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The Intelligent Homosexual’s Guide to Natural Selection and Evolution, with a Key to Many Complicating Factors

The views expressed are those of the author and are not necessarily those of Scientific American.

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June is Pride Month in the United States, and in communities across the country, lesbian, gay, bisexual, and transgendered Americans are celebrating with carnivals, parades, and marches. Pride is a rebuke to the shame and marginalization many LGBT people face growing up, and a celebration of the freedoms we’ve won since the days when our sexual orientations were considered psychological diseases and grounds for harrassment and arrest. It’s also a chance to acknowledge how far we still have to go, and to organize our efforts for a better future.

And, of course, it’s a great big party.

I’m looking forward to celebrating Pride for the first time in my new hometown of Minneapolis this weekend–but as an evolutionary biologist, I suspect I have a perspective on the life and history of sexual minorities that many of my fellow partiers don’t. In spite of the progress that LGBT folks have made, and seem likely to continue to make, towards legal equality, there’s a popular perception that we can never really achieve biological equality. This is because same-sex sexual activity is inherently not reproductive sex. To put it baldly, as the idea is usually expressed, natural selection should be against men who want to have sex with other men–because we aren’t interested in the kind of sex that makes babies. An oft-cited estimate from 1981 is that gay men have about 80 percent fewer children than straight men.

Focusing on the selective benefit or detriment associated with particular human traits and behaviors gets my scientific dander up, because it’s so easy for the discussion to slip from what is "selectively beneficial" to what is "right." A superficial understanding of what natural selection favors or doesn’t favor is a horrible standard for making moral judgements. A man could leave behind a lot of children by being a thief, a rapist, and a muderer–but only a sociopath would consider that such behavior was justified by high reproductive fitness.

And yet, as an evolutionary biologist, I have to admit that my sexual orientation is a puzzle.

There’s reasonably good evidence for a genetic basis to human sexual orientation–although the search for a specific "gay gene" has had mixed results. Gene variants, or alleles, associated with an 80 percent decrease in reproductive fitness should be naturally selected out of the population pretty quickly. So why aren’t all humans heterosexual?

Straight people are in the overwhelming majority, but gay men, lesbians, bisexuals, and transgendered people account for a non-trivial minority–the most recent survey I’m aware of found 7 percent of women and 8 percent of men in the U.S. identify as L, G, B, or T. We don’t have remotely comparable historical data, but mention of same-sex sexuality goes back to the dawn of recorded history. If natural selection is homophobic, it’s not particularly good at it.

Before I get much farther, here’s a disclaimer: I’m going to consider how same-sex attraction might persist in human populations in the face of its apparent selective disadvantages. In the absence of direct data–such as systematic measures of the total evolutionary fitness of gay men or lesbians in specific societal contexts–it’s easy to make up stories about natural selection, but much harder to determine which stories reflect reality. I’ll try to delineate which stories fit with what we know about how selection works, and with the little data we do have–but that’s the best I can do. If there’s one point I hope you take from all that follows, it’s that evolution is complicated, and human evolution doubly so.

Natural selection isn’t all-powerful

Natural selection causes traits associated with having fewer children to become less common over time. But natural selection is not the only evolutionary process at work in natural populations. Mutation introduces new alleles even as natural selection removes them. Furthermore, the effects of random chance in small populations creates an effect called genetic drift, which can interfere with the expected operation of natural selection.

Evolutionary biology has developed an excellent understanding of how mutation, selection, and drift interact over time to shape the genetic diversity of populations. That understanding allows us to do some back-of-the-envelope calculations to see how selection might operate on a gene associated with same-sex attraction. In setting this up, I’m following the lead of the evolutionary biologist Joan Roughgarden, who makes a similar point in her book Evolution’s Rainbow. Brace yourself for some math!

In an idealized population of infinite size, the balance between natural selection’s effect of removing disadvantageous alleles, and mutation’s effect of spontaneously re-creating them, means that the equilibrium frequency of the allele in the population should be about equal to the square root of the ratio between the mutation rate and the selective cost associated with carrying two copies of the disadvantageous allele.

A single base pair of human DNA has a chance of of mutating equal to about one in one hundred million every generation. Since there may be thousands of base pairs in a single gene, the probability of a mutation occurring somewhere in the gene is more like one in one hundred thousand. If we assume that it takes two copies of our hypothetical "gay allele" to make a person attracted to members of the same sex, and about five percent of people are attracted to members of the same sex, then mutation alone could balance a selective cost to being gay of 0.0002. That is to say, mutation-selection balance alone could explain the frequency of LGBT folks in the population if those attracted to the same sex had, on average, 0.9998 children for every child born to the average straight parent. That’s pretty weak selection.

This is where genetic drift enters the picture. Most natural populations don’t behave anything like the mathematical ideal assumed for the calculations in the preceeding paragraph, because most natural populations are not infinite in size. In finite populations, randomness–"mere bad luck" in the words of pioneering biologist J.B.S. Haldane–can prevent selection from operating efficiently. Smaller populations are more prone to genetic drift–the relevant number is not necessarily the number of individuals in the population, but the effective population size. The classical estimate of the human effective population size is about ten thousand, and more recent estimates have come up with even smaller numbers. This is because our population’s expansion to billions is a very recent phenomenon by evolutionary standards, and may reflect the fact that for much of our history we lived in smaller, isolated populations. For populations in this size range, selection may not operate efficiently.

Setting up an experiment that would take into account the effects of drift, mutation, and selection acting together on the human population is impossible both in practical and ethical terms. That leaves biologists two ways to approach the question of how a particular disadvantageous allele can persist in the human population: intensive study of the population’s genetics, and mathematical or computer-based modeling. Lacking easy access to massive amounts of human population genetic data, I’ve built a computer model.

The model is a script for the excellent open-source programming language R, and you can download it here if you’re interested. In an nutshell, it simulates the evolution of a population of critters that may have two copies, one copy, or no copies of a deleterious allele. Critters with two copies have their chances of reproducing reduced by a set amount, which is the selective cost of carrying two copies of the allele; critters with one or no copies experience no such cost.

Every generation, the critters who survive to mate pair with randomly-selected partners to form offspring, who then replace their parents to start the cycle all over again. This randomized mating allows for drift to occur–everyone who survives to mate has an equal chance to mate, but some are randomly paired more than once, and some miss out. Mutation occurs at the moment of reproduction, when the alleles passed on from parents to their offspring have a small chance of changing to the deleterious form, or back to the harmless form.

I set up the simulation with a population size of ten thousand, in which about five percent of the critters carry two copies of the deleterious allele. I set the cost of carrying two copies of the allele to 20 percent, and the probability of mutation each generation to one in one hundred thousand. Here’s what happens to the percentage of critters carrying two copies of the deleterious allele over fifty generations of sim-evolution. The blue line is the percentage of critters in the population carrying two copies of the deleterious allele; the dotted black line marks the starting percentage, for reference.

You can see that selection wins out, as we’d expect–but it doesn’t do so immediately. In fact, there are periods where the percentage of critters with two disadvantageous alleles increases. That’s the randomness of drift in action.

To get a sense of how this drifting randomness plays out in general, we need to run the simulation many times, and see what tends to happen. This is like flipping a coin over and over to see whether it really does land heads side up fifty percent of the time. Here are ten replicate simulations, with graphs like the one above superimposed on each other for comparison:

And here are a hundred simulations:

So drift and mutation complicate things–sometimes, rarely, the disadvantageous allele persists for 50 generations. However, we can say from these simulations that selection removes the allele far more often than not. These simulations assume selection quite a bit weaker than the widely-cited cost of same-sex sexuality–20 percent lower fitness instead of 80 percent–so you would expect selection to be even more effective against an allele that makes men gay and women lesbian. However, as I noted above, same-sex sexuality has been present in human populations for considerably longer than 50 generations. That suggests my simulations don’t accurately reflect the real-world evolution of human sexual minorities–not because I’ve simulated weaker selection, but because I’ve simulated selection that works too well.

It could be that I’ve modeled the genetics incorrectly. Most of what I’ve discussed up to now assumes that there’s only a single gene involved in determining sexual orientation. However, as the evolutionary biologist Douglas Futuyma has pointed out in his review of Roughgarden’s book for the journal Evolution, if multiple genes are involved, they would "share" the selective costs associated with same-sex attraction. Selection would then have proportionally less power to remove alleles for same-sex sexuality in the face of drift and mutation. Given that recent genome studies have not clearly identified a single gene region associated with sexual orientation, it seems likely that multiple genes are indeed involved.

The other possibility, however, is that selection against same-sex sexuality is not as strong as I’ve made it in my simulations. How is that possible? Well, simply put, the lives of gay men, lesbians, and transgendered people in Western societies over the last few decades might not be much like the lives we would live in other times and other places. And that could make all the difference.

Context matters

In the United States, the most well-known LGBT life story goes something like this: one discovers same-sex attraction in adolescence, and comes out of the closet before having much sexual interaction with members of the opposite sex. For committed same-sex partners, having biological children is possible via surrogacy or sperm donation, but it’s complicated by the lack of legal recognition and protection for the couple and for children they choose to have. All these factors tend to reduce the number of biological children gays, lesbians, and trans folks have–but they’re also all phenomena of our current historical and political moment. In different social contexts, LGBT fitness could very well be higher.

Before the gay rights movement, social expectations probably led many people who today would identify as gay or lesbian to enter into straight marriages and raise families. This kind of social pressure may explain why gay and lesbian couples are more likely to be raising children if they live in the conservative southern United States–not because adoption is more common in that region, but because in the South, gay men and lesbians are more likely to have heterosexual relationships, and children, before they come out.

On the other side of that coin, it’s possible to imagine that in societies in which same-sex relationships recieve the same legal recognition as straight marriages, gay men and lesbians might eventually have biological children at substantially higher rates than they do today. So if oppression might reduce the fitness cost to being gay, equality probably could, too.

Many non-Western societies, too, have accepted social roles for a "third gender" that can encompass identities approximating Western gays, lesbians, bisexuals, and transgendered folks. The roles and behaviors of third-gendered individuals vary considerably, but in some cases they may have biological children even if their primary relationships are with members of the same sex. As Christopher Ryan and Cacilda Jetha describe extensively in their book Sex at Dawn, pre-agricultural human societies may have been highly polygamous by modern standards, with children raised communally. In that context, Ryan and Jetha propose, same-sex sexual interactions may have provided social capital that could help to support children produced in relatively infrequent heterosexual couplings.

This is consistent with the picture Joan Roughgarden paints in Evolution’s Rainbow. Roughgarden’s book describes widespread same-sex activity that usually fosters social relationships in support of reproductive sex rather than instead of it. To pick just one example out of dozens, some male bluegill sunfish look and behave like females, and "court" more masculine males–then help attract a female and share in the resulting three-way reproductive opportunity by fertilizing some of her eggs. The wide range of non-reproductive sexual behavior in the broader animal kingdom suggests that human sexual minorities are just one manifestation of a phenomenon that could date back to the origins of sex itself.

Reproduction by proxy

It has also been suggested that LGBT folks might make up for a lack of biological children by boosting the reproductive success of their close relatives. My brother shares half of my genetic material–so if I help him and his future wife support more children than they would have otherwise, those nephews and nieces "count" towards the children I’m not making myself.

This kind of indirect fitness might offset a lack of direct reproduction, but I doubt that it can cancel it out. Genetically, a nephew "counts" towards my fitness about half as much as a son does. Therefore, if I would otherwise have two children on my own, I have to help my brother to have four additional children to make up for them. I’d like to think I’ll make a good uncle, but I won’t be that good. And, in fact, one survey of gay men has found that they aren’t significantly more generous toward their nephews and nieces than straight men are. (One objection you might make to this study is that it addresses our current societal context, not that in which humans originally evolved.)

A subtle twist on the indirect fitness idea that I haven’t seen in the scientific literature could be to consider not the fitness of gay men and lesbians, but their mothers. In order for my mother to have as many grandchildren as she would with two straight sons, I only need to help my brother and his wife support two additional children. That seems more achievable, and might work out in the context of the polyamorous, mutually-helping tribes described in Sex at Dawn. It’s also consistent with the observation that men are more likely to be gay if they have older brothers–a woman who has already had several straight sons might, concievably, have more surviving grandchildren by giving them a helpful gay uncle.

Finally, there is some evidence that genes associated with same-sex attraction in men might provide a fitness boost in women. Since any given gene has about a 50 percent chance of ending up in one sex or the other, a gene that makes men more likely to be gay but makes women more fertile might, on average, have no selective advantage or disadvantage. A 2004 study found that women related to gay men have more children, which supports this scenario. The biologists Sergey Gavrilets and William Rice used a mathematical model of selection on same-sex sexuality to consider this hypothesis in a 2006 paper, and found that a female fertility boost could indeed allow male same-sex sexuality to persist.

In the end, however, this is mostly storytelling–lots of possibilities, but much less hard data. What we need to test many of these ideas is detailed records of the total reproductive fitness of sexual minorities in specific social contexts–especially societies approximating the ones formed by our earliest human ancestors. The best we can say without this is that many societal contexts could have made the apparent fitness cost to same-sex attraction smaller than it appears at first glance.

So where does all of this leave the evolutionarily-aware gay man, lesbian, or transgendered person? As I noted at the start, figuring out the exact nature of our tenuous relationship with natural selection doesn’t tell us much about our moral stature, our value to society, or the best way to live our lives. It does, however, offer to answer the question that evolutionary biology can potentially answer for all human beings, regardless of orientation, gender, or race: how did we come to be what we are?

The best answer we have so far is complicated–it may be that we’re children of history and chance, not a clear-cut adaptive path. But easy lives and clear-cut answers aren’t, I think, what we celebrate in the history of our LGBT forerunners, or remember at Pride rallies. If we queer folk live our lives in the tail of a probability distribution, the the good news is that the company here is pretty good.

Thanks (or blame) for this article is owed to Steve Silberman, who got me thinking about this topic in an e-mail conversation last year; and to Chris Smith, who gave me some invaluable suggestions on an earlier draft.


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Rice, G., C. Anderson, N. Risch, and G. Ebers. 1999. Male homosexuality: Absence of linkage to microsatellite markers at Xq28. Science 284:665-7. DOI: 10.1126/science.284.5414.665

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Photo credits: Photo of motorcycles from San Francisco Pride 2008 via ingridtayler on Flickr; Human Rights Campaign Pride parade "equality" banner via PDX Pixels on Flickr; Bluegill sunfish via IcK9s [M. H. Stephens] on Flickr; Kissing sailors source unknown, image presumed in the public domain; simulation graphs and author photo via Jeremy B. Yoder.

About the Author: Jeremy Yoder is a postdoctoral associate studying population genetics at the University of Minnesota in Saint Paul. You can find more of his writing about evolution and ecology at his blog Denim and Tweed, and he’s also on Twitter. He hasn’t seen Tony Kushner’s newest play yet, but he’d happily sacrifice a fraction of his lifetime reproductive success for a couple tickets.

The views expressed are those of the author and are not necessarily those of Scientific American.


Comments 16 Comments

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  1. 1. thedarkdaimon 1:29 pm 06/21/2011

    What a fantastic article. I love that it shows how even a gene, that at first glance seems would not be selected for by natural selection, can stick around through genetic drift.

    But what really impressed me was showing how a "gay" gene could actually be beneficial and thus selected for.

    Link to this
  2. 2. jafunc 1:30 pm 06/21/2011

    Very well written article. Those who try to nail it down to one factor are troubled as it clearly exists that many factors come into play.

    Given that homosexuality has been around for so long in so many species, how can someone say it is a choice? It’s clearly natural and there is nothing wrong with it being a choice or natural.

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  3. 3. wolfkiss 2:01 pm 06/21/2011

    I third the accolades: great article.

    I appreciate that you disclaimed your speculations, but it’s useful to note that at least two of your examples are reasonably established ideas regarding evolution in the literature. Specifically:

    1) The phenotype of being gay selected for as a positive outcome is supported by the notion of Inclusive Fitness:

    2) The idea that one trait (e.g. women’s fertility) can be linked to another (e.g. gayness) is call Genetic Hitchhiking:

    I agree with jafunc that pinning most any trait to a handful of independent causes is "problematic", to put it euphemistically. I support reductionism as a tool in so far as it appreciates its intrinsic weakness in accounting for higher-order dependent interactions. For example, it’s quite probable that Genetic Drift works *with* selective and "hitchhiking" tendencies in a complex manner to sustain the ostensible stability of homosexuality and many other seemingly "deleterious" – from a moralistic/simplistic perspective – traits such as anger, jealously, aggression, and hate.

    Dynamic open systems can be a b*tch 8)

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  4. 4. Toronto Denise 2:49 pm 06/21/2011

    The author refers, perhaps automatically, to LGBT. But consider the T people for a bit. They are not all homosexual by any means. Many are heterosexual. And there are more factors involved in whether they can or do reproduce. Some so-called "MTF" transsexuals who are homosexual may, while trying to live a heterosexual life, reproduce. After genital surgery, many "MTF" transsexuals might no longer be able to reproduce. And there are a number of other possible complications. So while it is nice to be included, in this case including T people might make things muddier.

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  5. 5. BFC 3:05 pm 06/21/2011

    It is precisely this type of dribble ("And, of course, it’s a great big party.") that causes the bulk of people to not take a scientific basis for sexual orientation seriously. Is this supposed to be a serious scientific submission or the incomplete and speculative ramblings of what may or might be?

    "Lacking easy access to massive amounts of human population genetic data, I’ve built a computer model." Oh really? And that model is based on what if not real data? This is a wonderful piece for the local newspaper, but printing it in SciAm even as an editorial does a disservice to the subject and to those who are depending on a scientific explanation someday being readily explainable. If this is a summary of the best we can do on the science now, it demonstrates how pitifully far we have progressed to date.

    Jeremy, either get serious and do a real discussion of the science, or drop the pretense of babbling about nothing. As pointed out by the post above, your lumping together of categories only demonstrates a further lack of knowledge – or respect. SciAm, you do a disservice to the gay community and to your readers by publishing this type of pointless amalgamation of ideas haphazardly glued together to form nothing but a ball of semantic goo.

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  6. 6. CherryBombSim 5:51 pm 06/21/2011

    Your model accounts for the chance a gene will mutate back to it’s original sequence? Are you joking?

    You say the chance of a mutation somewhere in a gene is one in 100,000. OK, I’ll buy that as a ballpark stab at an order-of-magnitude guess, but the chance of an IDENTICAL mutation occurring somewhere else in the same population any time soon are infinitesimal. Likewise, the chance of a mutant gene mutating back to its original form are so close to zero that they can be ignored.

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  7. 7. August Berkshire 7:27 pm 06/21/2011

    Jeremy, welcome to Minneapolis. We invite you (and everyone else) to march with Minnesota Atheists in the Gay Pride parade in downtown Minneapolis this Sunday (and get a free Minnesota Atheists t-shirt), and to visit our booth at the Gay Pride festival in Loring Park this weekend.

    Link to this
  8. 8. ironjustice 11:54 am 06/22/2011

    One can use the common goldfish to show how this segment of our fishtank came to be. In the environment of that fishtank were ‘factors’ which causes some of the fish to turn from gold to black / heterosexual to homosexual and ‘in between’.
    We NOW have fish in the tank believing they are
    ‘normal’ / still gold when in fact they are NOW black / homosexual.
    These fish fail to understand they have NOT ‘evolved’ they have MUTATED and these mutations which affect their brains have caused more than just color change. Imho.
    "Iron overload in the endocrine glands and heart. The pituitary gland is one of the most vulnerable targets, leading to irreversible hypogonadotropic hypogonadism"

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  9. 9. Mong H Tan, PhD 3:52 pm 06/22/2011

    RE: Why make it hard for science — especially the science of homosexuality!?

    I thought this is a most self-reflective (psychologically mature) reporting that I have had ever read on the current issues of homosexuality science and philosophy!

    Unfortunately, being an aspiring "evolutionary biologist" — one who might have had trained under the influence of the pervasive neo-Darwinism and reductionism in biology* since the 1930s in our high academia worldwide — the writer has had presented a "wrong guide" or self-analysis in the current definition and comprehension of homosexuality issues, in our Homo sapiens population worldwide! — [*Please see my recent comments here -- -- a response to "Too Hard for Science? The Genetic Foundations of Intelligence -- RE: Absolutely not! -- The scientific answer may lie in the "neocortical foundation" of our brain: whereby our "intelligence" is primarily accumulated and expressed by learning and communications -- and not inherited or encoded in our "genes" as neo-Darwinists would have had persistently and reductively believed!?" (ScientificAmericanUSA; June 3).]

    Thus, as with the "origins" of our human "intelligence" and "knowledge" issues, there is no exception: That homosexuality — or "homosexism" as a result — may now be re-examined and better understood, from a perspective of both the "developmental biology" and the "evolutionary neuroscience-philosophy" issues, rather than from the opposite, neo-Darwinist, reductionist, or self-obscurantist sophistry in "evolutionary biology" issues — and therefore, scientifically and physiologically speaking, there would be no specific "gay" genes to be found in our genome or genotypes, at all; as rightly analyzed by Jeremy Yoder above!

    Whereas as far as homosexuality is concerned, whose origin may indeed be lying in the "zygotic foundation" of both the genetic and epigenetic expression and development processes; and their subsequent hormonal, cellular and molecular products generation and interactions, since the fetal development and growth phases and interactions in the womb; and from which and thereof the L, G, B, or T proclivity is so derived; and later "hardwired" and expressed (or self-reflected) in the brain — or the inclined sexual (or intellectual or spiritual) "mind" as a result — especially beginning at the L, G, B, or T individual’s puberty toward maturity age!? (To be continued below)

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  10. 10. Mong H Tan, PhD 3:54 pm 06/22/2011

    RE: Why make it hard for the homosexuality science!? (Continued from above)

    Thus biologically, spiritually, "anthropogenetically" or humanistically speaking, the LGBT individuals should now be fully recognized as our diverse persons among all human beings, especially in their inalienable rights and in their each zygotic foundation and development milieu — and not by "natural selection" as neo-Darwinists would have had reductively conjured up (please see the exemplary neo-Darwinist Richard Dawkins 1976 reductionist pseudo-genetic book "The Selfish Gene") — and the LGBT people should be mutually treated as sensible and sensitive human beings; and with respect that they are entirely capable of participating and functioning constructively, especially in our diverse societies and communities worldwide; in accordance with their each own, responsible, respectable, and respective, domestic and societal human relationships, norms, and evermore understandings of the homosexuality science-philosophy issues today (including the religiophobia vs. homophobia fallacy as stirred up — again — by Dawkins 2006 antireligious book "The God Delusion"); especially at a time when we are all matching on into the 21st century and beyond! — [Please see my seminal book "Gods, Genes, Conscience" (linked below) especially Chapter 12: Religions and Critical Analysis of Mind; Chapter 13: Afflictions and Remedies of Mind; Chapter 14: Sexuality, Spirituality, and Mind; etc.]

    Consequently, the evermore open and accurate — more self-reflective but not self-destructive — presentations of the homosexuality science and philosophy issues today (such as one presented by Yoder above) will indeed enhance the mutual understandings between our biosciences and religions; and further help alleviate the undue fears and antagonisms between the homophobia vs. religiophobia fallacy and controversy today, ever since the Renaissance; and the Advent of our Scientific Thinking, Reasoning, Communicating, and Learning; etc; etc; in Europe!

    Best wishes, Mong 6/22/11usct2:53p; practical science-philosophy critic; author "Decoding Scientism" and "Consciousness & the Subconscious" (works in progress since July 2007), "Gods, Genes, Conscience" (iUniverse; 2006 — ) and "Gods, Genes, Conscience: Global Dialogues Now" (blogging avidly since 2006 — ).

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  11. 11. LukeT 5:46 pm 06/22/2011

    Lets suppose you want to use the a Thought experiment to rationalize your homophobia. You use an example of sexual behavior between old men and young women that you find equally disturbing in the heterosexual population and use it as a basis for your thought experiment. However you don’t want to be labeled a narrow-minded bigot so you try to hide behind an extremely bias hypothesis.

    It is reasonable to expect that your behavior will marginalize you from the rest of society. People would hate you, but letting you wallow in your ignorance would be much more satisfying. Certainly some fathers, mothers, sisters, brothers or friends of homosexuals would be angry.

    It is your behavior that marginalizes you and attracts hate and loathing. The source of the negative attitudes and behavior is you and your narrow mindedness.

    It is the bigoted opinions of homophobes which attracts society’s reaction and they are better off not letting society know what their opinion is.

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  12. 12. GreenMind 6:25 pm 06/22/2011

    I would challenge the assumption that ordinary Mendelian inheritance, even with many genes, affects homosexuality in men. As Jeremy Yoder explains in this article very well, the existence of significant homosexuality rates is by itself a difficult thing to explain by evolution. But he then makes the assumption that most people in this debate make. The genetics of the gay man is not a factor in causing his homosexuality. It is very clearly the environment of the womb that causes it. Twins studies have shown that, somehow, for a male fetus, the more older brothers have previously occupied his birth womb, the more likely he is to be gay. The probability of being gay rises enormously with birth order, from 1% in firstborns to higher than 40% far down in the birth order. The fact that birth order has such a profound effect on the probability of being gay cannot be explained by any genetic factor in the gay man himself, but is easily explainable if it is caused by factors in the womb. I can probably find the research on this and cite it if anyone wants.

    This means that the if there is a genetic component to male homosexuality at all, it must be a "maternal effect" trait, determined by the genotype of the mother. This effect was first discovered in snails, in which the direction that a snail spirals is due to the genetics of the mother of the snail, not by the snail itself.

    In the article, Jeremy Yoder says that he has not come across any research that indicates that mothers have any higher fitness if they have gay children, but this has likewise been researched and found to be true. Mothers have more grandchildren if they have gay sons. Sorry, I can’t cite the actual paper because I only saw a summary of the article, quite a while ago.

    It might help to think of gay men as simply non-reproductive workers. Just as in the social insects, gay male humans may have developed as a kind of "worker bee" in the village or family. Gay sons might be non-reproductive "helpers at the nest" who increase the amount of resources coming into the family while reducing the danger of overpopulation. The statistics I have seen say that they tend to be smarter, less violent, more affectionate, and more creative than heterosexual men, and are less likely to be pedophiles, and these traits would be advantages in the role of staying home and helping with the family.

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  13. 13. ed2see 5:33 am 06/23/2011

    I enjoyed your article because it is well written and because I am gay also. But your point about natural selection and how it pertains to homosexuality pretty much dismisses the idea totally. I mean if gays do not pro create then it should have been weeded out of society a long time ago. The evolutionary track would say this since it does not an evolutionary purpose which is procreation. The fact that it still continues is another reason that natural selection and evolution is a theory which needs to be disreguarded. Gay men in general are very sexual and the fact that they cant have children is actually ok with most of them. There are the few exceptions though. The lesbians are different beccause they have the ability to procreate so that is why there are more children in lesbian relaitonships. Lesbians are also more nutruing and have feelings unlike men who are colder and more matter of fact. Lately I have been hearing that young boys are turning more femenim because of the chemicals in the air. Now with the radiation spreadinging around the world it may change more heteros to gay through mutations. Oh and i have 2 little nephiews and a neice who i buy alot for and treat them as if they are my own. So your comment on that does not apply to me hehe…The gay pride parade you are going to will probably be filled with half naked men on floats and drag queens and things of that nature which go against tradional views. I been to them before; this is the reason we get people to judge us as perverts. Have fun at it though if its your first time!

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  14. 14. SamGion 8:00 am 06/25/2011

    Very stimulating article, and very interesting analysis from GreenMind. I’d just like to add a few comments. (You’ll excuse my English but I’m French)

    - Jeremy, you say you’d have to help your brother raising four children in order to "counter" you having two kids. But during 99% of human history, we used to have much more than just one brother or sister. Imagine you have eight of them : if you just help half of them raising one more kid, it’s as effective as you raising two. (Without even considering the grand-parents interests. Adding them, homosexuality could sometimes reprensent a real evolutionary advantage.)

    - When ressources get scarce, it may be more effective to have more adults taking care of children than to have more children. Even if gay uncles don’t care, they could still have more odds to have their genes surviving by not reproducing, rather then causing the whole family to starve.

    - When monogamy isn’t well established, your kids may not be yours. But you’re always sure to share genes with your sister’s one. So, as a guy, 2 children of yours may be less "genetically valuable" than 4 ones from your brothers and sisters.

    - At last, during most of our history, all mothers and fathers didn’t live long enough to bring their children to the reproductive age. Couldn’t the LGBT be some kinds of "emergency dads or mums", always ready to take care of their nephews as they don’t have children on their own ? It remembers me of a story told by Konrad Lorenz. When duck fathers loose their spouse, they tend to "marry" (and copulate) randomly with another female or male. But little ducks raised by two males tended to have a better survival rate…

    Evolutionnary biology is one of the most fascinating science, and homosexuality is one of it’s most interesting paradox. I’ll be eager to read your next papers, here or on your blog, and make comments if I have any.

    Kind regards.

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  15. 15. Hominis 8:09 pm 02/18/2014

    Piffle, piffle and more piffle!!!

    Much of “homosexuality” is a learned trait by having example set in someones circle of friends or family.

    That is why there “seems” to be a genetic rather than psychological reason but only if you are forcing the issue to be pro gay rather than observe and report without bias on the facts.

    All this nonsense about evolutionary this that or the other is trying to force credibility on non normal nature.

    It is about as sensible as all the money and research to prove women were less clever than men, which has only relatively recently been dropped.(don’t mention driving skills!!!)

    It has never been an evolutionary issue, there is no gay gene to pass on.
    Evolution would have knocked it on the head a million years ago if it were the case.
    The behaviour is learned as is thieving and rape and chess and I do not believe there is a gene to be passed on for those either- said the XYY man.

    Carry on being confused by this so called “paradox”. Or you could just chuck out the very faulty “facts” trying to fit the round gene in a square evolutionary hole.
    Carry on mixing your elephants with lemmons and getting apples.

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  16. 16. wallyboy 1:29 pm 02/25/2014

    for thousands of years gay men have had just as many children as straight guys. I was married to a closet gay and had 2 kids, his father was also a closet gay and had 4 kids, both men were ashamed of being gay and didn’t come out of the closet until well into their 50′s…I divorced him but my ex still isn’t openly gay as he doesn’t want to upset his older sisters since they despised their father for coming out and ruining their mothers life….now men can be openly gay and happy about it I think we will see a massive reduction in gay men in the next hundred years…that is IF and only IF the gay gene is an inherited trait. Personally i’m glad gays are out so women are no longer taken in by them like I was…homosexuals should stop using the word gay, gay is only happiness to them not others.

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