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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.
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!)
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.
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.
"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."
Not "undetermined", but the result of a process complex enough that it's very difficult to predict the outcome or identify the root cause.
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.
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?
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.
It seems to me that any mental feature that can be explained should be explained.
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.
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?
Do they? Or is that just another assumption of yours? Do you have a good reason for believing this one?
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.
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.
From the perspective of the parents of the gay couple, it's a 0% fitness cost, though.
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.*
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.
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.
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.
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).
Monogamy isn't necessary; it works with polyandry and other arrangements such as levirate marriage too.