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Homo sapiens have menopause - as some other mammals (e.g. some Cetacea) - a phenomenon where older females live for decades after the end of their own reproduction.

Perhaps homosexuality, that looks to hold at 10% levels in any population, may be just the same sort of adaptation?

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    $\begingroup$ A quick scan of global and US data on wikipedia suggests the rate is closer to 5%. $\endgroup$ – user137 Apr 30 '15 at 18:03
  • $\begingroup$ To answer this question, we need to know the 4 values in the pay-off matrix, that is nb of babies of hetero sibling of a hetero, hetero sibling of homo, homo sibling of homo and homo sibling of hetero. $\endgroup$ – Remi.b Apr 30 '15 at 18:20
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    $\begingroup$ Also see: biology.stackexchange.com/questions/7669/… $\endgroup$ – kmm Apr 30 '15 at 19:22
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    $\begingroup$ I would put between 5 and 10%, @user137. But homophobia makes lots of homosexuals deny themselves (lowering the numbers in most researches). And other biological phenomena occurs at the 10% threshold (left-handedness, black jaguars, left-turning shells, etc.) $\endgroup$ – Rodrigo Apr 30 '15 at 20:04
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    $\begingroup$ Is there a reason you expect black jaguars and homosexuality to occur at the same rate? $\endgroup$ – canadianer Apr 30 '15 at 21:59
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Barnard's wonderful text "Animal Behavior: Mechanism, Development, Function and Evolution" offers some enlightening summary to the discussion of homosexuality in a biological context!

Brief Summary to ch 12, page 606:

What we know so far:

  1. homosexual behavior is recorded all throughout animal kingdom
  2. there are evidence for genetic basis to homosexuality in various species including humans

Three interrelated schools of thoughts:

  1. Homosexuality is maladaptive - external stress creates homosexuality

    • reduce resource stress on family unit, youngest child tend to be gay (E.M. Miller 2000)

    • *** mitochondrial DNA carries male killing genes, with male being a dead-end (Hurst 1991)

  2. Homosexuality as an adaptive early phase of development - homosexual acts are just target practice

    • homosexuality is just males practicing sex on each other, most homosexual acts are performed by bisexuals with just as high fertility as straight males (Baker & Bellis 1995)
  3. Homosexuality advantages in other aspects of reproduction - homosexuality may imply better parenting

    • homosexual genes induces feminine attributes in males such as sensitivity, empathy and kindness hereby increasing likelihood of long term pair bonding and paternal care (E.M. Miller 2000)

*** original text:

A completely different suggestion is that homosexuality may reflect the interests of selfish genetic elements (see 2.4.3.2) inherited maternally on mitochondrial DNA for which males are effectively a dead end. Some insects have been shown to carry 'male-killing' genes on their mitochondrial DNA, so perhaps homosexuality is a similar, if more benign, rout to sidelining males (Hurst 1991)

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  • $\begingroup$ keep in mind these are ot mutually exclusive either, non-exclusive homosexuality can increase reproductive success, exclusive homosexuality is rare enough it could be a side effect of this beneficial effect. homosexuality is not a single behavior. $\endgroup$ – John Jul 7 '18 at 23:25
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There is a study that found that female maternal relatives of homosexuals have higher fecundity than those of heterosexuals [1]. This suggests that an allele predisposing to homosexuality may propagate because it increases reproductive success in some situations, not necessarily in the homosexual individual.

However, many factors can influence a persons sexuality, such as birth order (in males each additional older brother increases the odds of homosexuality by approximately 33%) [2], so genetics only tells part of the story, and I found no comparable explanation for female homosexuality.

Here's an interesting article if you want further reading [3].

  1. Camperio-Ciani et al, 2004. Proc Biol Sci. Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity
  2. Cantor et al, 2002. Archives of Sexual Behaviour. How Many Gay Men Owe Their Sexual Orientation to Fraternal Birth Order?
  3. IFL Science. Born this way? An evolutionary view of ‘gay genes’
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Genetic view of adaptation:

Do note that this is looking at the view from the POV of a gene. Please read the comments below this answer, for a small discussion on the kin selection hypothesis. Do note that I do not study evolutionary psychology, or work on the level of the population.

This is a bit of a misconception, as nature really doesn't work like that. Nature works by allowing people to pass on genes, and then selecting those genes which are the best. Now the issue with homosexuality when viewing from an evolutionary angle is that, homosexuality by definition doesn't allow for genes to be passed on. I would also quote Richard Dawkins in this issue

“That’s a very tempting error,” Dawkins replied, “not with respect to homosexuality, but with respect to the idea that nature does population control. That’s not the way nature works.”

“It’s so tempting to think that things will be best for the species, so nature will do whatever it takes to keep the species going. But that’s just not the way evolution works.”

“Natural selection,” he continued, “works at the level of the individual gene. So in order for natural selection to favor population control, [it] would have to favor a gene or genes within an individual that limited that individual’s reproduction.”

“Homosexuality would do that, but of course, it wouldn’t actually be naturally selected because the individual [who] doesn’t have children doesn’t pass on the genes for not having children.”

“In order for natural selection to favor population control,” Dawkins said, “it would have to be the case that an individual who has too many children ends up rearing fewer” successfully.

He noted that this is exactly what ornithologist David Lack found when he studied avian clutch size — those birds that laid too many eggs were not able to care for all their hatchlings, whereas as those who laid too few were not selected for the obvious reason — they didn’t produce enough progeny.

Taken from: http://www.rawstory.com/2015/03/richard-dawkins-explains-homosexuality-didnt-evolve-as-natural-population-control/

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    $\begingroup$ The OP is referring to kin selection processes (or group level processes if you prefer). Selection does not only act at the level of the gene. Homosexuality might be beneficial if sibling of homosexuals have more babies. The allele would indeed increase in frequency. From Hamilton and Nash work, we can calculate the equilibrium frequency of homosexuality from the pay-off matrix. $\endgroup$ – Remi.b Apr 30 '15 at 18:25
  • $\begingroup$ @Remi.b Interesting you mention that. I was reading up on [Bobrow, David, and J. Michael Bailey. "Is male homosexuality maintained via kin selection?." Evolution and Human Behavior 22.5 (2001): 361-368.] and [Rahman, Qazi, and Matthew S. Hull. "An empirical test of the kin selection hypothesis for male homosexuality." Archives of sexual behavior 34.4 (2005): 461-467. ] and they seem to find no support for this? Do note that I am not an evolutionary psychologist, so I am not an expert in this field and am not upto date on literature. Has supporting evidence been documented ? $\endgroup$ – Rover Eye Apr 30 '15 at 18:35
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    $\begingroup$ It's also incorrect to assume that homosexuals never pass on their genes. There's nothing about being gay that makes you physically incapable of reproduction. $\endgroup$ – user137 Apr 30 '15 at 19:03
  • $\begingroup$ @user137 Yes, I agree that homosexuals can reproduce. But do note that homosexuals, can not reproduce, unless there is a partner of opposite sex involved (which goes against the scientific definition of homosexuality). What I mean is, if a homosexual chooses to reproduce by in vitro fertilisation, surrogacy or any of the other ways that currently exist, though legally a homosexual, there has to be a biological partner of the opposite sex involved. (I mean no disrespect to the LGBT community). $\endgroup$ – Rover Eye Apr 30 '15 at 19:12
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    $\begingroup$ Yes, I'm thinking about kin selection. Rover Eye, Bobrow & Bailey found that "homosexual men were somewhat more estranged from family members, especially from fathers and oldest siblings". This seems a consequence of homophobia, not of homosexuality itself. I don't think there was homophobia in the environment where homosexuality evolved. Rahman & Hull used a very reduced sample (n = 120) in a country well known for its historical homophobia. $\endgroup$ – Rodrigo Apr 30 '15 at 19:30

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