When does heritable low fitness need to be explained?

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.

This comparison is a category error--the comparison isn't "homosexuality" vs. "all birth defects," but "homosexuality" vs. "any individual birth defect."

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.

Note that anencephaly leads to early death, and you might think that in most evolutionarily relevant time periods, the costs of of raising a child to adulthood may have been significantly larger than the costs in bringing a pregnancy to term. A gene that halves your chance of giving birth to a child with anencephaly will have a very tiny impact on your overall fitness, and thus take a long time to spread through the population.

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.

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.

On the other hand, 'obligate' is a strong word to use for things as complicated as human behavior.

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.

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.

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.

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.

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.

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.

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.

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?

When you compare it to birth defects, you're comparing one problem against a host of problems. It's worth about the same to evolution to fix the one as to fix all the rest.

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.

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.

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:

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.

but the general expectation was that people entered heterosexual marriage and had children.

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?

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.

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?

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...

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.

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.

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.

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.