The normal at minimum is what passes the genes further. Otherwise there would be no humanity as it would go extinct.
This is an overly-simple criterion. We have many examples of people who have helped humanity prosper without themselves actually reproducing. Their sexual orientation was secondary to their other positive attributes. Yes, humanity does need a critical mass of hetero-normal people in order to survive, but perhaps that's not the whole story?
There's the popular idea that man can exist in a vacuum, individual; perhaps subsisting at best; but, in the fullest sense: free. Hobbes' "state of nature".
But we have direct evidence, from all related fields -- neurology, psychology, sociology, archaeology, etc. -- that such a state is utterly constructed, imagined.
Or if such a state did exist, it was on the order of, I don't know, 30 million years ago or more -- before our so very distant ancestors formed social groups in the first place.
Since then, survival of the species has always been tied to survival of the group. And neurological and psychological evidence shows humans have the exceptional capacity to understand the relationships between members of a group up to around 150 members (Dunbar's number), compared to 10-30 for closely related primates. (Come to think of it, I haven't heard how this number relates to the size of other social mammals, especially herding animals -- surely they must have some beef with each other past some critical size -- then again, they survive under a different dynamic, and probably they have a different fear/friend response to in/out-group individuals as a result. A farmer/rancher/shepherd would probably find the answer to this very obvious, but alas, one I am not.)
So the "state of nature" for humans, over the last couple million years that have most closely fine-tuned our evolution, has likely been as a superorganism consisting of increasingly many individuals, from a similar scale as our relatives (~10s), up to the ~150 we are at now (which is evidently about the critical mass to kick off agriculture and industrial revolution... assuming we could ever show causality from this property, heh).
And the survival of a superorganism is a very different thing from the survival of an individual. If the group is largely related*, then it doesn't much matter, genetically speaking, who dies, nor who passes on their DNA.
*Not like THAT
, of course I mean in contrast to neighboring villages; each one which might consist of many extended families, who are more closely related to each other, on average, than to their neighbors. But also not exclusive, as mixing and matching is also a necessary part of a healthy species. Whether by voluntary action (free exchange between villages), coerced (arranged weddings, strategic agreements?) or forced (rape, war raids taking prisoners, etc.). Evolution sadly doesn't mind any such distinctions.
And so we can easily explain -- I think -- "deviant" traits. Mind, this doesn't explain homosexuality
per se -- but it is nonetheless sufficient to debunk a position based on reproductive fitness.
Simply put: since the survival of a given individual is less important, while the survival of the group is paramount, then there is plenty of space for roles which help the group, without contributing to its reproduction.
Indeed, too much reproduction would be a big survival issue for a group. Reproductive control has always been an aspect of humanity, whether through use of natural medicines (contraceptives, abortifacients), or cold brutal infanticide. Harsh realities for individuals, but necessities of group survival.
And it's not like we have to go far to see living examples of such contributions. Like, can it get any more obvious -- explain old people! How can it be, that humans can live beyond childbearing age? Why don't they keel over and die when their gonads stop? (This obviously makes a stronger argument concerning women, but old men also experience reduced fertility.) Clearly, there are roles, required for group survival, that do not involve directly bearing children; else why waste good food on useless old people? Evolution found a use.
In fact, I'd be willing to bet, if you could survey such groups -- rather hard to do archaeologically, but there are few surviving isolated/uncontacted hunter-gatherer tribes, so, theoretically speaking, anyway -- and find that there is a negative correlation between homosexual population, and life expectancy. That is, if survival in the environment is more difficult, then there will be fewer old people, and a greater need for infertile people -- those not preoccupied with raising children.
And it won't be a strong correlation, but modest: there are some roles that young/adult bodies can serve, that old people simply can't; and some roles that both can. It's the exclusive roles which motivate this argument.
Notice the mechanism of this argument. It's not that births magically adjust themselves to conditions; well, maybe stress during pregnancy plays a role, that's plausible I suppose -- however, we'd have data on that today as well (which, I wonder if that particular correlation has been studied? something to look into..). It's that society itself changes, adapts, over many generations -- and along with it, the population balance, and the norms -- whether more or less "deviant" behavior is tolerated or accepted.
Mind, it's not even that a particular society need adapt itself in these ways -- as long as there's a reasonable "breeding population" of the superorganisms themselves -- the groups/villages -- selection pressures will force them, slowly but surely, either to change, to adopt norms of more successful neighboring groups, or to die -- perhaps outright, but perhaps more likely by fragmentation (individuals migrate to neighbors). Mind also, warfare plays a role here, as otherwise-healthy villages are always testing each other, sometimes in friendly sport, but also sometimes in deadly combat. If there is no environmental pressure, humans will find a way to create their own... go figure.
In contrast, where we are today: babies are, for the large part, wanted and loved, and infant mortality is at all-time lows. So we're getting the unfiltered natural occurrence that humanity ultimately evolved to (from the last 100kyr or so), without the reproductive and societal pressures associated with group survival. And as we destigmatize these formerly "deviant" traits, we're finding that, evidently the natural occurrence rate is around 15%.
And, mind, maybe (probably?) homosexuality
per se has nothing to do with it; that really just serves here as a proxy for individuals who are less fertile, by any reason, whether biologically or psychologically. Or maybe there are still other factors that would support this mechanism, that don't need to relate to I don't know, I'm not a demographer, or any of these other things. I'm just putting ideas together.
And we could still come up with explanations for homosexuality (or related traits), over other more direct traits. For example, if a group suffers from very low fertility, say in some very rough years (drought? poisoning? war?), more individuals can be pressured into reproduction, improving survival of the group; those individuals represent a reserve of reproductive capacity, that isn't normally active. Whereas some kind of congenital infertility, say, would supply the same bodies, but not offer such an emergency capacity.
[And, in case you were wondering, I use sex here, not gender, i.e., who today we might call AMAB/AWAB. Much as I'd like to, it's impossible to be culturally sensitive to imagined members of prehistoric groups, after all.]
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As for applying similar arguments today -- well, you're relatively welcome to pick and choose what your own personal morality is, these days, but the direct equivalent to prehistoric evolution would seem to be, the fitness of the whole Earth -- given how interconnected we are these days. Thus, the highest level of optimization seems to be the world economy, and we can attach some variety of utilitarianism to that -- whether it be by raw capitalist dollar value, or including some degree of constituent preference in there as well (rule utilitarianism, etc.).
Interestingly, the world seems to be made up, recursively, of superorganisms in turn. While the success of the individual, rewards success to the group, and in turn, the neighborhood, city, state, country, and world economy; in the same way that the survival of a superorganism doesn't necessarily depend on survival of any given individual, so too, we have the case that individuals, groups, neighborhoods, cities, states or countries, can come and go in search of greater global fitness. And, indeed, that is the observation; often through exceedingly, unprecedentedly violent means, at that... death of a state (or, most levels of superorganism, really) being better known as genocide.
So, given the level we've reached already -- it's not at all necessary that we continue along some imagined line of progress -- we could simply be happy enough with things as they are now. Any interest to go further, necessarily indicates interest in optimizing the superorganism. Which means, if you'd like to see, say, cybernetics, or "hard" AI maybe a singularity or something, maybe colonizing other planets/systems -- well, that'll take a more advanced level of technology than we have now, and so you must support the superorganism in aim of that goal.
However, if you hold such values, and also hold values favoring individuality, "freedom", resourcefulness -- you hold a contradiction. You cannot be in favor of both: one is towards progress, one is against it. There is no progress, nothing like the global system we have today at least -- from lone individuals hiding in little grass huts. That is, if you remove everything we have to day, that's where you'd be, worse off indeed than the real human "state of nature" in ~150 size villages -- instead playing the true Hobbesian nightmare, scared, cold and starving in a hut in the wilderness.
(And, to be clear, such a state, is indeed possible; many have certainly done so, even without the benefit of modern materials, i.e. not even taking knives, pots, etc. with them. But a means of propagating the species, it is not.)
Tim