jump from other species or evolving. The rate at which new viruses evolve is greater than before due to the very large and connected human populations they evolve in.

If later-born children are more likely to be homosexual, then it seems trivially true that the more children one is inclined to have, the higher percentage of them will be gay. If any genetic mechanism encourages having more children, we'd expect there to be an apparent genetic link as well.

Alternatively, if error-checking causes spontaneous abortions of homosexual children (assuming homosexuality is an error in construction rather than in the instructions), then we would expect more children to be born to those without that error-checking. (And false positives of whatever error-checking is encoded may make it prohibitively expensive to do so.)

Short answer - no, this is a hard, ongoing problem.

I think you're looking for the concept of 'mutational variance'. This is the amount of variation in a trait that is generated by random mutation. The variance in a trait is going to be determined by the balance of mutational variance and selective effects. Things with lots of genes effecting them will have a large 'mutational target size'. So for instance intellectual disability has a large mutational target size because there are so many different ways to break a brain, while some kinds of haemophilia hav... (read more)

This brings to mind the notion of Heterozygote advantage for certain traits. For example, there is the sickle-cell trait. One allele makes you highly resistant to malaria. Two gives you sickle-cell anemia. In a population where malaria is a grave threat, the trait is worth it in the general population even if some poor saps get shafted with the recessive genes. For reference, Wiki quotes a rate of 2% of Nigerian newborns having sickle-cell anemia.

If there's some process where homosexuality is a fail mode, then as long as it confers a net overall advantage one would expect it to persist.

For obvious political and social reasons, it's hard to be sure how many people are homosexual.

Without defining exactly what you mean by homosexual the question of how many people are homosexual isn't very meaningful. Different definitions are going to give you different answers.

We're interested in homosexuality that lowers fitness by making people not marry and not have children, in societies where same-sex couples are not accepted. A necessary condition is the lack of attraction to the opposite gender.
What exactly does "lack of attraction to the opposite gender" mean? How do you measure it? Why is this important? The standard political definition of homosexuality is about self identification and not about attraction.
It's not the political definition that anyone here is using. It's the meaningful one related to behavior. Talking about the "political definition" is like turning up to a group of electrical engineers talking about "positive" charges and complaining that electrically charged things can give you shocks hence aren't very "positive"(political definition).
There not a single one but multiple different ones that you can pick that relate to behavior. That's why when having a discussion like this it makes sense to explicitly define terms.
This reminds me that people do complain about a related issue.
It's important because we're trying to make a definition of homosexuality-that-reduces-fitness. If fitness is not reduced, there is no need to explain evolution not reducing homosexuality rates.

We don't wonder how it is possible that selection pressure allows anencephaly to occur in 1 in 4859 births.

Oh, but we do.

I'd recommend a couple of West Hunter posts: this and this.


Probably the biggest well-understood case involves two common variants of APOL1, a gene that mostly transports lipids, but also zaps trypanosomes – the cause of sleeping sickness. Humans with the standard form of APOL1 are immune to most trypanosomal infections, but two strains have evolved resistance to the standard form of APOL1 – they’re the ones that cause slee

... (read more)
These don't seem to be related to anencephaly in particular? Not sure if you meant to imply that they do. Certainly some mutations trade off a harm against a benefit, sickle-cell anemia is the classic example, but that doesn't mean all or even most of them do.
No, I don't imply any connection to anencephaly. The point is just that many deleterious outcomes that we observe are a trade-off against something. In some cases we know against what -- as you mentioned, sickle-cell anemia is an example. But in most cases we do not know. I would expect that a prior of "it's a trade-off against some advantage, we don't know yet which one" to hold as a rule, but I also expect to find some exceptions to it as well.
Not all but most very common obviously harmful mutations involve some kind of tradeoff or somehow fails to affect reproduction. If something kills you in older age it's free to spread, if something isn't so terrible it can also spread through founder effects in a population but if some trait obviously hurts people at a young age but is still common it's a good sign that it's giving or did give some kind of advantage. .
Is homosexuality very common, at 1-3%? This requires quantitative analysis.
1 in 30 of the population counts as very common in genetics terms.

I remember reading about schizophrenia being a group of similar of genetically distinct diseases. This is a paper on the subject. It also results in lowered fitness yet has a genetic component. It turns out that polygenetic traits can't be understood using simplistic theories of selection.

Being a man greatly reduces reproductive fitness, compared to the reproductive success of women. E.g., at age 12, for example, the death rate for boys is 46 percent higher than the rate for girls. And there are probably other factors that add to less reproductive success among males besides death. Being both gay and male doesn't seem like that much of a difference.

If this were true, then the human species would acquire an unequal ratio of men to women (with more women), until the fitness of both was the same (because men would be in greater demand). There are species which work that way, like sea lions. This is known as Fisher's Principle.

Men and women have, on average, the same number of children. Gender doesn't affect reproductive fitness in and of itself.
Gender can affect reproductive fitness, in a population that isn't at a stable sex ratio. For instance, if people kill most girl babies at birth because they prefer sons to daughters, then the few women who do grow up will necessarily have higher fitness than the average man - because most men won't reproduce at all.
You're correct about stable sex ratios. (It's unimportant, but your example doesn't apply to stable sex ratios as far as evolution goes.)
What do you mean?
The 50% who get killed lowers the reproductive fitness of having a girl by the exact amount that the reproductive fitness of having girls is raised by the lower percentage of them relative to men. Or to take it from another route, the average number of children had by men and women must remain equal.
That's why I said the women who grow up have higher fitness, not all women born. The detail of my example, that girl babies are born and then killed, is easy to modify. Imagine a drug that, when taken by a woman before sex, selectively kills XY sperm. Or a sex-selective early abortifact. The average number of children by men and women must remain equal, but the average number of men children and woman children doesn't have to.