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The Chimera Hypothesis: Homosexuality and Plural Pregnancy

July 14th, 2008 · 31 Comments


On Christmas Day last year I was reading an article about cattle breeding, recalled the novel Brave New World by Aldous Huxley, and suddenly an explanation for human homosexuality came to mind. As a fairly new parent, this issue, along with others concerning how people grow and what they ultimately become, has been on my mind. There are many theories, both religious and scientific, that have been proposed as explanations for the occurrence of homosexuality, but none seemed all that compelling to me. Some of the scientific theories are rather far-fetched and go through all sorts of convoluted reasoning to explain how something that renders people less fertile could be selected for by evolution. The most popular religious explanation of today is that it is a choice, which, being a man and knowing how little control I have over my own sexual desires, I cannot accept. The best idea I’d seen was Cochran’s pathological origin hypothesis. In fact, I pretty much accepted that explanation for some time. However, when reading a 19th century book on American Indians and their societies just a few days prior to coming up with this idea, I found that one of the pieces of data he’d relied on was strongly contradicted by the behavior of certain tribes. Cochran suggested that among hunter gatherers homosexuality was rare, even unheard of. However, the Koniagas of Kodiak Island and the Aleuts both practiced homosexuality to the extent that it was a culturally-sanctioned institution. Both groups could fairly be described as hunter gatherers, as could the Californian tribes who were also notorious for homosexuality. So he was wrong at least on that point, and that gave me second thoughts, which led me to keep my mind open to other possibilities.

When I first came up with the idea I had a “Eureka!” moment, but when I looked further into the subject of embryology I came to realize just how strange and complex an environment the womb is. It seems we can say more certain things about the Martian environment than we can about where we all started our lives, so I began to realize why it is so difficult to understand certain aspects of human development, especially those things that occur before birth. Convinced that someone else must already have thought of this I searched diligently through the literature on homosexuality, but although I found plenty of good information and numerous studies, the idea that homosexuality may be a result of chimerism did not come up. Eventually, it was a search into the phenomenon of twinning, which is the main cause of chimerism (excluding fetal cell chimerism in mothers), that led me to the work of Dr. Charles Boklage, a geneticist who is at the forefront of research into chimerism and twinning. Boklage is of the opinion that chimerism is at the root of many poorly-understood conditions and traits, including homosexuality. His research, although considered controversial, was essential to a better understanding of the complex subject of embryology. Furthermore, Richard Lippa, who discovered associations between hair whorls, handedness and homosexuality, introduced the idea of selection for a gene that might predispose people to homosexuality, counter-clockwise hair whorls and left-handedness. His explanation was that perhaps what’s being selected is phenotypical diversity, but he left out the association between twinning and the characteristics he studied. Twinning suggests increased fertility, which is always a factor in selection. So perhaps homosexuality is incidental to increased maternal fertility in the rather unique environment of the human womb, which is not designed for plural pregnancies.

What fascinates me is the idea that homosexuality may be the combination of genetic and environmental factors working together, and that it could be explained in terms of human evolution. Whether it is actually a result of sibling stem cells or not, the association of homosexuality with plural pregnancies is a fascinating phenomenon, and provides a potential link between a genetically advantageous trait and homosexual expression.

The Chimera Hypothesis

For over a century, scientists and thinkers have been trying to figure out the origin of homosexuality. Sir Richard Burton, one of the first to propose a biological basis for homosexuality, theorized that it was prevalent in a geographical region he called the “sotadic zone.” Psychologists attempted to explain it as a result of the action of various mental influences, and religious leaders believed it to be a moral issue.

Recent research has focused on genetic and environmental variables. Could it be the result of hormones in the womb? Could it be a genetic condition? Gregory Cochran has proposed that, because homosexuality is mathematically counterselective on a genetic basis, it must be due to another factor. He posits that it is the result of an infection at some point very early in human development, when the brain is developing sexual characteristics.

However, I see another explanation. Homosexuality is not the result of solely genetic or environmental factors, but rather a combination of the two. This combination results from the complex conditions in the womb during early development, when cells are exchanged in a medium in which the individual is not yet fully defined.

As the science of genetics has advanced in recent years, the existence of human chimeras has surprised a number of researchers. There are varying degrees of chimeric expression, and the effects span a fairly wide spectrum. In most cases, they are the result of twinning or blood exchange between mother and fetus. The most extreme form of chimerism, called tetragametic chimerism, is the merging of two genetically distinct embryos to form one organism. Although commonly thought of as mythological beings, men have long known of the existence of real-life chimeras. The freemartin, a chimeric cow with varying degrees of male characteristics, was used for centuries by farmers in the identification of fertile cows. Aldous Huxley’s Brave New World envisioned a society in which 70% of human females were deliberately made to be human “freemartins” through hormonal manipulation (DNA had not been discovered at the time he wrote the book) for the purposes of population control.

Genetic testing has revealed some spectacular examples of human chimeras. In one case in England, it was discovered that a woman’s children were actually not genetically her own, but rather her sibling’s. Apparently, she had absorbed stem cells from a vanishing twin early in embryonic development, leaving some of the cells in her ovaries with genes from her twin. This is referred to as “germ-line” chimerism, and, although very rare in humans, has been found in up to a third of marmosets — which, like humans have a uterus designed for single pregnancies — but through evolution have acquired the tendency to have multiple births. In another case, a man was referred to a doctor for an undescended testicle and found to have not a testicle, but rather an ovary from a sister with whom he had merged! These are extreme examples of human chimeras; in most cases the effects are far more subtle.

Chimerism has been found to be exceedingly common in humans. Sensitive genetic tests have shown that at least 50-70% of healthy adults are chimeric to some extent. Certainly, most mothers are. When women are pregnant, they receive a number of fetal stem cells in their bloodstream, and these cells are found in various tissues throughout the body — in some cases actively repairing damage and creating new tissue. Perhaps women are healthier than men in old age due to the beneficial effects of lingering fetal stem cells. It is even thought that these cells could be involved in the regeneration and repair of brain tissue. But not all of these fetal stem cells are necessarily beneficial to mothers. Some diseases — scleroderma in particular — may be a result. Scleroderma affects women 10 times as often as men, and is similar to a disease that occurs in some bone-marrow transplant recipients, where people receive stem cells from donors’ bone marrow.

However, the key issue here is the structure and function of the brain. Some physiological differences have been found between heterosexual and homosexual men suggesting a slight, overall feminization of homosexual men, but the most compelling and obvious difference is behavior, which is neurologically determined. Humans are extraordinarily complex beings, and much of what we are is expressed by our minds. Our likes and dislikes, our tastes — both artistic and physical — are determined by our neurological makeup. Some of us are timid, some bold. Some are meticulous and some spontaneous. And, as it happens, some of us prefer women and others men. In most cases this can be determined by sex, i.e., most men prefer women and most women prefer men, but this obviously isn’t a hard-and-fast rule: some men clearly prefer men to women and some women prefer women to men.

From a conventional evolutionary standpoint this approaches heresy. How could a man prefer to mate with another man, who cannot bear children? One would think this tendency would be bred out of the population very rapidly, as those with a same-sex attraction would have fewer children. But what if there is a factor that actually contributes a certain degree of selection for this trait? Although the concept is counterintuitive, characteristics of the human womb and embryonic development may hold the key to an explanation.

Recently, it has been shown that blood-type chimerism amongst fraternal twins is at least 8%. This is only one expression of twin chimerism, which suggests that others may make the overall rate of chimerism between twins much higher. Other expressions of the tendency have not been fully investigated, but additional effects are certain to show up in time. Additionally, with the advent of early ultrasound, it has been found that “vanishing twins”, i.e., twins that do not make it to full term, are far more common than previously thought. An ultrasound study showed that one out of eight pregnancies was a multiple pregnancy, and this may underestimate the number, as many embryos probably do not grow to the point where they can be detected by ultrasound. Another study found that only 15% of observed multiple pregnancies actually resulted in multiple births. Thus, it is certain that many people are the result of twin pregnancies that turned out to be single pregnancies due to the failure of one or more sibling embryos. Intriguingly, one of the complications listed for children whose opposite-sex twins vanished is gender-identity confusion. It is probable that humans, who typically give birth to only one child at a time, have evolved an intrauterine pruning mechanism to ensure the survival of only one embryo in most cases. It appears that intrauterine fetal competition is part of this mechanism.

Vanishing twins are said to be absorbed by the mother, but it is likely that their siblings absorb some of their twins’ cells as well. Stem cells are passed from twin to twin not only through blood-vessel anastomoses, but also through the amniotic medium, which recycles nutrients back to the placenta. There is also the possibility that cells could go through a mother’s bloodstream and end up in the other twin. If one twin fails to develop, it disintegrates and is resorbed. Although rather disquieting to think of, absorption of a failed twin could be advantageous to a competing embryo, so absorption would not necessarily be only back into the mother. The absorption of a twin’s stem cells is what causes chimerism among fraternal twins. If the vanished twin is female, some female genetic material will be incorporated into any male twin that absorbs her cells.

Homosexuality has high twin concordance, especially in monozygotic (identical) twins, where it approaches 50%. In fraternal twins it significantly lower, but still high, at 22%. Aside from genetic similarity, one of the factors that sets identical and fraternal twins apart is that most identical twin pregnancies are monochorionic, meaning that they share a placenta, whereas most fraternal twins are dichorionic, meaning that they each have their own placenta. Twins that share a placenta are naturally more likely to receive similar amounts of stem cells if any happen to be circulating through the placenta. Twins with separate placentas are still likely to be exposed to the cells, since they are both in the same uterus. However, each one has a distinct supply of nutrients and cells, so the ratio each twin receives would tend to vary more than in the case of identical twins. Interestingly, in fraternal triplets, the blood-type chimerism rate, at 21%, is significantly higher than that in twins. One thing to keep in mind when considering blood-type chimerism is that it is by no means the only type of chimerism exhibited by twins; it simply happens to be the only kind tested for with equipment sensitive enough to measure microchimerism.

One of the functions of the human immune system is to eliminate foreign cells. This is why compatibility is such a big issue with bone-marrow and certain organ transplants. The closer the genetic match, the better. However, even with close matches such as siblings there can be problems. It is likely that many stem cells, even from siblings, would eventually be rejected as foreign in a developing fetus, or would be weeded out when the child is born and begins to develop a mature immune system. However, certain tissues are exempt from this, including those in the brain. This means that stem cells that made their way into the incipient brain of an embryo could stay there safely, even if they are eliminated from other tissues. In fact, they could, and probably do, persist in the brain throughout the life of the organism. Female stem cells in a developing male infant’s brain could have profound implications for its ultimate form. Just as much of the rest of the body is shaped by our gender, so is the brain, and tests with rats have shown that hormones alone are not sufficient to masculinize the brain. The anatomical differences discovered in regards to homosexual brains, most of which show feminization, could be explained by pockets of genetically female cells, or even a few interspersed throughout areas critical to masculinization. Chimerism in the brain is probably more common than in other regions, and does not usually result in homosexuality, but can when cells colonize certain regions.

Genetic researchers have been using the receptiveness of the brain to stem cells to produce interspecies chimeras for some time. Mice and sheep brains have been injected with human stem cells to produce chimeras for study, and they readily grow genetically human brain tissue. Another example of the brain’s propensity to absorb stem cells is that female mice have been found with their offspring’s stem cells in their brains, happily working away creating tissue and repairing damage, a process that is thought to occur in humans as well.

So how could humans select for chimerism that expresses itself as homosexuality? Fertility is obviously a genetically positive trait, and fertility is the result of eggs being both available and fertile. This means that more fertile eggs during ovulation would be genetically advantageous, but due to human child-rearing logistics the survival of only one fertilized embryo would also be advantageous. Therefore, women prone to having more than one egg fertilized, but whose pregnancies only resulted in only one live birth, would have the optimum level of fertility. A side effect of this could be an increased incidence of chimerism in human children, and in the case of multiple embryos with different genders, this could lead to physiological changes that result in homosexuality in a proportion of the population. The key is that although homosexuality lowers male fertility in a couple percent of the population, even a relatively small increase in female fertility, which is really the limiting factor in human population growth, could outweigh the disadvantage of less fertility in a small number of male infants.

An Italian study that finds higher fertility in the maternal female relatives of homosexual men seems to bear this out. If these women are more likely to have plural pregnancies, then sibling chimerism would be more common among their offspring. In any case, people who are the result of multiple pregnancies are more likely to be homosexual, whether chimerism is the cause or not.

Chimerism may also be the cause of female homosexuality. That more homosexuals are male further supports twin chimerism as a cause of homosexuality, because in cases of twinning the male embryo is more likely to survive to term than the female. Female homosexuals also show opposite-sex characteristics in certain regions in their brains and some generalized masculinization.

Supporting points:

- Men with twin sisters are more than twice as likely to be homosexual as those without

- Children known to have had a vanished twin are more likely to have gender-confusion issues

- Both twins and homosexuals are more likely to be left-handed

- At least 8% of twins have blood-type chimerism

- Older women are more likely to have plural pregnancies, as well as have homosexual children

- The brain does not reject stem cells, whereas most other organs’ tissues do

- Maternal relatives of homosexuals have been found to be more fertile than their paternal relatives

- Most plural pregnancies result in a single birth

- Female twins are more likely to vanish

So far, the possibility of maternal stem cells has not been discussed, but it is worth mentioning. Because we all have mothers, a maternal source of chimerism might be the most logical thing to look for, and indeed maternal stem cells have been implicated in some pediatric diseases. However, embryonic stem cells would be more likely to contribute to ultimate phenotypical expression than adult stem cells, and barriers between maternal cells and embryos appear far stronger than those between twin siblings.

Tags: Health/Science · Ideas

31 responses so far ↓

  • 1 Jingo S. // Jul 17, 2008 at 12:20 pm

    I’m no expert but it seems like with men homosexuality is either 1 or 0, whereas with women it’s a gradual continuous spectrum, or at least I’ve heard Steve Sailer say that. How would your theory explain that if it were true.

  • 2 Bill // Jul 17, 2008 at 1:17 pm

    Female sexuality is likely the default, and the brain undergoes a masculinization at some point during fetal development. I think the presence of female cells could prevent this through hormonal or structural influence (homosexual brains appear to be structurally similar to female brains) or a combination of the two.

  • 3 Matt // Jul 18, 2008 at 8:23 am

    Very interesting hypothesis, but there’s no basis for the assumption that selection is at play. It’s more likely to be an unselected disorder (like hypogonadism, dwarfism or diabetes).

  • 4 Lukobe // Jul 18, 2008 at 9:58 am

    It’s not an assumption, it’s a conclusion, and Welmer laid out his supporting points, so there’s certainly a basis for it. Whether or not it’s correct is another matter, but what’s your basis for your claim that it’s an unselected disorder? Why so quick to dismiss?

    Perhaps it’s something more like sickle-cell anemia, which kills a lot of its carriers but also provides protection against malaria. Indeed, Welmer points out that “an Italian study… finds higher fertility in the maternal female relatives of homosexual men.” I believe it has been hypothesized that homosexuality is the result of selection for androphilia, and that the decreased (in many cases zero) fertitility of male homosexuals is more than made up for by the increased fertility of their female relatives.

  • 5 TGGP // Jul 19, 2008 at 5:11 pm

    Dwarfism is rare enough to be a mutation. Diabetes has a lot to do with diet.

  • 6 Goffutor71 // Sep 8, 2008 at 9:39 am

    I’ve found it on the web…sure it’s not proven but it’s funny and makes sense…

    Dr. Shlain thinks homosexuality probably evolved in hunting tribes to ensure that there would be more men to hunt for the food but wouldn’t produce hungry offspring.

  • 7 PatB // Oct 1, 2008 at 11:22 am

    I too was initially persuaded by the Cochran disease hypothesis. If male homosexuality is a disease only seen in humans and sheep then its easy to conclude that this disease crossed the species boundry from sheep to sheepherders.

    That seems possible and even plausible. However sheep domestication probably arose in the Middle East about seven thousand years ago as part of the neolith revolution. We know also that the Americas were populated from the Old World at around ten thousand years ago with the Clovis culture people. Furthermore the land bridge closed down and kept more recent migrations from occuring. This means that the American Indians who met the Europeans in the 15th century did not herd sheep and were therefore untouched by any hypothesized sheep pathogen that caused male homosexuality in humans.

    Yet as far as I can determine, American Indians did have male homosexuality.

    The sheep disease theory can be saved if the Clovis chronology is wrong, there were other contacts after Clovis, or homosexuality was introduced by the white man like Small Pox and spread silently and quickly. Or perhaps the sheep disease was already in the wild Rocky Moutain Big Horn Sheep for millenia and domestication was not important.

    If a homosexuality pathogen is isolated and identified in sheep, I will again take the Cohran hypothesis seriously. Until such a time, I;m sceptical.

  • 8 two faced Nick // Oct 20, 2008 at 10:18 pm

    possible a homosexual lifestyle or a bisexual lifestyle is just easier for a tetragametic chimera man because the majority of women are just not turned on or attracted to them when given the real life opportunity. If they aren’t gay they are outed by society anyway. I can think of a few weirdo pop and film stars that this could of explain. Some on record as having the AB blood type. Does anyone know any australian’s specialising in the field of chimeraism? I have AB blood, coarse facial hair and hard lines (Caucasoid features) on the right side of my face while the left cheek is rounder with very little hair. I have even after years of low rep, high weight training a very feminine tall thin frame. I am not a gay man or even a metro-sexual generation y’er, but people still have opinion’s about me.

  • 9 Chimerism Common in Sheep | Welmer // Nov 2, 2008 at 3:51 pm

    [...] is relevant to the Chimera Hypothesis due to evidence for homosexuality – particularly male homosexuality – in sheep. The chimerism [...]

  • 10 Brian // Jan 23, 2009 at 4:52 pm

    Hello Bill,

    I see this is an older post, but I thought I’d chime in, because I’ve given some thought to this in the past. I’ve never been satisfied with the stark, either-or hypothesis posited by militant homosexuals on one side and by fundamentalist Christians (among other orthodox religions) on the other – that homosexuality is either entirely biological or entirely a lifestyle choice. My opinion is that there are probably a multitude of origins for homosexuality (and other aberrations of the human type). I had never considered the possibility presented by your chimera hypothesis, but I think it is well thought-out and very plausible. However, I believe that there are also environmental causes of homosexuality. I refer in this instance to studies that illustrate that a very high percentage of homosexuals were subject to sexual molestation/abuse at a young age. I am of the opinion that adolescents approaching or passing through puberty are actually in the process of developing what one might call a sexual fingerprint – and that various experiences that (for whatever reason) arouse the adolescent or that become associated with sexual arousal essentially form part of the sexual fabric of that person’s life. Thus, young boys who are molested by men (or who by accident or happenstance find themselves in a situation they perceive or sense as sexual around a male or males) may be inclined towards homosexual feelings or behaviors. I think this is also a potentially strong explanation for other sexual perversions (various fetishes, etc). Without going into details, I can identify specific events in my life that I am fairly sure were the precursors to my own (fairly tame) sexual predilections. I think this, general theory can go a long way towards explaining many such perversions, including pedophilia. I further posit that people with such formative experiences frequently exacerbate their influence on their lives by nourishing (for lack of a better word) these sexual feelings, rather than resisting them and trying to redirect their sexual energies towards more normal stimuli. I would point out that contrary to popular belief, I understand that few homosexuals are purely homosexual – and that many homosexual men have relationships (and often children) with women. Similarly, lesbianism seems to often be a choice of women who suffer emotional stresses due to failures of their heterosexual relationships (I know more than one lesbian who was at one time happily married – and whose husband left them…they seem to be emotionally traumatized women who find solace in the arms of a woman in a similarly vulnerable state of mind). Many of these relationships may indeed be largely platonic relationships – but regardless the level of active sexuality, they are homosexual. Lastly, I would mention women (and even men) who seem to dabble in homosexuality on an almost faddish level (much more common in females). This last point makes me wonder if female bi-sexuality might even be an evolutionary advantage! In a species with some tendency towards polygynism, the willingness or interest in females to engage in sexual activity with (or in proximity to) other women might help avoid frictions within a family group comprised of multiple females and a single male.

    And of course, here we arrive at the #1 male fantasy – two hot chicks going at each other like randy cats, with you in the middle of course!

  • 11 AMac // Mar 4, 2009 at 11:04 am

    Thanks for posting this very interesting idea. I have no insight into whether chimerism is a strong contributor to homosexuality, but the more important virtue is that you’ve framed the question as a testable hypothesis. Some very do-able experiments would show whether this line of inquiry is worth exploring.

    A couple of points.

    1. With the rapid advancement of genomics, there ought to be a lot of new information about the extent and nature of human chimerism. Much of this information should fall out from the sorts of SNP studies that are becoming more common in clinical trials. For instance, a chimera should have unusual signatures at many loci on a typical chip. (e.g. Illumina and Affymetrix chips with over 500,000 features are used for many clinical studies, and by personal genomics companies like 23andMe.) For instance, a nonchimeric individual will either be homozygous or heterozygous at a given SNP locus. If the common alleles are, say, A and C, then that person would have a signal that is all A, all C, or 50%A and 50%C. A chimeric individual might have an intermediate signal, depending on the originating cells and their proportions in the tissue sample queried.

    People will have noticed results like this, though not being in this field, I don’t know if they have or not. You could try contacting Daniel MacArthur at Genetic Future or one of the bloggers at GNXP for some insight on this point.

    2. ” It is likely that many stem cells, even from siblings, would eventually be rejected as foreign in a developing fetus, or would be weeded out when the child is born and begins to develop a mature immune system.”

    You’re referring to the concept of “self-tolerance,” the process by which the young individual’s immune system learns to reject foreign antigens (e.g. antigens from bacteria and viruses) while not rejecting one’s own antigens. Peter Medawar won the Nobel Prize a while back for showing that this happens through “clonal deletion.” If I remember correctly (I shouldn’t have dozed in immunology in grad school!), this process takes place right around the time of birth. Thus, one would expect that stem cells from a vanished twin (etc.) if integrated into the embryo or fetus well before that point (e.g during the first and second trimesters) should be completely tolerated once the immune system is activated. No need to invoke the blood-brain barrier.

  • 12 JohnP // Mar 4, 2009 at 12:39 pm

    And of course, here we arrive at the #1 male fantasy – two hot chicks going at each other like randy cats, with you in the middle of course!

    I just want to point out that IMO, the randy cats part is window dressing. The #1 male fantasy is banging tons of pretty, voluptuous women. This is difficult to reduce to a “scene” since it involves, by definition, a series. So, the next best thing is a proxy, i.e., banging two pretty, voluptuous women. I.e., the harem fantasy writ small. Whether or not the two muff-dive is immaterial. On the other hand, whether or not two generally-heterosexual women muff-dive IS germane to whether or not they’ll let you bang them both at the same time.

    Add in the PC pressure to accept homosexuality and presto.

  • 13 robert61 // Mar 7, 2009 at 3:47 pm

    Interesting and plausible hypothesis. It is slightly undercut by one of its own central insights, that “even a relatively small increase in female fertility…could outweigh the disadvantage of less fertility in a small number of male infants”. Any gene that significantly increases female fertility would likely prosper, even if its phenotypic expression in XY babies has a higher chance of being homosexual. The specific mechanism of chimerism might be important or it might not; female fecundity is the issue.

    Incidentally, my father was a fraternal twin. His twin brother was homosexual. Whether they were dichorionic I don’t know, but if so, and if your hypothesis holds, there was a hell of a party going on in my dour, petite old Scots grandmother’s womb back in the day.

  • 14 Who Cares? // Mar 24, 2009 at 3:34 pm

    Let me put in my two cents. The goal is not to produce as many children as possible as much as it is to have as many surviving children who reproduce succesfully. Perhaps homosexuality is natures way to provide more resources to a fewer but healthier number of offspring.

    By having fewer breeding males in a given population to the number of females (my guess is that lesbian females produced the same number of children as straights) there is more genetic choice for the females assuming poligamy is a good thing. It also means that there are more male providers to children. This might be critical in societies where proteen is needed and and the balance of life is more percarious.

    On the other hand …

  • 15 Marcus // Mar 24, 2009 at 3:43 pm

    One conclusion that a chimera theory implies is that homosexuals are more likely to be chimeras than non-homosexuals are. This makes the theory testable. Test homosexuals for chimerism. Test non-homosexuals for chimarism. Compare.

    You could argue that this is not a good test – that chimerism could be equally prevalent among both homosexuals and non-homosexuals. But that seems unlikely because the theory suggests that most homosexuals are chimeras, whereas chimerism is rare. You would need to prove that chimerism is a lot more common than we currently suspect before you could defend the counter theory.

  • 16 Natural Manhood // Mar 24, 2009 at 11:59 pm

    (plz publish this comment instead of the previous one)
    The statement — “most men prefer women and most women prefer men, but this obviously isn’t a hard-and-fast rule: some men clearly prefer men to women and some women prefer women to men” — is not validated by empirical evidences from contemporary non-westernized societies, from ancient pre-christian societies, and from the mammalian world.

    Mammalian males do not, as a rule have a heterosexual orientation. They just have heterosexual sex once in a blue moon, and that too only for reproduction. Only a small proportion of males engage in reproduction in any given mating season, and a big proportion of males never mate to reproduce at all. Also, male-female interaction is in most cases limited to depositing the semen for reproduction… its like wham bam thank you mam. Mammalian males and females have no romantic or even social interaction with each other as adults (apart from possibly as mother-son or siblings).

    On the contrary, mammalian males have lots of sex amongst each other the entire year. And its near universal. So, if there is any truly constant orientation amongst mammalian males, it towards other males not towards females. Also, the only romantic, bonds as committed couples that exist in the wild amongst mammals are between two males… and these can be of short duration as well as life-long, could also be monogamous.

    I think the Western science needs to get over trying to fit natural facts into its preferred theories and for a moment try to adjust their theories — and reinvent them if need be — to the natural facts.

  • 17 Natural Manhood // Mar 25, 2009 at 12:04 am

    [...]most men prefer women and most women prefer men[...]

    The above statement is not validated by empirical evidences from contemporary non-westernized societies, from ancient pre-christian societies, and from the mammalian world.

    Mammalian males do not, as a rule have a heterosexual orientation. They just have heterosexual sex once in a blue moon, and that too only for reproduction. Only a small proportion of males engage in reproduction in any given mating season, and a big proportion of males never mate to reproduce at all. Also, male-female interaction is in most cases limited to depositing the semen for reproduction… its like wham bam thank you mam. Mammalian males and females have no romantic or even social interaction with each other as adults (apart from possibly as mother-son or siblings).

    On the contrary, mammalian males have lots of sex amongst each other the entire year. And its near universal. So, if there is any truly constant orientation amongst mammalian males, it towards other males not towards females. Also, the only romantic, bonds as committed couples that exist in the wild amongst mammals are between two males… and these can be of short duration as well as life-long, could also be monogamous.

    I think the Western science needs to get over trying to fit natural facts into its preferred theories and for a moment try to adjust their theories — and reinvent them if need be — to the natural facts.

  • 18 Natural Manhood // Mar 25, 2009 at 12:10 am

    Response to Brian (comment no. 10)

    Brian: [...]Thus, young boys who are molested by men (or who by accident or happenstance find themselves in a situation they perceive or sense as sexual around a male or males) may be inclined towards homosexual feelings or behaviors. [...]

    You are actually very right. But, going by the same explanation, don’t you think, it is also possible that the societies in general and the West in particular makes boys heterosexual by forcing proxmity with women on them, and through various social mechansisms that make sure that boys channel their sexuality towards heterosexuality rather than towards other males.

    Now, for a second, if we look at it objectively, without saying which sexuality is ‘healthier’ or more natural… then heterosexuality can be explained in much the same way as ‘homosexuality.’

  • 19 Zoe Brain // Mar 25, 2009 at 2:46 am

    References to primary sources please! An excellent and thought-provoking article.

  • 20 georgesdelatour // Mar 25, 2009 at 2:56 am


    As Welmer explains, homosexual men tend to show certain physical characteristics, such as more feminized brain structures, which strongly suggest their sexuality has a real physical basis, and is not simply a response to childhood abuse. But by all means let’s look for evidence for your theory too. If you’re right, the pattern should be pretty hard to miss once the data’s collected.

    Natural Manhood

    The important thing about humans is that their children need parental nurture and protection for far longer than most other species. This gives a genetic advantage to mothers and fathers that form long-term bonds. So comparing human sexuality with all other mammals may not be relevant.

    There’s a distinction between homosexual acts occurring in segregated environments (prisons, monasteries, nunneries etc), where it’s the only sex available, and homosexuality in the midst of heterosexual possibility.

  • 21 George // Mar 25, 2009 at 10:35 am

    What about KRISTANNA LOKEN, who is bisexual, and has a lesbian twin sister? Besides the vanishing twin, do you think that a substantial number of twin births have a “vanishing triplet”?

    (Loken is an actress who was in a relationship with Michelle Rodriguez, which was in my opinion the best looking lesbian couple ever. )

  • 22 The end remains in the hand of the puller - Sam Hardwick | The straight line // Mar 31, 2009 at 9:57 am

    [...] mentioned a theory about the biology of homosexuality I’d heard about to my sister the biologist a while ago [...]

  • 23 Miss Bhetti // Jul 16, 2009 at 4:25 pm

    After preliminary consultation with my colleagues, it is as I thought: the Elsevier group that the journal belongs to is respectable, at least. I think you already know this, though.

  • 24 Michele S // Oct 25, 2009 at 2:40 pm

    Why isn’t anyone testing your theory?

    I have g/g/b triplets and I see A LOT of feminine characteristics in my boy. He’s only five, so it’s not like I have any way of knowing at this point if he will be gay, but I would consider it a future possibility.

    I know a tremendous amount of triplet families with fraternal mixed set triplets. Why not follow us and see what ends up happening in 10-15 years? What percentage of our g/g/b or b/b/g triplets will end up gay or lesbian due to environmental conditions in the womb? We are the perfect group to pose this question to.

  • 25 Travis Cottreau // Dec 16, 2009 at 3:59 pm

    Another explanation for male homosexuality – it also explains all the factors, and predicts known male homosexuality rates:

    This doesn’t say that the chimera explanation is not involved – it may well be, just that sexual antagonism long known in animals may also be present in humans and explain male homosexuality.

    Also, it’s believed that female homosexuality is a different phenomenon, since sexual antagonism doesn’t match observations in female homosexuality.

  • 26 The Chimera Hypothesis « Bayesian Investor Blog // Mar 2, 2010 at 2:07 pm

    [...] The Chimera Hypothesis: Homosexuality and Plural Pregnancy makes the surprising claim that: at least 50-70% of healthy adults are chimeric to some extent. [...]

  • 27 Iridium // Jun 14, 2010 at 9:57 am

    As the main subject of this theory (I’m an exclusively homosexual male) I’ll pitch in some of my own experiences:

    1) I can have sex with women if really needed. It’s a popular misconception that just because we deem it extremely unappetizing that we somehow have lost reproductive capabilities. We haven’t.

    2) I have very little effeminate characteristics. I have male interests and have no wish at all to be a woman despite being primarily attracted to men. I know a lot of the similar ‘invisible gay’ population who are the same. Homosexual males who do not exhibit the (wrongly) stereotyped behavior of what it meant to be homosexual. Historical records show the same thing with cultures which openly tolerated homosexuality – Celts and Greeks for example. It is in fact quite common among warriors. Note that I am not distancing myself from our more effeminate brethren. A lot of my friends are very flamboyant, but people need to realize that homosexuality is not defined by effeminacy. It is quite true though that gay men are more often than not more perceptive emotionally than straight men, but that can be explained by the fact that most of us underwent extensive constant trauma in our teenage years hiding who we are for fear of total rejection from society.

    3) I have the same protective (and fatherly) instincts when it comes to women and children as your average heterosexual man. Maybe even more so because that instinct isn’t tainted by sexual attraction. Since most of us are either never given a chance or choose not to have children, we often lavish this fundamental ‘protector’ instinct on pets, our nephews and nieces, and adopted children instead. The evolutionary advantage of such a behavior is undeniable. In a tribal society, we would have provided much needed extra hunter/gatherer numbers for the women/children of the tribe but without the drive to compete with other males for the attention of the females. There’d be no danger of us fighting with the males over the females and risking a further decline in the population of providers.

    4) While it is true that unlike most lower animals, we have the power to actually choose what we do consciously, homosexuality again has never been a conscious decision for us. I had my first same sex crush at the age of 7 before I even realized what sex was. I realized that the same attraction meant I was gay at 12. Most gay men I know have similar stories, almost all of them realizing they were homosexual at about the same time heterosexual males start having interest in girls. Even men who have been forced to marry women and only came out in their middle ages. Bisexuals may have different experiences. And yes, as soon as I realized that my behavior was considered ‘sinful’ and ‘aberrant’ I tried as hard as I could to change my behavior, even going so far as to date girls. The result is still the same – there is absolutely no way for me to change my sexual orientation unless I allow myself to be subjected to extreme forms of behavioral conditioning (i.e. brainwashing). In which case, the risk of becoming psychologically damaged (in some cases becoming totally asexual) is very great indeed. Since my behavior does no harm to anyone but my own capability of propagating my genes. I do not see why I should even attempt to ‘correct’ my sexual attraction at all, nor why most heterosexual males seem to be threatened by it.

    Because really… when you think of it. Why is there so much antagonism towards homosexual males? Particularly in regions which follow a patriarchal/male-centered culture/religion? The amount of [murderous] hatred directed at us makes no sense. The rather flimsy excuse of being unnatural is not enough (why do you then not execute people born with congenital deformities? More importantly, the fact that we have existed continuously and independently in human populations in all parts of the world all throughout history on circumstances not of our choosing means it IS a natural). We do not threaten the heterosexual male propagation of their DNA. To the contrary we actually help ensure the survival of their offspring.

    As such it is a far more interesting question to identify the cause of male homophobia rather than the cause of homosexuality itself. Sexuality and gender is, after all, just an instinctual vehicle for reproduction. A biological mechanism humans have long lost the need for when they became fully conscious.

    In contrast to say, a lot of teleost fish species who often change sex just to ensure their species’ survival, or to whiptail lizards and social insects who have either completely eliminated or relegated the status of the male gender as a DNA randomizer, we do not anymore need the urge to copulate with a woman in order to survive extinction. We already know the mechanics of reproduction. Our brains are starting to break free of the mindless robotic drive of the DNA to make copies of itself in any way possible even if it meant the death of the original carrier or the death of several other individuals of the breeding population. We are even beginning to start modifying our own DNA by consciously choosing our reproductive partners (consciously guided sexual selection which results in certain perceived advantageous traits in races to be magnified without having to rely on random mutation/recombination/complex mating rituals) and direct genetic manipulation itself. Rewriting the book so to speak.

    Who we are emotionally/sexually attracted to has lost its significance to our survival as a species. Unless we suddenly lose our cognitive power, even if a homosexual man and a homosexual woman are the last humans left on earth after an unimaginable apocalypse scenario, they are both still perfectly equipped to reproduce the human race. And while they may not like it, they have brains that know what they have to do regardless of what they feel like doing. So why is it such a big deal?

  • 28 Iridium // Jun 14, 2010 at 10:09 am

    And yes, until a scenario forces me to have to have sex with a woman in order to save the human race from extinction, I’d rather not, thank you very much. LOL. Overpopulation as it is, we are actually one of the few reasons why we haven’t (yet?) destroyed our own biosphere.

  • 29 Rebecca Love // Aug 1, 2010 at 9:23 am

    What a cool theory! I was so pleased to find something written on this topic. As a psychologist, I am well aware of statistical trends in the expression of homosexuality related to birth order, twins, and sib -ships. I know this speaks to biological explanation however, not being a medical doctor, my efforts towards understanding have never gone beyond psychological and relational curiosities until today, as I just saw a show on Chimera mothers not matching the DNA of their children and that more than one set of DNA may be dispersed and dominant or recessive in different parts of the body! My immediate thought was the possible link to homosexuality and I was disappointed the show didn’t explore this. Very cool to find this article online though. I am glad others have had this same train of thought and hope that more biological explanations for homosexuality are discovered as these have proven crucial for social acceptance of differences and may greatly assist my clients who struggle with gender identity issues.

  • 30 Christine Martins // Jan 18, 2011 at 3:44 pm

    In ”Embryogenesis Of Chimeras, Twins And Anterior Midline Asymmetries”, Boklage posits Chimeras are not as rare as previously thought and in fact 1 in 8 of us may be chimeras and are not aware of our conditions.

    What can be considered rare is the motivation to detect a chimera. Most chimeras look completely normal. Chimerism need not be present in the blood. So without physical abnormalities or strange blood typing to act as a motivator, there is little or no reason to pursue the diagnosis.

    While there may be no physical signs of chimerism, the condition might be expressed in the behavior of a chimera as you have suggested. The creation or artifical chimeras lends creedence to this idea.

    The creation of artificial “brain chimeras” is becoming a routine practice in the field of neurobiology. One such study conducted by German scientists created a brood of Japanese quails with female brains and male physiology.

    “Male Japanese quails with female brains do not show male sexual behaviors”

    The male quail chimeras with female brains did not display mounting or crowing behavior. It is a scientific fact the hypothalamus–pituitary–gonad (HPG) axis coordinates displays of sexual behavior with reproductive physiology. Sexual brain development therefore defines the ability of brain areas,including the neural part of the HPG axis, to respond to particular physiological signals in adulthood.

    My take is that the formation of a spontaneous human chimera could be remarkably similar to the artificial lab process. When merging occurs in the womb, the embryos are little more than brain primordia.

    The brains merge and create a hybrid brain structure or more appropriately a hybrid HPG axis – – both male and female at once. I believe sometimes the merging creates a human with same-sex attraction, other times merging creates a human with gender confusion, and sometimes perhaps no effect at all. It would largely depend on the physics of the collision of the two embryos as to how the resulting chimera would form.

  • 31 Natural manhood // Apr 26, 2012 at 10:46 am


    It’s funny how you westerners go on making one invalid assumptions upon another to fit nature into your preset whims and fancies — starting from the assumption that the homo-hetero divide is a reality, when only a forcefully heterosexualized society like the west has these categories.

    “As Welmer explains, homosexual men tend to show certain physical characteristics, such as more feminized brain structures, ”

    There is no such thing as ‘homosexual men.’ Those the west calls ‘homosexuals’ are nothing but ‘third genders,’ (and they can like men or women or other third genders). They have female like brain structures and other qualities because they are third genders and not because they like men.

    If you start with wrong assumptions, you’d arrive at wrong conclusions. But, this is what you people want to do — distort the fact about male gender and sexuality, using pseudo science.

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