Over
the years, the controversy of the nature versus nurture debate has extended beyond childhood behavior to
intelligence and also sexual preference. Today's article aims at uncovering the roots of male homosexuality.
On July 16th 1993, the Daily Mail ran the headline: "Abortion hopes after the 'gay genes' findings". The author, Jason Lewis, claimed that, thanks to recent advances, it might soon be possible to predict the sexual orientation of a baby and give parents the option of abortion. Irrespective of the intentions of the author and possible implications of this article, let us first consider whether this will ever be possible. Since male homosexuality is much more common in nature than female homosexuality, most of the research on sexual orientation concerns only male homosexuality. Years into research on the topic, scientists cannot give a conclusive answer to the question of reasons for our sexual orientation. Simply put, it does not exist.
The Evolutionary Mystery
Homosexual behavior is natural in the animal world, with over 1500 species practising it [1]. Does this give us a first clue that biological factors play a crucial role in establishing sexual orientation? Possibly. In 1991, a twin study conducted by Michael Bailey and Richard Pillard showed that among monozygotic male twins, one gay brother increased the probability of the second being gay by up to 52% [2]. In contrast, this percentage was as low as 22% for dizygotic twins and 11% for adoptive brothers. Hamer and colleagues proposed that an Xq28 allele influences sexual orientation, as its sharing between gay brothers was 64%, not 50% as would be expected by chance [3].
Yet, evolution would intuitively not promote a gene that is anti-reproductive. Why would it then survive in our population at all? The hypothesis here is simple and elegant – the gene that in males promotes homosexuality, in females contributes to their fecundity. Families with homosexual members have been shown to reproduce more than those without [1]. We must, however, note that genetic studies conducted on a large group of participants showed that the genetic changes can explain only a small fraction of the occurrence of homosexuality [4].
If Not Genes, Then What?
If genes do not explain 100% of homosexuality, there must be some additional factors. Interestingly, a fraction of male homosexuality might be explained by immunology. A phenomenon called the fraternal birth order effect shows that homosexual males have a higher number of older brothers. To put it differently, having a son increases the probability of the next male child being homosexual.
This is explained by the maternal immunity hypothesis. It states that male offspring immunize the mother to male cells, thus producing anti-male antibodies during the next pregnancy with a male. The theory posits that those antibodies cross the placenta and blood-brain barrier, changing the brain’s development. This, in turn, may cause diverted sexual attraction and sexual orientation in the male offspring [5]. This phenomenon is the most thoroughly established factor in the field of human homosexuality [1]. However, we cannot forget that more than half of homosexual males have no older brothers [1].
The Neuroscience of Homosexuality
Concerning the ‘homosexual brain’, a number of brain regions have been shown to be, either functionally or anatomically, different in homosexuals compared to heterosexuals. Research has shown that brain regions such as the interstitial nuclei of the anterior hypothalamus show similar anatomy in homosexual men as in heterosexual women, that is clearly different from heterosexual men [6]. Yet it must be noted that even if homosexual males’ brains are similar to those of heterosexual women, it is not clear whether those changes precede the development of homosexuality, or whether attraction to males has elicited changes in the brain.
Environment and Sexual Orientation
In his paper, John Bancroft argues that homosexuality is caused by environmental factors [1]. He starts with an argument that homosexual males usually experience sexual attraction earlier than heterosexuals. They might therefore develop same-sex attraction because at this point of time, boys spend most of their time with other boys. This is, however, only a speculation.
He goes further and says that homosexuality is often just a phase in human development and that the final phase is heterosexuality. To support his hypothesis, Bancroft gives an example of a people in Papua New Guinea: Young boys around the age of ten are taken to all-male dormitories. They are told to fellate older boys and swallow their semen (in a belief that it is crucial for their proper development). At a particular age, they leave the dormitories and lead a heterosexual life that leads to marriage. He claims that this proves his argument of homosexuality being just a phase. Yet, the author fails to mention that activities performed in all-male dormitories might be merely a cultural concept and that in no way are they a proof of homosexuality.
Additionally, nurture theorists claim to have identified characteristic familial patterns that are supposedly related to children’s homosexuality. Those include low paternal presence or high maternal cues in the case of male homosexuality. However, this can be easily reversed – perhaps those familial patterns were caused by the homosexuality of children [1].
Clearly, homosexuality is a multifaceted and extremely complicated phenomenon. But why do we even think about it? The Daily Mail headline from 1993 seems unnecessarily harsh and, at least now, incorrect. We are investigating the basis of homosexuality, but can it have detrimental outcomes? Being able to predict one's sexual preferences might mean denying the right to live. So maybe, we should just let it go and try to accept that people are different no matter where this difference comes from.
[1] Jannini et al, J Sex Med, 2010
[2] Bailey and Pillard, Arch Gen Psychiatry, 1991
[3] Hamer et al, Science, 1993
[4] Mustanski et al, Hum Genet, 2005
[5] Blanchard and Bogaert, Am J Psychiatry, 1996
[6] LeVay, Science, 1991
by Filip Morys, PhD student Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig
This article originally appeared in CNS Volume 7, Issue 3, Nature vs Nurture
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| digitalart / FreeDigitalPhotos.net |
On July 16th 1993, the Daily Mail ran the headline: "Abortion hopes after the 'gay genes' findings". The author, Jason Lewis, claimed that, thanks to recent advances, it might soon be possible to predict the sexual orientation of a baby and give parents the option of abortion. Irrespective of the intentions of the author and possible implications of this article, let us first consider whether this will ever be possible. Since male homosexuality is much more common in nature than female homosexuality, most of the research on sexual orientation concerns only male homosexuality. Years into research on the topic, scientists cannot give a conclusive answer to the question of reasons for our sexual orientation. Simply put, it does not exist.
The Evolutionary Mystery
Homosexual behavior is natural in the animal world, with over 1500 species practising it [1]. Does this give us a first clue that biological factors play a crucial role in establishing sexual orientation? Possibly. In 1991, a twin study conducted by Michael Bailey and Richard Pillard showed that among monozygotic male twins, one gay brother increased the probability of the second being gay by up to 52% [2]. In contrast, this percentage was as low as 22% for dizygotic twins and 11% for adoptive brothers. Hamer and colleagues proposed that an Xq28 allele influences sexual orientation, as its sharing between gay brothers was 64%, not 50% as would be expected by chance [3].
Over 1500 species practice homosexual behaviors
Yet, evolution would intuitively not promote a gene that is anti-reproductive. Why would it then survive in our population at all? The hypothesis here is simple and elegant – the gene that in males promotes homosexuality, in females contributes to their fecundity. Families with homosexual members have been shown to reproduce more than those without [1]. We must, however, note that genetic studies conducted on a large group of participants showed that the genetic changes can explain only a small fraction of the occurrence of homosexuality [4].
If Not Genes, Then What?
If genes do not explain 100% of homosexuality, there must be some additional factors. Interestingly, a fraction of male homosexuality might be explained by immunology. A phenomenon called the fraternal birth order effect shows that homosexual males have a higher number of older brothers. To put it differently, having a son increases the probability of the next male child being homosexual.
This is explained by the maternal immunity hypothesis. It states that male offspring immunize the mother to male cells, thus producing anti-male antibodies during the next pregnancy with a male. The theory posits that those antibodies cross the placenta and blood-brain barrier, changing the brain’s development. This, in turn, may cause diverted sexual attraction and sexual orientation in the male offspring [5]. This phenomenon is the most thoroughly established factor in the field of human homosexuality [1]. However, we cannot forget that more than half of homosexual males have no older brothers [1].
The Neuroscience of Homosexuality
Concerning the ‘homosexual brain’, a number of brain regions have been shown to be, either functionally or anatomically, different in homosexuals compared to heterosexuals. Research has shown that brain regions such as the interstitial nuclei of the anterior hypothalamus show similar anatomy in homosexual men as in heterosexual women, that is clearly different from heterosexual men [6]. Yet it must be noted that even if homosexual males’ brains are similar to those of heterosexual women, it is not clear whether those changes precede the development of homosexuality, or whether attraction to males has elicited changes in the brain.
Environment and Sexual Orientation
In his paper, John Bancroft argues that homosexuality is caused by environmental factors [1]. He starts with an argument that homosexual males usually experience sexual attraction earlier than heterosexuals. They might therefore develop same-sex attraction because at this point of time, boys spend most of their time with other boys. This is, however, only a speculation.
He goes further and says that homosexuality is often just a phase in human development and that the final phase is heterosexuality. To support his hypothesis, Bancroft gives an example of a people in Papua New Guinea: Young boys around the age of ten are taken to all-male dormitories. They are told to fellate older boys and swallow their semen (in a belief that it is crucial for their proper development). At a particular age, they leave the dormitories and lead a heterosexual life that leads to marriage. He claims that this proves his argument of homosexuality being just a phase. Yet, the author fails to mention that activities performed in all-male dormitories might be merely a cultural concept and that in no way are they a proof of homosexuality.
Additionally, nurture theorists claim to have identified characteristic familial patterns that are supposedly related to children’s homosexuality. Those include low paternal presence or high maternal cues in the case of male homosexuality. However, this can be easily reversed – perhaps those familial patterns were caused by the homosexuality of children [1].
Homosexuality is multifaceted and extremely complicated
Clearly, homosexuality is a multifaceted and extremely complicated phenomenon. But why do we even think about it? The Daily Mail headline from 1993 seems unnecessarily harsh and, at least now, incorrect. We are investigating the basis of homosexuality, but can it have detrimental outcomes? Being able to predict one's sexual preferences might mean denying the right to live. So maybe, we should just let it go and try to accept that people are different no matter where this difference comes from.
[1] Jannini et al, J Sex Med, 2010
[2] Bailey and Pillard, Arch Gen Psychiatry, 1991
[3] Hamer et al, Science, 1993
[4] Mustanski et al, Hum Genet, 2005
[5] Blanchard and Bogaert, Am J Psychiatry, 1996
[6] LeVay, Science, 1991
by Filip Morys, PhD student Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig
This article originally appeared in CNS Volume 7, Issue 3, Nature vs Nurture
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