Sexual orientation – wired that way

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In a recent post, I presented the evidence that sexual preference is strongly influenced by genetic variation (http://www.gnxp.com/wp/uncategorized/sexual-orientation-–-in-the-genes). Here, I discuss the neurobiological evidence that shows that the brains of homosexual men and women are wired differently from those of their heterosexual counterparts.

First, we must consider the differences between the brains of heterosexual males and females. These differences are extensive and arise mainly due to the influence of testosterone during a critical period of early development (see Wired for Sex). They include, not surprisingly, differences in the number of neurons in specific regions of the brain involved in reproductive or sexual behaviours as well as differences in the number of nerve fibres connecting these areas. But they also involve areas not dedicated to these types of behaviours, such as the cerebellum, for example, which is involved in motor control among other things, and which shows a very large difference between men and women. Another area that shows prominent differences is the corpus callosum, the very large sheet of fibres that connects the two cerebral hemispheres, which is larger in females, despite lower overall brain size. Indeed, females show greater and more efficient connectivity in cortical networks than males, on average.


It should be emphasized that all of these differences are apparent only in group averages and there is very substantial overlap in the distributions of the measures of different brain regions in males and females. This is a similar situation to height, where, while it is true that men tend to be taller than women, on average, the distributions overlap and the average difference is not diagnostic – if the only thing you know about someone is their height then you have little predictive power as to which sex they are. That is because height is affected by many other variables besides sex and so sex simply shifts the mean of a wide distribution. Similarly, it is not possible to tell from one measurement from a brain scan what sex a person is, because brain structure is also affected by many other variables (primarily the rest of the genome). Nevertheless, just as with height, the group sex differences in brain structure are very robust and reproducible. They are also correlated with average differences in many different aspects of cognition, perception, emotion and any number of other psychological domains as well as large sex differences in susceptibility to various psychiatric diseases.

With that as background, what have studies of the brains of homosexual men and women found? The naïve hypothesis would be that the brains of homosexual men might look more like heterosexual women and vice versa for homosexual women. In fact, this is exactly what has been found, for the most part, not just in structural measures but also in measures of brain activity. Starting in the early 1990’s, a number of studies by Swaab, Le Vay and others found differences in the size of specific regions of the hypothalamus between homosexual men and heterosexual men, with homosexual men showing a more female pattern. As the hypothalamus is involved in regulating many sexual and reproductive behaviours, and given that brain activity feeds back onto the organization of brain circuits, it was possible that such differences arose due to differences in behaviour, rather than the other way around. Experiments in animals argue strongly against that conclusion, however, given that similar differences can be induced by the manipulation of sex hormones during a critical period of early development.

A number of other studies have found similar results in other areas of the brain where sexual dimorphism is observed, including the size of the corpus callosum and also of the anterior commissure, another tract connecting the two sides of the brain. Both of these are larger, on average, in homosexual than in heterosexual men, mirroring the difference between heterosexual women and men. Conversely, various regions in homosexual women tend to show a more masculine pattern.

Interestingly, there is also strong sexual dimorphism in the degree of lateralization of brain structures and of brain activity – in general, men show greater lateralization than women (e.g., for language or face-processing areas or indeed, for the overall size of the cerebral hemispheres), and this trend is reversed in homosexuals. This is not limited to the brain itself but also extends to facial symmetry– males tend to show greater asymmetry in facial features than females, but the opposite is true for homosexual males and females.

Differences in brain activation have also been observed, for example in response to pheromones or to visual presentation of male or female faces. In both cases, homosexual men respond in a way that is more similar to heterosexual women, and homosexual women show responses more like heterosexual men. That may not be a surprise, you say – in fact it may seem obvious that that must be true and is not necessarily evidence for innate differences.

The study by Savic and Lindstrom referred to below extends these observations to another brain system, the amygdala – a region involved in emotional processing, but not directly linked to sexual behaviour. In heterosexual men and homosexual women, the right amygdala tends to be slightly larger than the left, while the opposite was found in homosexual men and heterosexual women. Differences in resting state functional connectivity were also observed (this refers to which areas are active in synchrony with the region of interest while the subject is at rest – not performing any specific task). In the first group there were more connections from the right amygdala and they were stronger to a different set of brain regions (including prefrontal cortex, caudate and putamen) than in the second group (which showed connections with the contralateral amygdala and cingulate). These results show that differences in brain wiring and functional activation between homosexuals and heterosexuals are not restricted to brain regions directly linked to reproductive behaviours or to responses to sexual cues.

Taking the genetic and neurobiological evidence together thus provides a clear picture of the biological basis of sexual orientation, though the details remain unknown. It should not be long however before some genetic variants are discovered that are associated with sexual orientation and these should give clues to the genesis of brain wiring differences between the sexes and how they control sexual preference.

Savic I, & Lindström P (2008). PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. Proceedings of the National Academy of Sciences of the United States of America, 105 (27), 9403-8 PMID: 18559854

Swaab DF (2008). Sexual orientation and its basis in brain structure and function. Proceedings of the National Academy of Sciences of the United States of America, 105 (30), 10273-4 PMID: 18653758

20 Comments

  1. I worry about what will happen if/when societies incapable of performing the research themselves are given information capable of identifying homosexual-tending gene patterns. The implications are disturbing.

    Especially if homosexuals (particularly gay men) are just extreme cases of generally diminishing sexual dimorphism in humans.

  2. societies incapable of performing the research themselves

    Which societies are you thinking of?
    :-)

  3. I certainly don’t doubt that there’s a biological root to homosexuality, at least the kind in which a male just isn’t turned on by females, and I doubt that other gnxp readers harbor doubts, but the “genetic variation” term is problematic.

    First, however, the brain scans–If I remember, the Savic/Lindstom sample size was less than a hundred, half heteros, half homosexuals. I do remember that the hemispheric size differences they noted in straight men and lesbian women were “slight” while they said that the hemispheres were somewhat more symmetrical in homosexual men and heterosexual women. Those adjectival describers are hardly strong evidence although even “slight” and “more” symmetrical might be meaningful with a larger sample size and if the “slight” had high predictive value.

    If it turns out that these brain scans do indeed show structural differences that actually mean something, are these differences directed and established in utero? Or soon after birth? Or, in early childhood? Are they directed by “genetic variation”? Or by some triggering agent(s)?

    We’ve still the question of the identical twin concordance rate (isn’t it about 45-50%?). In order for the “genetic variation” argument to make sense, you’d have to argue that something is turning off/on a gene(s) in one twin, but not in the other. If so, what? What usually causes such epigenetic occurrences? I have to admit having a big problem with the use of the germ “genetic” since one group seems to use it in one way and other groups in another way. Is it accurate to speak of any such switching on or off as “genetic” rather than as “biological”?

    For all the studies, I still get the feeling that we are missing the forest for the trees on this one. Or, perhaps it’s more accurate to say we are missing the tree for the forest.

  4. One thing I’ve noticed, as a gay man who knows a lot of other gays, is that most of the gay men I know who have an ADD diagnosis have the inattentive (i.e. the non-hyperactive) variety, whereas a lot of lesbians with an ADD diagnosis the hyperactive variety (i.e ADHD, not ADD). It’s interesting because in the general population men are much more like to be diagnosed with ADHD, and woman with ADD.

  5. BTW, I thought that the huge brothers’ DNA study being done at Northwestern by Sanders and Bailey was supposed to have been completed and the results published way over a year ago, but I haven’t seen anything unless I missed it.

    Anyone know what’s up with that? They were trying to determine once and for all by amassing a huge sample if Hamer’s linkage claims had any validity.

  6. Which societies are you thinking of?

    The very first one I thought of was Iran. But there are parts of America that would qualify just as well.

    Think of the degree to which female fetuses were aborted in certain parts of the world, because male children were more valued, once they received the technology of amniocentesis.

    If ‘gay infants’ could be identified in vivo, or even male fetuses predisposed to be somewhat more feminine than whatever the societal preference was…

  7. “Which societies..?”

    You mention only one country, Iran, and you also have “parts of America” in mind.

    Then you mention an activity, selective abortion of females, that does NOT take place in Iran or anywhere in America, but rather among Buddhist, Daoist, or non-religious Chinese, and among Hindu Indians.

    Don’t get me wrong: I abhor the values and behavior of the Iranian Islamist tyrants, and I disagree with many religious Americans on gay issues, but that doesn’t justify accusing people of crimes they do not commit.

    The communities in America that take religion most seriously, whether Baptist, Orthodox Jewish, Amish, etc., or the like, have low levels of abortion in general, as does Iran, and do NOT have a disproportionately high male population, which is a conclusive sign of female abortion.

    Selective abortion of females is an issue that many American pro-choice activists have tried to avoid. Since they are not on record as fighting this horrifying trend (tens of millions of females aborted in recent decades), they will not be in a strong position to oppose selective abortion of homosexuals. We’ll have to count on the Christian right to oppose such a practice if it ever takes root.

    In any case, abortion is a less likely outcome that medical intervention in utero to reduce the likelihood of homosexuality. See this article in the New Scientist about an intervention that is already taking place to address a genuine medical condition that also affects sexual orientation: http://www.newscientist.com/article/dn19151-debate-over-gender-disorder-drug.html.

    Here’s a prediction: at some point in the medium future American society will be ready to accept homosexual orientation as morally equivalent to heterosexuality; at just about that point, medical intervention will be available. If even a minority of parents take that route, homosexual orientation will begin to decline, as fewer and fewer such individuals emerge from the womb.

  8. The dashes break the hyperlink to your previous post. You should enclose the url itself with an href.

    I will again plug Greg Cochran’s pathogenic theory.

  9. Differences in fetal environment from one twin to another can explain the twin disparity. Corpus Callosum atresia is regulated by Testosterone (T). T receptors are also adrenaline receptors, so if one fetus is stressed at the time when the Corpus Callosum is breaking down, then the T doesn’t get through because the receptors are occupied with adrenaline. Atresia is down regulated and now you have a male baby with a more female brain.

    Nucleus

  10. “Selective abortion of females is an issue that many American pro-choice activists have tried to avoid. Since they are not on record as fighting this horrifying trend (tens of millions of females aborted in recent decades), they will not be in a strong position to oppose selective abortion of homosexuals. We’ll have to count on the Christian right to oppose such a practice if it ever takes root.”

    A great irony, for sure. Feminists and pro-abortion activists would not be vocal in opposing selective abortion of fetuses that might tend toward being homosexual. It’s always amazing when new circumstances make political bedfellows into political opponents very quickly.

    “In any case, abortion is a less likely outcome that medical intervention in utero to reduce the likelihood of homosexuality.”

    My guess too. Fundamentalists would welcome this, but so too will most people, including the non-observant and the non-believers.

    “Here’s a prediction: at some point in the medium future American society will be ready to accept homosexual orientation as morally equivalent to heterosexuality; at just about that point, medical intervention will be available. If even a minority of parents take that route, homosexual orientation will begin to decline, as fewer and fewer such individuals emerge from the womb.”

    Growing numbers of people see homosexuality as having a biological basis so they don’t see it as a moral failure, I agree; however, this does not mean they view it as a neutral or positive trait. It’s definitely not a trait they’d prefer for their own kids. They will gladly opt for medical intervention for their offspring for this as they would for any number of other anomalies or atypicalities.

  11. “Differences in fetal environment from one twin to another can explain the twin disparity. Corpus Callosum atresia is regulated by Testosterone (T). T receptors are also adrenaline receptors, so if one fetus is stressed at the time when the Corpus Callosum is breaking down, then the T doesn’t get through because the receptors are occupied with adrenaline”

    Yes, levels of stress hormones affect testosterone “baths.”

    I also remember reading that stress increases the permeability of the blood brain barrier–Cochran’s pathogen could come into play here.

    Still, if stress responses are the major cause, one would think that studies would show that there were an unusually high number of births of what turn out to be gay people during historical periods of great stress for a population. I think there have been some such studies and they came up empty.

  12. “….unusually high number of births of what turn out to be gay people during historical periods of great stress for a population. I think there have been some such studies and they came up empty.”

    Not at all:

    Dörner, G., Geier, T., Ahrens, L., et al. 1980. Prenatal stress as possible aetiogenetic factor of homosexuality in human males. Endokrinologie 75: 365-368.

    Dörner, G., Schenk, B., Schmiedel, B. & Ahrens,
    L. 1983. Stressful events in prenatal life of bi- and homosexual men. Exp Clin Endocrinol 81: 83-87.

  13. EW,

    Thanks for the references. I’ll check them out. When I get a chance, I’ll see if I can find something more recent and whatever sources I read that referred to older studies.

    A close friend of my mother and father bore three sons by two different husbands. Two sons are gay–the ones with different fathers. The first son was quite gender atypical as a kid, dressing up in his mom’s apparel, complete with make-up, at every chance when he was small. The second son was, as is often said, “all boy.”

    The third son, the one by a different father, is quite a bit younger than the other two, eleven and twelve years younger. He was not gender atypical as we think of it, but he was a kid who seemed to prefer his own company rather than playing too much with others his age, withdrawing into his room to play with all the toys his middle class parents threw his way. This one married, had two daughters, then astounded pretty much everyone by heading for the city and, well, being gay.

    It’s a good example of a familial connection researchers have noted.

  14. I don’t think that there are more recent data. There’s now a certain tendency against such studies, because the gays suspect, that with influences known, parents would select against potentially gay fetuses or apply measures against such an outcome. As the above linked article in the New Scientist describes. Or this one on the same subject:
    http://www.newsweek.com/2010/07/02/the-anti-lesbian-drug.html

  15. but that doesn’t justify accusing people of crimes they do not commit.

    I didn’t accuse Iranians of selectively aborting female fetuses. I do suspect that if homosexual fetuses could be conclusively identified, Iranian society would be one of the first to implement their destruction.

  16. Some funny Comments here :)

  17. EW,

    You get a variety of what seems, at least on the surface, to be conflicting research on the maternal stress hypothesis as it relates to life events: two for example,
    http://www.jstor.org/pss/3812874 versus
    http://www.springerlink.com/content/q6q2180250341315/

    How hard would it be to collect data from a random sample of women who were pregnant, say, during the Northridge or Loma Prieta earthquakes? Or, if those events are considered too short-lived in duration, what about women who were pregnant during their husbands tours of duty in Viet Nam? Women who were pregnant and living close to the 9/11 attacks have children now approaching 9–are the number of male gender atypical children born to them significantly higher than in the general pop? Trimester of pregnancy would be of interest, of course.

    It seems there are limitless historical events that could be used for such studies to establish whether a pattern exists or not. One could even collect data from a series of populations of relatively small towns in which stress-causing events took place to see if patterns emerge in the % of homosexual males v. heterosexual males born to women pregnant during those events.

    Since the immune system is compromised by stress of all kinds it makes sense that atypicalities during pregnancy would occur during such times of high stress, but even the common cold, the flu, and any number of other common infections may have the same effect if any of them strike during a period of important fetal or neonatal development.

  18. Regarding the condition of CAH–http://www.newscientist.com/article/dn19151-debate-over-gender-disorder-drug.html

    Concerns about the safety of treatment and the knowledge of the parents involved are legit, but “concerns” from those who think parents have no business caring about or preventing such a condition in their child are the selfish rantings of a political minority. Anyone who’d prevent me from taking action to prevent CAH, (again, assuming it had been deemed safe) would be subjected to fist in his or her face.

    This reminds me of how some in the deaf community fight against coclear implants in children, maintaining that deafness is not a disability. They can argue whatever they want; it’s none of their business what I decide for my child. Perhaps it has happened, but I have never heard those pretty much confined to wheelchairs argue that children born with disabilities that make it difficult to learn to walk not be fitted with devices that enable them to do so.

  19. “It should not be long however before some genetic variants are discovered that are associated with sexual orientation and these should give clues to the genesis of brain wiring differences between the sexes and how they control sexual preference.”

    Where have I heard this before?

    If they are going to find something, I’d not be surprised if it was related to the immune system.

  20. What we call ‘homosexuality’ is more accurately classified as ‘transgenderism’, which is what it really is. It may be a milder form than the kind which leads to crossdressing and/or surgery, but it is clear that (nearly) the entire brain, not just the sexual part, has shifted to the oppposite-gender.
    I actually think that sexual desire and behavior is the least interesting aspect of this, but for some reason, the whole complex is labeled with reference to that, maybe due to the sex-obsession of modern Western societies, or the lack of a traditional transgender category in the recent Western past – which then made the political move for recognition occur under the umbrella of “sexual preference” which was seen as more acceptable/comprehensible.
    In theory true (male) “homosexuality” would be two men attracted to each other, not two transgenders. But according to current teachings, this is impossible, since a truly male brain is hard-wired to be (exclusviely?) gynephillic.

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