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The Chimerism Theory of Male Homosexuality
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Steve Sailer has entered this debate with a post on the chimerism theory, i.e., the idea that male homosexuality arises when a male fetus absorbs cells from a female twin that has died during early fetal development. Chimerism appears to be more widespread than previously thought, but its actual incidence remains unknown (also see this post).

What do I think? I’m frankly skeptical. For one thing, to keep the male brain from masculinizing during prenatal or neonatal development, it isn’t enough to have a ‘female cell’ somewhere in the male body. The cell must be positioned at a critical point on this developmental pathway. Otherwise, the brain will develop normally.

But I have a more basic objection: chimerism is nothing new. So the human organism has had eons of evolutionary time to adjust. We come back to Greg Cochran’s point: natural selection tends to remove any condition that seriously hinders reproduction. And ‘tends to’ is an understatement. If an organism cannot reproduce, its characteristics will not survive into the next generation. Game over.

Finally, if the chimerism theory is true, exclusive homosexuality should be more common in populations with a higher incidence of twinning, such as sub-Saharan Africans. In southwest Nigeria, twin births are 3 to 5 times more common than in Europe (Akinboro et al., 2008). Presumably, there would also be proportionately more ‘phantom twins’, i.e., those that die in the womb. Is exclusive homosexuality likewise 3 to 5 times more common in Nigeria?

The post’s comments brought up another point: there is nothing evolutionarily puzzling about male homosexuality in and of itself:

 

I’m still not convinced that human homosexual activity is anything that needs to be ‘explained’ any more than whistling or square dancing or checkers need to be explained.When you strip away the family values or evolutionary or gay rights hysteria, homosexual activity is just the co-occurrence of two traits that are separate but widely dispersed in humanity:

a) enjoyment of recreational sex

b) emotional bonding with members of the same sex.

 

The evolutionary puzzle lies elsewhere, as another commenter pointed out (while citing a previous comment):

I don’t think that the issue is homosexual behavior as much as exclusive homosexual orientation. –Glaivester
Yes, it’s not so much the homophilia, but the heterophobia. Or, as I once saw scribbled on a university (men’s) bathroom stall, “I’d rather die than go to bed with a woman.” I doubt this was John the Baptist speaking.

It is thus exclusive homosexuality, and not homosexuality per se, that needs explaining. Facultative homosexuality does not preclude reproduction, nor does it preclude normal heterosexual development of the male brain. It may arise simply because access to women is limited (as in prisons or in polygynous societies where older men monopolize the pool of fertile women). It may also arise in hypersexual men who have low thresholds for sexual excitement, i.e., who are turned on by anyone or anything that remotely resembles a woman.

Reference

Akinboro, A., Azeez, M.A., and Bakare, A.A. (2008). Frequency of twinning in southwest Nigeria, Indian Journal of Human Genetics, 14, 41-47.

(Republished from Evo and Proud by permission of author or representative)
 
• Category: Science • Tags: Chimerism, Male Homosexuality 
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  1. Anonymous • Disclaimer says:

    I read the blogger’s theory on chimerism quite a while back.

    I guess I give a guy like Greg Cochran a bit more credit than might be wise, but somehow I figured that discriminating thinker and scavanger of new ideas that he is (and I say that with respect, not scorn), he’d have surely read about it by now, and that if he felt it had legitimacy, he’d have done some investigating into it.

    For all I know, maybe he has but has dismissed it. Maybe not.

    It would be interesting to hear his take on it–just another goofy idea or one that has enough going for it to at least consider it?

  2. Anonymous • Disclaimer says:

    As a follow up, I clicked the link you offered to Sailer's blog and in one of the comments was this link to a study I'd seen before about blood type and Rh factor,in which the researchers found a higher than average frequency between blood type and Rh factor in gays and lesbians.

    Any thoughts about this? If this is so, could there be a connection between blood type/Rh factor and the ability of certain pathogens to attack the brain, particularly in the neonatal stage or even later?

    It certainly wouldn't take much of a study to establish if there really is such a correlation.

    Arch Sex Behav. 2008 Feb;37(1):145-9.Click here to read Links
    Eye color, hair color, blood type, and the rhesus factor: exploring possible genetic links to sexual orientation.
    Ellis L, Ficek C, Burke D, Das S.

    Department of Sociology, Minot State University, Minot, ND 58707, USA. [email protected]

    "Using a sample of over 7,000 U.S. and Canadian college students supplemented with additional homosexual subjects obtained through internet contacts, we found no significant differences between heterosexuals and homosexuals regarding eye color or hair color. In the case of blood type and the Rh factor, however, interesting patterns emerged. Heterosexual males and females exhibited statistically identical frequencies of the A blood type, while gay men exhibited a relatively low incidence and lesbians had a relatively high incidence (p < .05). In the case of the Rh factor, unusually high proportions of homosexuals of both sexes were Rh- when compared to heterosexuals (p < .06). The findings suggest that a connection may exist between sexual orientation and genes both on chromosome 9 (where blood type is determined) and on chromosome 1 (where the Rh factor is regulated)."

  3. Anonymous • Disclaimer says:

    So, since we really don’t know what brain cells control attraction (the “center,” let’s call it), we’d have to assume that the female chimeric (is that a word?) cells had somehow not only settled in the male fetus’ brain, but had also overridden/destroyed the male fetus’ own “center,”? Yes? No?

    It hasn’t been established that there actually is a “center” of attraction however, right? I am not arguing against the notion of a center, just asking.

    I have read the term “search image,” and I think, but I am not sure, that it is Cochran’s term. I do know he has hypothesized that there may be a small group of cells that have been knocked out by a pathogen. He points to the few number of cells knocked out in narcolepsy as evidence that it doesn’t take much to change behavior radically.

    If a small group of cells controls our attractions, I wonder how he or anyone else subscribing to the idea that a small number of cells has been knocked out but everything else is has been pretty much left undisturbed would explain the overall feminine behavior of most gay boys and men?

    Do these hypothetical cells account for male feminine behavior as well as for sexual attractions, or is it more likely that the relatively feminine behavior of these boys is socially learned?

    Does this youngster who is attracted to males behave in a feminine manner because he is emulating behavior that he has noticed males find attractive, i.e. feminine behavior?

    Or, is it more likely that the “center” of attraction is also a neurological center for all kinds of behaviors and ways of feeling and thinking…ie, feminine ways of feeling and thinking? If this is so, is there a corresponding male “center” to account for not only attraction to females but also for masculine behavior?

    In a developing fetal or neonatal brain is it more likely that there are two circuits(one for females, one for males) or only one, the female, and if it doesn’t get turned off, does the fetus/child remain (in neurological search image terms) “female”?

    As you can tell, I have no knowledge in any of this and am quite confused. I am just throwing out questions in case anyone cares to help explain the hypotheses to me.

  4. Tod says:

    It may also arise in hypersexual men who have low thresholds for sexual excitement, i.e., who are turned on by anyone or anything that remotely resembles a woman
    That would not apply to taking the passive role I think

    Gregory Cochran’s rationale for dismissing a new factor explaination other than a bug rests on his assertion that it (homosexuality) has been around a long time. Basically he is saying passive or active is the same thing.

    Rh Factor in Pregnancy Hmmm, but it’s not new.

  5. Anonymous • Disclaimer says:

    “Gregory Cochran’s rationale for dismissing a new factor explaination other than a bug rests on his assertion that it (homosexuality) has been around a long time.”

    I’m not saying I buy the chimera thing, but haven’t plural pregnancies been around for a long time? That a fetus dies in the uterus doesn’t have to be weeded out by natural selection, does it, since another child survives.

    New technologies have revealed that plural pregnancies are more common than once thought.

    Google chimerism in sheep,which are, as Cochran points out, a species known to have a fairly high number of rams who are disinterested in ewes.

  6. Bill says: • Website

    Hi Peter,

    my intention in writing the Chimera Hypothesis was to get people to look into the connection I saw between plural pregnancy and homosexuality.

    A couple points about the African twinning issue:

    It seems that twinning is actually repressed by non-African populations (especially East Asians) through a brake on the development of female embryos.

    Africans are also the least likely to have monochorionic twins, and Asians the most likely. Europeans fall in between.

    There is clearly something different about African embryogenesis.

    Anyway, I phrased my theory as a hypothesis because it will obviously take some research to determine whether chimerism actually does cause homosexuality in humans (as it does in cows), or whether it is simply incidental to some other factor (e.g. hormonal effects in mixed-sex twinning) that does.

    The real authority on plural pregnancy and chimerism is Dr. Charles Boklage, who is one of the world’s foremost experts on twinning. He agrees that chimerism is probably responsible for homosexuality:

    Embryogenesis of chimeras, twins and anterior midline asymmetries

  7. Chris says:

    “Anyway, I phrased my theory as a hypothesis because it will obviously take some research to determine whether chimerism actually does cause homosexuality in humans (as it does in cows)…”

    Whoa! We know that chimerism causes homosexuality in cows? I’ve never heard that. Sources, please????

  8. Chris says:

    Okay, sorry. I don’t know much about cattle. I did a Google. So a freemartin is a female twin of a male, born sterile and masculinized. Not that uncommon.

    But, isn’t this condition analogous to the human intersex phenomenon rather than to homosexuality?

    Or….on second thought, maybe….

  9. PT says:

    From Peter: “What do I think? I’m frankly skeptical. For one thing, to keep the male brain from masculinizing during prenatal or neonatal development, it isn’t enough to have a ‘female cell’ somewhere in the male body. The cell must be positioned at a critical point on this developmental pathway. Otherwise, the brain will develop normally.”

    This was my thought as well.

    Also would this really be likely to occur in something near 4% of men?

    I still think the germ theory is the likely explanation although admittedly this chimerism stuff is very interesting and compliment the person who offered it.

    I wouldn’t be surprised to find that all kinds of us who aren’t gay have suffered, probably in early childhood, a pathogenic brain insult that has left its mark to one degree or another. We just don’t know it.

  10. Anon,

    As I understand Greg’s theory, the pathogen would cause more or less the same damage to the population of neurons that controls sexual orientation. Some men, however, would be more vulnerable to a change in sexual orientation because they initially have fewer of the neurons in question, i.e., they are already close to borderline in their heterosexuality. And this could be where Ed Miller’s theory comes in.

    I suspect that Rh factor is regulated by a gene that is close to whatever genes control the growth of this population of neurons.

    ‘Search image’ is a term that has long been used in studies of animal behavior. It is a mental image generated by a population of neurons for recognition of a potential mate or some other purpose (e.g., recognizing a potential prey or predator).

    The general thinking is that the human body plan is initially female. In the absence of modification, a fetus will develop into a female, both physically and psychologically, even if it is genetically male.

    Given that exclusive male homosexuality correlates, to some extent, with other signs of feminization, we may be looking at a common factor (e.g., estrogenization) that is not simply targetting a single population of neurons.

    Tod,

    Yes, I was thinking of gays who play the active role. Many of them may be hypersexuals who simply want to have as much sex as possible.

    Bill,

    Are you saying that the twinning rate is the same in all human populations, but that more fetuses get ‘culled’ before birth in non-African populations? Do you know of any references for this?

    I can think of a number of ways to test the chimerism theory. The most obvious one would be to examine two sample populations, one hetero and the other homo, for signs of chimerism. I would also expect to see a lower incidence of exclusive male homosexuality among twins, and even fewer among triplets (since there would be proportionately fewer phantom twins).

  11. Tod says:

    (Folic acid is said to increase twinning).
    Chimerism and viruses are maybe just more kinds of fetal stress. Virus theory gets best supported for schizophrenia (e.g. a 7 fold risk with influenza exposure in first trimester) but the effects are generalized. Those with schizophrenia and its spectrum disorders have poor working memory and attention.

    Homosexual men show none of this mental deficit, which suggests if it’s down to a bug it’s a specific one that evolved away from collateral damage. Apart from evolution towards manipulation (the original GC theory) what explains the precise targeting of certain neurons? So the ‘original’ GC theory is the only one that can be true for homosexuality I think.

    Here, we review new results obtained in aromatase and ?-fetoprotein knockout mice showing that estrogens can have both feminizing and defeminizing effects on the developing neural mechanisms that control sexual behavior. We propose that the defeminizing action of estradiol normally occurs prenatally in males and is avoided in fetal females because of the protective actions of ?-fetoprotein, whereas the feminizing action of estradiol normally occurs postnatally in genetic females

    Fetal stress interfering with the prenatal defeminizing action of estradol is the mechanism best supported by signs of developmental instability in utero (fluctuating asymmetry) associated with later homosexuality.

    The “feminizing action of estradiol” could take place in males postnatally if an environmental source was available.

    Re Ed Miller’s theory,- are human males really such a threat to their own wife and children that ‘caring and sharing’ genes would spread though the population

  12. Anonymous • Disclaimer says:

    Around 1% of the population is a lifetime “asexual”. In case you didn’t know asexuals have no interest in sex at all. How does Chimerism explain that phenomenon? Does the cotwin cancel out the sex drive of the surviving twin? Good grief.

    AVEN: The Asexual Visibility and Education Network

    It’s time for people to stop being silly about sexuality. As far as Mother Nature is concerned “straight” is the only winning answer.

  13. Anonymous • Disclaimer says:

    Is Schizophrenia the result of Chimerism because one twin absorbs the other twins brain? The hallucinations must come from the extra brain?

    Maybe Autism works the same way. The extra twin cancels out the surviving twin’s ability to read facial expressions.

    sigh… (face palm)

  14. Bill says: • Website

    Peter Frost said…

    Bill,

    Are you saying that the twinning rate is the same in all human populations, but that more fetuses get ‘culled’ before birth in non-African populations? Do you know of any references for this?

    I can’t say that it is the same, but the loss of female twins accounts for much, if not all of the difference. This is clear if you consider the male/female birth ratio in different populations. It is lowest in Africa, highest in Asia, and, once again, Europeans fall in the middle. The reason so many more males are born than females in Asia is the loss of female embryos in mixed-sex twin pregnancies. Consider that Asia has the lowest rate of mixed-sex twinning, and Africa the highest.

    Here’s a quote from the link in my first comment:

    The excess of males in human births appears due to a paternally
    imprinted X-chromosome retarding female embryogenesis
    relative to that of males (Boklage, 2005b). The male excess
    at birth is lower for fathers of African descent than for white
    European fathers, and higher for Asian fathers. This could
    mean that the more permissive the paternal X-imprint, the
    more females, the more twins, and the more male-female twins
    reach term birth. Also differing over these populations in the
    same order: average female age at menarche, at first birth and
    at last birth. Also in the same order, the earlier the trophoblast
    differentiates, and the greater is the fraction of dichorionic
    pairs among same-sex pairs and the lower the fraction male
    among monochorionic pairs and still more so among monoamnionic
    pairs. The more permissive the paternal X-imprint, the
    faster apparently moves every aspect of reproduction in
    females.

    I can think of a number of ways to test the chimerism theory. The most obvious one would be to examine two sample populations, one hetero and the other homo, for signs of chimerism. I would also expect to see a lower incidence of exclusive male homosexuality among twins, and even fewer among triplets (since there would be proportionately fewer phantom twins).

    The only problem with testing for chimerism is that it is best done on deceased subjects. The reason for this is that chimeric tissue often shows up in patches, and not in every organ. So you may have someone with a chimeric organ, such as a kidney or the brain, but not chimeric blood or skin. To do a full test for chimerism would involve testing samples from tissue throughout the body.

    However, testing available fluids and tissues would probably be sufficient to demonstrate a correlation, even if it couldn’t answer the question definitively.

    It is interesting that one commenter brought up autism, because autistic individuals have a far higher rate of Blaschko’s lines on their skin (up to 10%), which suggests chimerism.

  15. Anonymous • Disclaimer says:

    Bill, I respect you and I know that you mean well. But…

    .66% of the children have Autism because of Chimerism
    3% are gay because of Chimerism
    1% are Schiz because of Chimerism

    etc. etc. etc.

    Wouldn’t natural selection step in at some point and stomp out Chimerism?

    The only way this makes sense is if Chimerism is caused by a disease agent.

  16. Tod says:

    Signs of fetal stress such as abnormalities and low IQ are more common in twins. If generalized fetal stress is the cause of homosexuality then homosexualty should be more common in twins

    If chimerism actually does cause a significant amount of preferential homosexuality in humans then singletons should be more likely to be homosexual than men born as one of a set of twins. Handedness, dyslexia and twinning in homosexual men. found “The incidence of twins was lower than the population “

    (I am assuming that those who are born as one of twins are far less likely to exibit chimerism than a singleton)

    Chalk one up to the chimerism hypothesis. Still, chimerism (as opposed to vanished twin syndrome) would have to be vastly more common than it is currently thought to be.

  17. Anonymous • Disclaimer says:

    Tod

    Twins are more likely to be left handed as well. But that’s probably due to pathological left handedness, i.e. damage in utero. Mom’s aren’t optimized for two babies like they are for one.

  18. Tod says:

    The anomaly is left-handed people make up the extremely gifted and the extremely compromised, “The rest of us make up the middle ground

    Hand Asymmetry in Heterosexual and Homosexual Sexually Dimorphic Anatomical Traits The third conclusion was that homosexuality is unlikely to be a result of increased developmental instability Although limited in scope, the present evidence actually suggests that homosexuals have lower FA than heterosexuals, raising the question of whether the positive fitness components associated with low FA may contribute to selection that maintains homosexuality in a population.”

  19. Bill says: • Website

    Anonymous said…

    Bill, I respect you and I know that you mean well. But…

    .66% of the children have Autism because of Chimerism
    3% are gay because of Chimerism
    1% are Schiz because of Chimerism

    etc. etc. etc.

    Wouldn’t natural selection step in at some point and stomp out Chimerism?

    The only way this makes sense is if Chimerism is caused by a disease agent.

    I’m not saying that Autism is caused by chimerism, but rather that there is a fairly clear correlation. From what I have found, autism appears to be on the rise due to IVF and fertility drugs, which may cause an increased incidence of chimerism.

    Chimerism is just one of many developmental anomalies that occur in plural pregnancy. Although I strongly suspect it plays a role in homosexuality and certain psychoses, there could well be other aspects of twin embryogenesis that cause these phenomena.

    It is actually the evolutionary advantage of increased fertility that leads me to suspect that, despite the reduced fitness in a certain proportion of people resulting from plural pregnancies, there is still a benefit to plural pregnancy. If there is an increase in female fertility, even if only a small one, there would be no evolutionary pressure to stomp out the tendency.

  20. Anonymous • Disclaimer says:

    http://www.welmer.org/2008/07/14/the-chimera-hypothesis-homosexuality-and-plural-pregnancy/

    That explains more about chimerism and homosexuality. I mean the whole thing makes sense that homosexual people have dna of the opposite sex in them. A big argument is that some of the dna settles in the brain which is one of the rare organs of the body which wouldn't not be rejected as a liver or kidney would with different dna.

  21. Anonymous • Disclaimer says:

    how is twinning linked to the incidence of chimerism? wouldn't the successful separation of twins to birth be selectively advantageous over chimerism, and thus likely to be less frequent in populations with high degrees of twinning?

  22. Anonymous[268] • Disclaimer says:

    While I am not a scientist, I have read quite a bit on the subject so I offer what I have read.
    As research for associations between animals & humans have a good bit of value, I suggest a consideration of chimera in cattle where a male twin can masculinize his female twin making her infertile.
    This is apparently fairly common.
    There are different types of chimera from micro on to absorbing a twin where your body is the dna equivalent of two people and may not be a hundred percent blended. For instance, your reproductive system can contain the dna of your opposite sex twin so that the dna passed to your child could be his/her uncle or aunt for the most part.
    Human chimerism can exist more prevalently than previously thought because we don’t typically dna check each organ and system of our bodies and double dna isn’t always in the bloodstream but can be elsewhere within the body.

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