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.
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...
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.
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.
Ulcers are far more heritable than homosexuality, and genetics matters: but you don’t get the ulcers without h. pylori.
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...
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...
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.
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...
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...
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...
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 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.
Sample:
...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
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.
Gay historian Rictor Norton vehemently disagrees with the notion that gay identities are recent. Here is his basic position:
He takes a position against social constructionism:
It is very easy for historians to establish that most of the sexual categories which are supposed to have arisen under modern capitalism in fact existed much earlier. ...
One of the reasons why many contemporary lesbian and gay theorists fail to appreciate that homosexuals existed before 1869 is the politically correct view that terms such as ‘queer’ and ‘faggot’ and ‘queen’ are not nice, and especially since the late 1960s people have endeavoured to use the phrase ‘gay and lesbian’ wherever possible. There are some men who lived before 1869 whom I would feel uneasy at calling ‘gay’ or ‘homophile’, but I would not hesitate to call them queer or even silly old queens. Many of the mollies of the early eighteenth century were undoubtedly queens, whose interests and behaviour are virtually indistinguishable from queens I have known in the early 1960s (and later). ...
‘Queer’ was the word of preference for homosexuals as well as homophobes for the first half of the twentieth century, and of course is being reclaimed today in defiant rather than defensive postures. In English during the eighteenth and most of the nineteenth century the words of preference were ‘molly’ and ‘sapphist’, for which good modern equivalents are ‘queer’ and ‘dyke’. During the seventeenth century and earlier the commonest terms were ‘Sodomite’ and ‘tribade’, for which, again, good modern equivalents are ‘queer’ and ‘dyke’. In ancient and indigenous and premodern cultures there were many terms for which good modern equivalents are ‘queer’ and ‘tomboy’. And the nearest modern equivalent for the nineteenth-century term ‘homosexual’ is: queer. ...
I add my voice to the widespread dissatisfaction with social constructionist thought, that seems to have been based on nothing and to have lead nowhere in the past twenty years. Its initial premises have been constantly reinforced by restatement and incestuous quotation amongst constructionist colleagues rather than supported by scholarly research.
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.
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:
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?
Notes:
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.