Why aren't homosexuals becoming extinct?

[fluiddruid]

@charizard

This is a good way to put it, but if homosexuality were genetic, the basic paradox still stands. If some number of your offspring don’t have offspring themselves, from a purely evolutionary point of view they’re dead ends and will reduce the effective number of your offspring. If a genetic trait causes this a significant portion of the time then it should, over time, be weeded out.

That’s assuming that being gay reduces the effective number of offspring, which, as has been noted, is not necessarily the case (with the possible exception of perhaps the last generation or so). Historical pressures to marry and reproduce certainly could have impacted this.

How many gay men have had sex with women at some point in their lives? I’d wager, even today, a significant number.

[Koxinga]

@brossa

Also, please consider that evolutionary pressures favor those who have the most grandchildren, not the most children. Suppose (and this is totally hypothetical, by the way) a set of parents have four children, one of whom has no sex drive at all. Three of the children go on to have four children each, while the fourth does not, but may contribute to the welfare of its nieces/nephews in a significant way. So in that case 12 grandchildren are supported by 4 children, rather than 16 grandchildren. But the difference between 1/3 of an adult’s production and 1/4 of an adult’s production may be quite significant, and more of those 12 grandchildren will survive to have their own kids and grandkids.

eta: like the ‘grandma effect’ mentioned above

I may not be getting something here, so pls be patient with me. But if we’re talking about a genetic trait that (by definition) is passed down via an individual’s offspring, how could evolutionary pressures favor something that doesn’t get passed down? It’s nice that this generation has Uncle Steve to help out around the house, but Uncle Steve’s genes aren’t going anywhere. (evolutionarily speaking, smartypants, oh stop it!)

[brossa]

@Koxinga

I may not be getting something here, so pls be patient with me. But if we’re talking about a genetic trait that (by definition) is passed down via an individual’s offspring, how could evolutionary pressures favor something that doesn’t get passed down? It’s nice that this generation has Uncle Steve to help out around the house, but Uncle Steve’s genes aren’t going anywhere. (evolutionarily speaking, smartypants, oh stop it!)

In this hypothetical, gayness is a recessive trait, so the nongay children can still be carriers, and have gay children themselves.

[Revenant_Threshold]

@Koxinga

I may not be getting something here, so pls be patient with me. But if we’re talking about a genetic trait that (by definition) is passed down via an individual’s offspring, how could evolutionary pressures favor something that doesn’t get passed down? It’s nice that this generation has Uncle Steve to help out around the house, but Uncle Steve’s genes aren’t going anywhere. (evolutionarily speaking, smartypants, oh stop it!)

Yes they are. Uncle Steve shares a quarter of his genes with his nephews and nieces. Less of his genetic material’s getting passed on, sure, but some is.

[Guinastasia]

@Fireclown

Most people nowadays believe that being gay is genetic. If this is indeed true, given the fact that homosexuals do not reproduce near as much as heterosexuals, why have their numbers not dwindled?

Couldn’t the same be said for alot of conditions? Mental retardation, for example.
Besides, what about straight people who have gay children-where did the genes come from there?

ETA: and what about bisexuality?

[scotandrsn]

If homosexuality were in fact strictly genetic in origin, and were the result of a single point mutation, basic genetics tells us that about 2% of the population would be gay, regardless of the heritability of the “gay gene”.

However, there is every indication that there is more going on than simple genetics, as this article last year from the PNAS states:

Sigmund Freud has a lot to answer for. As the 20th century began, he convinced medical professionals that abnormal family relationships caused children to develop a homosexual orientation, which could be “cured” by psychoanalysis. Not until the 1970s would psychiatric and psychological associations finally repudiate this idea that gay people have any illness that needs treating. Since then, a host of reports bolster the idea that prenatal events, rather than family relations, affect the likelihood that a person will grow up to be straight or gay. For example, for each additional brother that precedes him, a boy’s chance of growing up to be gay increases by a third. If Freud had known that, he might have suggested that the presence of older brothers shifts family dynamics, subjecting the youngest son to a social milieu that leads to homosexuality. However, in this issue of PNAS, Anthony Bogaert (1) provides evidence that the social influence of an older brother is irrelevant to whether his younger brother will develop a homosexual orientation. It is the number of older biological brothers the mother carried, not the presence of older brothers while growing up, that makes some boys grow up to be gay. Older stepbrothers in the home have no effect, although older biological brothers raised apart still exert their influence. These data, by elimination, strengthen the notion that the common denominator between biological brothers, the mother, provides a prenatal environment that fosters homosexuality in her younger sons.

(emphasis mine).

[Darwin_s_Finch]

@brossa

Also, please consider that evolutionary pressures favor those who have the most grandchildren, not the most children.

Not really. Evolutionary success is determined retroactively, but NS only operates in the here and now. Natural selection only serves to give those with some sort of advantage a higher-than-average chance of reproducing and thus spreading those advantageoues traits throughout a population, while also reducing the chance of reproduction for those with disadvantageous traits. It doesn’t “care” at all what one’s chldren or grandchildren might do (except to the extent that it will scrutinze them individually if when they are produced).

[matt_mcl]

Incidentally, there’s evidence for a variety of fixed homosexual identities going back much further than just this century. They would have been thought about differently than homosexuals are this century, it’s true, but they were there, in the sense that there were definitely people who identified themselves as having a long-term, exclusive or predominant sexual and affectional preference for people of their sex. Hamish would be able to tell you more about it than I would, though.

[WhyNot]

@scotandrsn

It is the number of older biological brothers the mother carried, not the presence of older brothers while growing up, that makes some boys grow up to be gay.

Which is really freakin’ interesting, viewed in the light of the “gay uncle provider” hypothesis. The more brothers, the more likely that you’ll have lots of nephews and nieces that could use some extra looking after.

Man, I know nature doesn’t “design” anything, but sometimes it sure seems like it!

[scotandrsn]

Here is a recent overview article on the biology of homosexuality by Simon LeVay, the neuroscientist who first discovered a difference in the hypothalamus between gay men and straight men. (Page is wonky, scripting seems to screw up the loading of it sometimes).

He mentions and cites many sources for various biological origins of homosexuality. Here’s what he has to say on the subject of the OP:

Gay genes and evolution. The existence of genes promoting homosexuality is counter-intuitive, since such genes should reduce their owner’s reproductive success and thus, over many generations, they should be eliminated from the gene pool. A number of people have considered the various ways in which gay genes might persist (Wilson 1978; Weinrich 1987; Ruse 1988; Hamer and Copeland 1994; Ridley 1994; Bailey 2003). Here are some of the ideas that have been put forward:

1.                          Gay genes might persist if gay people, though having few children themselves, promote the reproductive success of their siblings (“kin selection”).
   

2.                          A gene might cause homosexuality and thus reduce reproductive success when present on two homologous chromosomes (homozygous state) but have some other, positive effect when present on one chromosome (heterozygous state). The analogy is to the sickle-cell gene which causes anemia when homozygous but confers resistance to malaria when heterozygous. If the heterozygous advantage is sufficiently great the gene will persist in the population.
   

3.                          A gene for sexual attraction to men would cause homosexuality in men but might cause a “hyper-heterosexuality” in women, thus increasing their reproductive success—and vice versa. The positive effect on the reproductive success of one sex might balance the negative effect in the other sex. Consistent this hypothesis, an Italian study reported that the female maternal relatives of gay men have more offspring than those of heterosexual men, as if a gene predisposing simultaneously to male homosexuality and female “hyper-heterosexuality” were being transmitted on the X chromosome (Camperio-Ciani, Corna et al. 2004).
   

4.                          It’s possible that, through much of human evolution, people have been socially compelled to marry and have children regardless of their sexual orientation. In this case, the negative effect of a gay gene on reproductive success might be small, and might be outweighed by some other, unknown benefit conferred by the gene.
   

5.                          The elimination of gay genes from the population (by non-reproduction of gay people) might be balanced by the occurrence of new mutations. For this to be the case, the mutation rate for gay genes would have to be exceptionally high.
   

Comment: None of these theories are particularly persuasive. The evolutionary value of gay genes may become clearer if and when such genes are identified and their mechanism of action determined.

[John_Mace]

Twin studies have shown that homosexuality is about 50% genetic. That’s high, but not as high as one might think if you buy into the genetic/not genetic dichotomy. However, it shouldn’t be surprising that something as complex as sexual attraction in humans doesn’t fit neatly into that dichotomy. There may be a genetic component and an environmental component. In fact, studies have shown that birth order of males influences the odds of being homosexual-- the more older male siblings you have, the more likely you will be gay. Bottom line, though, we simply do not know the cause or causes of homosexuality.

It is also important not to get hung up on the dichotomy that any given individual is either gay or straight. There could very well be a continuum of behaviors that we artificially put into two boxes. And as others have pointed out, even if there is a genetic component, it might have other effects that increase the reproductive success of individuals with all or some of the “gay genes” (assuming such genes exist).

[Walloon]

Missing: Research showing that having a gay uncle makes any significant difference in one’s life.

[WhyNot]

@Walloon

Missing: Research showing that having a gay uncle makes any significant difference in one’s life.

Well, he’s not gay, but I (an only niece) got a lot of really cool expensive presents from my uncle (stereo, guitar, bike) until the bastard went and got married and started breeding and spent his money on his own kids!

[Shagnasty]

As a former Ph.D. student in the neuroscience of sexual differentiation, the genetic meme irritates the hell of me because it based on a flawed view of human and animal development. Biologically determined <> genetic.

Genetics are only an indirect cause how anything develops sexually. The direct cause is the sex hormones and things can go wrong there in many ways. Genes determine whether something has ovaries or testes and these are supposed to trigger the proper sexual differentiation events but it isn’t rare for them not to. The brain goes through a sexual differentiation process on its own during development through a mechanism called critical periods. All brains (and bodies for that matter) default to the female state and need androgens like testosterone to be present during certain windows in development so they can “decide” whether to become male or female.

There are many of these windows and once they are closed, that is the end of it. It is perfectly possible for someone with male genes and testes to default to a female pattern when it comes to sexual preference even if all the other critical periods caused a male pattern of development. It is also completely possible for a genetic female to have high levels of androgens and develop in the male pattern for any given critical period.

The part that irritates may is that most people use “genetic” as equivalent to biological determinism and that simply isn’t true. The reason you follow the male or female pattern is caused directly by the sex hormones and not genes. Genes just help kick off those processes. This demonstration is easy to do in animals and has been since the 1970’s through hormone injections during development. It is also easily demonstrated in humans through some conditions ranging from androgen insensitivity disorder to pseudohermaphroditism and many more. Taken together, these disorders aren’t exactly rare and you have undoubtedly met or known people whose genetic sex does not meet their phenotypical sex. It is somewhat common in the grand scheme of things for the person not to know either until they have something like a fertility problem and find out that they have the same gonads as their partner.

It is pretty simple to produce “gay” rats through hormone injections at the right times. I have done it myself and research like this has been done in monkeys and many other types of animals. That is most likely why the genes for homosexuality haven’t died out. It is more of a biological condition rather than a genetic condition although we can’t rule out that genes contribute indirectly to the condition in some cases.

[Wendell_Wagner]

I have a theory of how homosexuality is inherited. This is only a theory, and I’m no expert in genetics. Furthermore, I’m not saying I’m certain it’s true, and I will point out some possible potential evidence against it.

Suppose sexual attraction is inherited in the same way that height is inherited. What happens for height is (something like) the following: You are born with a sex-neutral potential height inherited from your parents. The sex-neutral potential height you’re born with is approximately the average of your parents’ sex-neutral potential heights. If you’re a boy, the sex-neutral potential height you’re born with is your adult height minus two and a half inches. If you’re a girl, the sex-neutral potential height you’re born with is your adult height plus two and a half inches. As you grow up, the sexual hormones circulating in your body inhibit or enhance your growth potential. If you’re a boy, the sex hormones tend to make you grow two and a half inches more than your sex-neutral potential height. If you’re a girl, the sex hormones tend to make you grow two and a half inches less than your sex-neutral potential height.

This is why you inherit your height from both of your parents, despite the fact that your father is (on average) five inches taller than your mother, so if you’re a boy, most of the time you’ll be taller than your mother, and if you’re a girl, most of the time you’ll be shorter than your father. That’s why the best way to guess the adult height of a baby is, for a boy, average the father’s height with (the mother’s height plus five inches), and, for a girl, average the mother’s height with (the father’s height minus five inches). Equivalently, for a boy, subtract two and a half inches from the father’s height, add two and a half inches to the mother’s height, average the two numbers, and add two and a half inches. For a girl, subtract two and a half inches from the father’s height, add two and a half inches to the mother’s height, average the two numbers, and subtract two inches. Note: This is the best guess for the baby’s eventual height, not an exact prediction. It has a standard deviation of about two inches. Look up the term “standard deviation” if you don’t understand it. So if a father is 6’0" and a mother is 5’7", their boys will be between 5’10" and 6’2" two-thirds of the time, between 5’8" and 6’4" 95% of the time, etc., while the girls will be between 5’5" and 5’9" two-thirds of the time, between 5’3" and 5’11" 95% of the time, etc.

Suppose then that everyone as an adult has a sexual attraction number. The adult sexual attraction number is between (let’s say for the purposes of this argument) -10 and 10. If your sexual attraction number is -10, you’re solely attracted to women. If your sexual attraction number is 10, you’re solely attracted to men. If your sexual attraction number is 0, you’re equally attracted to men or women, and so forth from all numbers between -10 and 10. As with height though, what you inherit from your parents is something we’ll call a sex-neutral potential sexual attraction number. Your sex-neutral potential sexual attraction number is the average of your parents’ sex-neutral potential sexual attraction numbers. Let’s say, for the purposes of this argument, that a man’s sex-neutral potential sexual attraction number is his adult sexual attaction number plus 5. Let’s similarly say, for the purposes of this argument, that a women’s sex-neutral potential sexual attraction number is her adult sexual attraction number minus 5. As children grow up, sex hormones in their body affect their sexual attraction number. For a boy, the sex hormones cause his adult sexual attraction number to be 5 points lower than the sex-neutral potential sexual attraction number he was born with. For a girl, the sex hormones cause her adult sexual attraction number to be 5 points lower than her sex-neutral potential sexual attraction number.

So suppose a completely hetrosexual man and a completely hetrosexual woman have children. The man’s adult sexual attraction number is -10, so his sex-neutral potential sexual attraction number is -5. The woman’s adult sexual attraction number is 10, so her sex-neutral potential sexual attraction number is 5. The average is 0, so (on average) either a boy or a girl will inherit a sex-neutral potential sexual attraction number of 0. There’s probably a standard deviation to take account of here. Let’s say that the standard deviation is 2 points. So two-thirds of the time the child will inherit a sex-neutral potential sexual attraction number between -2 and 2, 95% of the time they will will inherit a sex-neutral potential sexual attraction number between -4 and 4, etc. This means that, as adults, the boys in this marriage will have sexual attraction numbers between -7 and -3 two-thirds of the time, between -9 and -1 95% of the time, etc. The girls, as adults, will have sexual attraction numbers between 3 and 7 two-thirds of the time, between 1 and 9 95% of the time, etc. Nearly all the time then, the children will be hetrosexual, but there is some reasonable chance that they will be homosexual.

On the other hand, suppose a completely hetrosexual man (adult sexual attraction number -10 and sex-neutral potential sexual attraction number -5) marries a mildly hetrosexual woman (adult sexual attraction number 2 and sex-neutral potential sexual attraction number -3). The sex-neutral potential sexual attraction number of their children will be, on average, -4. This means that the boys will have an adult sexual attraction number between -10 and -7 two-thirds of the time, between -10 and -5 95% of the time, etc. (Note that when this theory predicts an adult sexual attraction number lower than -10 or higher than 10, just round it to -10 or 10, respectively.) The girls in this marriage will have an adult sexual attraction number between -1 and 3 two-thirds of the time, between -3 and 5 95% of the time,. etc. This means that while the boys in this marriage will very likely be hetrosexual, there’s a substantial chance that the girls will be homosexual.

Go the other way now. Suppose a completely hetrosexual woman (adult sexual attraction number 10 and sex-neutral potential sexual attraction number 5) marries a mildly hetrosexual man (adult sexual attraction number -2 and sex-neutral potential sexual attraction number 3). The sex-neutral potential sexual attraction number of their children will be, on average, about 4. This means that the girls will have an adult sexual attraction number between 10 and 7 two-thirds of the time, between 10 and 5 95% of the time, etc. The boys in this marriage will have an adult sexual attraction number between -3 and 1 two-thirds of the time, between -5 and 3 95% of the time,. etc. This means that while the girls in this marriage will very likely be hetrosexual, there’s a substantial chance that the boys will be homosexual.

This theory predicts that male hetrosexuality will run in some families while female sexuality will run in other families. There will not be a tendency for male hetrosexuality and female hetrosexuality to run in both families. Is that true? I don’t know. The good thing about this theory is that it predicts that there is a genetic component to homosexuality and hetrosexuality. This theory can be disproved easily though if it turns out that male and female hetrosexuality run in the same families. In any case, what this theory predicts is that what is being inherited is not homosexuality/hetrosexuality, but attraction to men/women.

[Darwin_s_Finch]

@Walloon

Missing: Research showing that having a gay uncle makes any significant difference in one’s life.

Also missing: evidence that a person who might be genetically gay assisting with someone who is not genetically gay would have any effect at all on the gay individual’s genome’s persistance. The whole “gay uncle” thing only works if the “gay gene(s)” are found throughout the population, gay or not, in which case, the “gay uncle” hypothesis becomes moot…

[Moirai]

@irishgirl

Ummm…correct me if I’m wrong, but most gay people have straight parents, no?

All the ones I know certainly do- 100%. So, I guess that hetero marriages are to blame for teh gay!

[John_Mace]

@Darwin's Finch

Also missing: evidence that a person who might be genetically gay assisting with someone who is not genetically gay would have any effect at all on the gay individual’s genome’s persistance. The whole “gay uncle” thing only works if the “gay gene(s)” are found throughout the population, gay or not, in which case, the “gay uncle” hypothesis becomes moot…

But no thread about this subject would be complete without someone offering this “hypothesis” up. Frankly, it sounds more like a just so story to me, and if it were true we’d be able to document it pretty easily in any number of the more primitive societies that have been studied. Sure, some of those societies reserve special rolls for gay men, but that’s not the same thing. And I’m not aware of any primitive society that recognizes gay women-- a woman unattached to man in such a society stands little chance of survival.

[magellan01]

@irishgirl

Ummm…correct me if I’m wrong, but most gay people have straight parents, no?

Since people keep having babies and their babies keep turning out both gay and straight, I don’t think we’re ever going to live in a totally gay or straight world.

That, to my mind, is a good thing.

This raises an interesting question; Why is that a good thing? Miind you, I’m not arguiing at all the converse. But why would not every individual being heterosexual, and thereby experiencing both procreation and parenthood, be the good thing? Or the better thing?

[Harriet_the_Spry]

@John Mace

And I’m not aware of any primitive society that recognizes gay women-- a woman unattached to man in such a society stands little chance of survival.

Although a woman’s sexual preferences would have relatively little to do with her reproductive behavior in such a society, as they have for much of history.