Sexual orientation – in the genes?

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Mirrored from http://wiringthebrain.blogspot.com

Is homosexuality a lifestyle choice or an innate biological disposition? The idea that it is a choice is certainly widespread – a part of several mainstream religious doctrines and political ideologies – and is used to condone significant discrimination against homosexuals and the criminalization of homosexual behaviour. But what does the science say?

The broad conclusions are that sexual orientation is an innate disposition – no different from whether you are left or right-handed – that it is affected by genetic influences and that it reflects differences in brain structure and function. I will consider the evidence of genetic effects on sexual orientation here, including some recent additions – a later blog will look at the neurobiological findings.

A number of family and twin studies of the heritability of sexual orientation, starting in the 1950’s, found significant genetic influences: the statistical likelihood of an individual being homosexual increased somewhat if they had a homosexual dizygotic (fraternal) twin and dramatically if they had a homosexual monozygotic (identical) twin. However, these studies have generally suffered from some methodological limitations, including small sample sizes, the possibility of ascertainment bias due to the methods of recruitment of participants and an assumption that homosexuality in males and females is likely caused by the same mechanisms.

This assumption reflects a common idea that heterosexuality represents the same default state in both males and females – that it is the “normal”, baseline condition, one that requires no active processes. In fact, there is not a single rule: “be attracted to members of the opposite sex” – there are two rules: either be attracted to males or be attracted to females. These “rules” are embodied in anatomical and physiological differences in neural circuitry controlling sexual desire and behaviour, which differ between heterosexual males and females. (See: Wired for Sex for more on these processes in male brains). Understanding that both these behavioural rules require active and possibly distinct neurodevelopmental processes to establish makes it much easier to appreciate how alterations to those processes can lead to exceptions to how those rules are expressed.

Three recent twin studies have largely overcome previous methodological issues, demonstrate clear genetic influences on sexual orientation and argue strongly that homosexuality in males and females is due to distinct mechanisms. These studies all used large, population-based samples – that is, the subjects were not recruited to the study based on sexual orientation – in Sweden, Finland and Australia. In each study, rates of homosexuality were compared between pairs of monozygotic or dizygotic (same-sex or opposite-sex) twins. Each study had several thousand participants and several hundred twin pairs, making them well-powered statistically to detect genetic or environmental effects on sexual orientation.

These studies differed significantly in how they assessed sexual orientation, however, which may be reflected in their results. Zietsch and colleagues in the Australian study used a questionnaire to assess sexual attraction on a seven-point “Kinsey” scale, from exclusive heterosexual attraction to some degree of homosexual attraction. In response to this question, 11% of men and 13% of women, were rated as non-heterosexual. Alanko and colleagues in the Finnish study used a composite measure of same-sex attraction and same-sex sexual contact. In this survey, 6.1% of men and 6.6% of women reported a homosexual orientation. Interestingly, the quantitative measures used demonstrated a much more bimodal distribution in males than in females, mirroring previous observations that bisexuality is much more common in females than in males (males tending to be either strongly heterosexual or strongly homosexual). Langstrom and colleagues used a direct question about lifetime number of same-sex sexual partners, with 5.6% of men and 7.8% of women reporting at least one – among those reporting at least one, men reported significantly more same-sex partners.

Estimates of genetic influences were high across all three studies. The Australian study found heritability of 48% for sexual orientation across males and females together. The Finnish study estimated genetic influences on sexual orientation of 45% and 50% for men and women, respectively. Neither study found any evidence of an effect of shared environment. The Swedish study gave somewhat different results – the heritability for male heterosexuality was quite high, 39%, with no effect of a shared environment. However, the estimated heritability for female heterosexuality was lower in this study, around 18-19%, and a significant contribution from the shared environment was found for females in this study (16-17%). These differences could reflect sampling effects, population genetic or cultural differences, or the differences in how sexual orientation was assessed (based on actual same-sex sexual behaviour in the Swedish study). It is important to note that a shared family environment for dizygotic twins includes a shared uterine environment, which may impact on neural development.

Importantly, both the Australian and the Finnish studies found zero correlation of homosexuality across opposite-sex dizygotic twin pairs, while same-sex dizygotic twin pairs showed substantial correlations. So, if a male has a fraternal twin brother who is homosexual, there is a significantly increased likelihood that he will also be. This is not the case if his twin sister is homosexual (and vice versa).

The major conclusion from these studies corroborates previous findings: sexual orientation is strongly influenced by genetics. Whatever the underlying biological processes, they are likely different for males and females, as reflected in differences in reports of same-sex attraction and expression of sexual behaviour, with males showing a more bimodal distribution. The potential neurobiological processes involved will be the subject of a later post.

Those are the scientific conclusions. My personal interpretation is that dispositional homosexuality is no more a choice than left or right-handedness. Most heterosexuals certainly can not point to the time when they “chose” to be straight no more than someone can say they chose to be right-handed. Natural left-handers can certainly learn to write right-handed but that will not change the inherent disposition, nor is there any good reason to try and change it. At the other extreme, these findings do not suggest that homosexuality is a biological “disorder”. Conditions are only defined as a disorder if they have a negative impact on someone’s life – by this definition, homosexuality is only a disorder if society’s reaction makes it one.

Långström N, Rahman Q, Carlström E, & Lichtenstein P (2010). Genetic and environmental effects on same-sex sexual behavior: a population study of twins in Sweden. Archives of sexual behavior, 39 (1), 75-80 PMID: 18536986

Alanko K, Santtila P, Harlaar N, Witting K, Varjonen M, Jern P, Johansson A, von der Pahlen B, & Sandnabba NK (2010). Common genetic effects of gender atypical behavior in childhood and sexual orientation in adulthood: a study of Finnish twins. Archives of sexual behavior, 39 (1), 81-92 PMID: 19172387

Zietsch BP, Verweij KJ, Bailey JM, Wright MJ, & Martin NG (2009). Sexual Orientation and Psychiatric Vulnerability: A Twin Study of Neuroticism and Psychoticism. Archives of sexual behavior PMID: 19588238

30 Comments

  1. Interesting article. The elephant in the room still remains however in explaining how this genetic influence on homosexuality is maintained in the population, or could have been selected for in the first place.

    My assumption is that the genetic influence found in these studies is on other characteristics that lead to homosexuality, but not homosexuality itself. For example, all individuals vary in sex hormone levels and degree of masculinity-femininity. Genes leading to less masculine characteristics than average in men and more masculine characteristics than average in women could lead to them becoming homosexual. These starting differences could also be exaggerated by the environment.

    I agree with you that choice seems to play no role here, and your analogy with heterosexuals is apt. Why don’t you ever hear anyone saying that a person chose to be straight? Because its a ridiculous idea.

  2. Why don’t you ever hear anyone saying that a person chose to be straight? Because its a ridiculous idea.

    Well of course it’s a ridiculous idea – we’re an organism that requires heterosexual activity to reproduce itself. By the same token, the notion that homosexuality – in an organism that requires heterosexual activity to reproduce itself – is “naturaal” is equally ridiculous. These studies that suggest genetic bases for homosexuality do not solve that riddle – they only make it more puzzling.

  3. What no one seems to want to point out is that, while these studies do suggest a strong genetic component to homosexuality, they also argue for an equally strong environmental component — about 50/50 actually. If you are trying to think of homosexuality as entirely genetic, how do you explain that in so many cases where one twin was gay the other was straight?

    I don’t believe many people consciously choose to be gay (although I think a few actually do), but I do believe that many gay people might have ended up straight if it had not been for some influence early in life — perhaps an improper encounter, or perhaps they just saw something that intrigued them and turned them in a certain direction. (Children are allowed to see so little, but they are very interested, so what little they do see can have a big impact). Whatever the actual mechanics, a 50 percent environmental influence is certainly big enough to worry about!!!

  4. @ziel:

    It doesn’t seem like that big of a riddle to me. It is fairly well-established that non-heterosexuality correlates with increased masculinity in women and increased femininity in men. Other research (from Zietch and colleagues, for example) shows significantly greater number of sexual partners for both heterosexual men with more feminine traits, and heterosexual women with more masculine traits. In short, it seems that typical patterns of sexual attraction serve to keep gender dimorphism from “going off the rails”, which means that many of the same traits which predispose someone to homosexuality are advantageous when they turn out heterosexual.

  5. significantly greater number of sexual partners for both heterosexual men with more feminine traits, and heterosexual women with more masculine traits. In short, it seems that typical patterns of sexual attraction serve to keep gender dimorphism from “going off the rails”, which means that many of the same traits which predispose someone to homosexuality are advantageous when they turn out heterosexual.

    Interesting. So there would be a negative feedback on sexual dimorphism, which would result in occasional overshoot and homosexuality – itself operating as a “negative feedback on the negative feedback” due to its obvious negative impact on reproduction, and thus indirectly maintaining said sexual dimorphism. That’s actually pretty cool if it can be confirmed.

    Also, I take it that some of the previous commenters are unaware of the concept of developmental stochasticity. 50% genetic doesn’t imply that the other 50% are cultural – biological factors can be non-genetic too.

  6. significantly greater number of sexual partners for both heterosexual men

    That seems like the linchpin for not finding it to be much of a riddle. That certainly flies in the face of what I have observed in life and what the tabloids seem to assume would be attractive to women.

    Has anyone ever calculated what the added fertility of more feminine men would have to be to balance out the reduced fitness of homosexuality in the population?

  7. In mammals there are two prenatal or perinatal testosterone surges in males but not females. The first of these follows the differentiation of the gonad as a testis early in development. This burst guides external genitalia and internal plumbing as male.

    The second burst occurs around the time of birth. This is associated with the survival of a significant sets of neurons that normally die in females, and with increased dentritic arbors in males relative to females. Oddly, this testosterone is first converted into estrogen that is used to signal the nerve cell precursors to live and/or increase the number of dendrites. Tests of adding estrogen or testosterone to perinatal females (mice) leads to masculinization of behavior in the females and castrated pups.

    This shares some similarities with some behavioral pathways in Drosophila. In flies the basic sexual behavior pathways are largely genetic. It appears that a significant fraction of the neurons involved in sex-specific behaviors actively transcribe a sex-focused promoter for the fruitless gene. In males this is spliced in a way that generates a set of four proteins differing in C-terminal Zn-finger cassettes. In females, an alternative splicing pathway yields an apparently non-functional transcript. In females, a number of these cells die while they are spared in males.

  8. I think what J. Goard is referring to is something like the “Johnny Depp” theory of male homosexuality. And equally for females, it wouldn’t surprise me if women with more masculine traits (i.e. more testosterone and higher sex drive) had more reproductive success than those without.

    @ ziel, evolution is not a perfecting agent. It isn’t a prior a ridiculous idea that some sort of natural tradeoff (e.g. balancing selection) could keep genetic variance in homosexuality in the population. Look at other “imperfections” in the human race like sickle-cell anemia, schizophrenia and myopia, that all have a large genetic component. Indeed, sexuality would seem to be an area of human behavior particularly suited to this form of selection, because its not an entirely quantitative trait.

  9. It seems like the research suggests fairly equal numbers of male and female homosexuals.

    But I’ve always wondered why the impression we seem to get from society is that there are far more gay men than lesbians — is this just media portrayal?

  10. I think what J. Goard is referring to is something like the “Johnny Depp” theory of male homosexuality.

    In part yes, but appearance and mannerisms are the tip of the iceberg. I would guess that there are many other traits which are significantly stronger in females but may not come to mind as easily when we hear the word “feminine”. Verbal facility, ability to manage complex relationships, memory for social details, etc., seem absolutely vital for the would-be playboy. OTOH, overly male minds, at least on one prominent theory, become autistic.

  11. Since when is “It is fairly well-established that non-heterosexuality correlates with increased masculinity in women and increased femininity in men…?”

    What are your sources? I haven’t seen this in my circle of friends, granted I mostly pay attention to men like a good straight girl would.

  12. “these findings do not suggest that homosexuality is a biological “disorder”. Conditions are only defined as a disorder if they have a negative impact on someone’s life – by this definition, homosexuality is only a disorder if society’s reaction makes it one’

    Your main argument in relation to schizophrenia was the perfectly reasonable one that – “natural selection only cares how many offspring you have who survive to reproduce themselves”. ( though it is the relatives of schizophrenics (the ‘schizotypal’) who are hypothesized to have some fitness advantage)

    This post is saying the opposite and in effect arguing that wrecking fecundity is not a relevant criterion for deciding if something is a disorder. Sorry, but if homosexuality is normal so is schizophrenia; they are both far more common than any genetic disease.

    Saying free will does not exist in relation to sexuality makes one wonder if that is a special case or if you are suggesting that there is no such thing as free will. Even if nobody believed in free will in relation to anything there would still be sexual orientations regarded as disorders and greatly disapproved of and the disapproval of society is an effective way of curtailing undesirable behavior.

    Very few people get to go through life without their sexual behavior being criticized.

  13. I don’t think this level of analysis is very informative. It doesn’t explain the range of sexual characteristics. The issue is not nearly so simple as attraction to males vs. attraction to females. There are many other issues sexual proclivity such as “top” vs. “bottom” that are not explained this simply. Why do some men become homosexual while others become transgendered?

  14. About 1 in 1500 kids are born deaf: about 1 in 3000 are born blind.

    Why so low? I mean, it would hardly be surprising if those numbers were 1 in 30, right? Just as no one would be surprised if a few percent of birds flew north – or east – for the winter: odd that this isn’t the case.

  15. If ‘a few percent of birds flew north – or east – for the winter’ it would make me think that such behavior had once paid off big time.

    It would be surprising if a bug had no other effect than to make them fly north or east – unless flying north was connected with them getting infected with the bug in the first place.

  16. “It would be surprising if a bug had no other effect than to make them fly north or east – unless flying north was connected with them getting infected with the bug in the first place.”

    Assuming that there is “no other effect,” I feel compelled to point out it’s a mighty big effect, don’t you think?
    That said, the sexual behavior which results in fewer offspring or no offspring may not be the only effect. After all, there are other behavioral effects noted in gender atypical people. And, there may be other physical effects as well. The latest study (Danish or Swedish study, I believe) showed higher rates of depression in gays– the study did control for things such as societal attitudes.

    If 2-4% our unneutered cats or dogs showed no interest in the females of their species when those females came into heat, if female fish began laying their eggs in places where male fish could not reach them to fertilize them, or if those male fish released their fertilizer in areas that would not allow it to find those eggs, we’d not look to genes or some evolutionary strategy, would we?

    It seems not at all likely that natural selection resulted in a male of the species who comes equipped with a working sperm delivery system but a brain that provides for the delivery of that sperm to an unproductive recipient.

  17. I don’t accept the analogy with fish. It would indeed be very surprising if fish showed a non workable fertilizing behavior because fertilising is all they do so there would be no way for genes causing such behavior to compensate for the fitness deficit in other ways in (male) fish.

    Humans males have a lot to do with their children and in circumstances where male provisioning was crucial genes ‘for’ increased male provisioning which sometimes resulted in outright homosexuality could have given enough of a fitness advantage to outweigh the occasional (2%) individuals in who provisioning genes resulted in a total failure to procreate.

  18. “Humans males have a lot to do with their children and in circumstances where male provisioning was crucial genes ‘for’ increased male provisioning which sometimes resulted in outright homosexuality…..”

    Just what evidence have you that males who are homosexual provide increased provisioning? A feminine man was not likely to “bring home the bacon,” and any extra help he gave back at the hearth wouldn’t make up for that unless he was so darned effective a mother that he was better than a mother –maybe he allowed mothers to breed and breed and breed so that they didn’t even have to take care of their kids? He’d have to be responsible for the survival of more children than even a mother. Doesn’t make sense.

    The ‘gay uncle helps out’ thesis doesn’t cut in a whole host of ways. Look around. See many gay men rearing their sibs from infancy to adulthood? And of those who might, what production of offspring do they account for? After all, just to come out even in the genetic reproduction department, they’d have to help survive more children than parents would. And when these men hit puberty, they have a sex drive like any male’s, not like a female’s. Their minds are not on child rearing. Doesn’t make sense.

    It seems your argument comes down to this–that genes that made some males content to provide care for children and the tribe sometimes resulted in an overshoot which resulted in tsuch a male being so much like a woman to the point he didn’t reproduce because he didn’t find women sexually attractive.

    How has this trait been maintained in the gene pool? Why hasn’t this overshoot been corrected by natural selection? It’s costly. The trade off isn’t neutral.

  19. In response to Yawnie’s comments and some others: Homosexuality clearly will typically lower fecundity and therefore evolutionary fitness. This does not make it a “disorder” in a clinical sense.

    It does raise the question of why it persists in the population at such a high prevalence if it is both heritable and likely to be selected against. My guess is the explanation for this is the same as that for some common psychiatric disorders – individual predisposing alleles probably are strongly selected against, but new ones arise all the time due to ongoing mutation. (See: http://www.gnxp.com/wp/uncategorized/is-mental-illness-good-for-you).

    The alternative- that there is some kind of balancing selection is certainly conceivable, and probably cannot be ruled out given that odd things can happen when sexual selection is active, but I do not know of any evidence supporting it.

  20. Regarding the studies on MZ twins

    It would be interesting to know if in MZ twins where one is gay and one is straight, does the straight male exhibit more feminine traits than straight males even though his sexual attraction is for females? For example, was the straight male of the twin pair less aggressive physically than the average boy? Did he exhibit gender atypicality even though he turned out straight? Are his career choices more like those of gay males than straight males?

    I’ve never seen this addressed in the studies about concordance or lack of concordance in MZ twins.

    A guy like Bailey or Sanders would probably know just from having worked with the twins.

  21. You expect higher failure due to mutational load in extremely complex systems, ones that depend on the action of many genes: thus, you see a higher frequency of genetic retardation than you do, say, genetic albinism. Such a syndrome would be caused by mutations in many genes: we know of more than 100 that can cause congenital deafness. Of the many mutations causing such a grab-bag mutational load syndrome, some will be syndromic – i.e. will have other distinctive phenotypic effects, such as the white streak of hair in Waardenburg syndrome deafness, or macroorchidism in fragile X. In the case of congenital deafness, about 15% of causal mutations are syndromic.

    Is there any reason to suspect that sexual orientation is extraordinarily complex? No. Birds do it. Bees do it.

    Do we see syndromic forms of homosexuality in homosexual men? No. In principle they may exist, but they must then be very rare.

  22. I think any male that has a high level of testosterone and doesn’t find a (especially ovulating) woman attractive has had some serious problems in their childhood.

    Similarly, I think that any male that is an estrogen dominant castrato and doesn’t find something enviable and attractive about a masculine male body is oblivious.

    My point being: sexuality is hormonal. Hormones can be manipulated through diet, exercise, genetics, environment.

    And sex between males is used as a form of domination. I am what is considered a pretty boy. I have had gay men follow me for blocks making comments to me. They were no doubt ‘tops’ who like to mount other men, much the same as male dogs will mount other male dogs as a way to dominate them.

    To argue that sexuality is entirely genetics is absurd to me.

  23. hmoore, consider two fathers

    Married man who preferentially spends time and money with wife and kids.

    Married man who prefers to be out pounding beers and trying to pick up women.

    Which one of the above would be most likely to have a homosexual brother?

    All the instincts are far weaker in humans than in animals so I can see that the attenuated drives of humans may be more easily neutralized by an environmental interference such as infection. But why would instincts so often be not just nullified but inverted in such a specific way. Asexual men are orders of magnitude less common than one very specific orientation so there is some kind of selection going on – in bugs or people.

  24. Sorry, finishing up my thesis so don’t have much time to comment. Here’s one example of a twin study addressing several commenters’ points.

    http://www.ehbonline.org/article/S1090-5138(08)00068-8/abstract

    Zietsch et al. (2008):

    “We show that psychologically masculine females and feminine men are (a) more likely to be nonheterosexual but (b), when heterosexual, have more opposite-sex sexual partners. With statistical modelling of the twin data, we show that both these relationships are partly due to pleiotropic genetic influences common to each trait. We also find a trend for heterosexuals with a nonheterosexual twin to have more opposite-sex partners than do heterosexual twin pairs. Taken together, these results suggest that genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population.”

  25. Neat article J. This is the kind of evidence of balancing selection kjmtchl was looking for. I favor this hypothesis as opposed to the mutation-selection balance, which seems to make more sense for psychiatric disorders. Although of course it doesn’t have to all-or-nothing in either of these cases.

  26. One thing I keep wondering about is how big a fitness effect homosexuality has in most environments in which our ancestors evolved.

    In the case of lesbians, a woman who “does her duty” with her husband enough to fill the pipeline of children she can bear and nurse and care for, but whose romantic/sexual preference is other women, isn’t obviously less fit than a woman who does the same thing reproductively, but prefers romantic/sexual connections with men. (This works the same way as for a woman with relatively less sexual interest generally.) The issue here is simply that how often a woman has sex isn’t the constraining factor on her fertility, it’s how many kids she can bear and nurse and care for.

    In the case of men, it seems more likely that the difference could have fitness effects, because if you have lots of partners, more sex correlates with more offspring. I’m not sure how bisexuality affects this. Romantic attachments to men seem like they’d have more impact on fitness than sexual attraction, at least if you’re in an environment where you’re not able to get sex with lots of different women. I gather plenty of conquerors who left a fair-sized genetic mark in the world were perfectly happy to have sex with both the boys and the girls in their conquered territory. (And the livestock probably didn’t escape unscathed either.) For men, sex is (in evolutionary terms) incredibly cheap, as long as it doesn’t come with commitments to care for the offspring.

  27. I’m with TGGP (above). The pathogen hypothesis should be discussed by any new study. The sibling concordance stuff, superficially anyway, doesn’t distinguish between genetic factors and environmental ones such as pathogen exposure.

  28. Albatross, what dude wants a mate who’s indifferent to sleeping with him. Some dude may wind up stuck with her anyway – but it won’t be a superior specimen of a man, it will be an inferior one. That makes all the difference.

  29. Hmoore made an interesting point about cats. I am curious if there has ever been a study done on male cats “ignoring” female cats in heat. Isn’t that “true” homosexuality? Just having intercourse with a member of the same gender could be nothing more than “wanting it bad”. For animals and humans. But ignoring the opposite gender all together, that is fascinating.

    I remember I was jogging with one of my friend a year or two ago, and we passed another jogger, this one a female probably in the 18-20 year range wearing tight spandex, and she was easy on the eyes to be polite about it. My friend made the comment “even a gay guy would like that.” It was crass, but I think it is relevant to the topic. Gay men may say they prefer men, but do they “actually” ignore an extremely attractive member of the opposite sex? I think that is worthy of study.

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