17 February 2014
Last updated at 19:37 ET
Macklemore and Ryan Lewis's hit song Same Love, which has
become an unofficial anthem of the pro-gay marriage campaign in the US,
reflects how many gay people feel about their sexuality.
It mocks those who "think it's a decision, and you can be cured with some treatment and religion - man-made rewiring of a predisposition". A minority of gay people disagree, maintaining that sexuality is a social construct, and they have made a conscious, proud choice to take same-sex partners.
But scientific opinion is with Macklemore. Since the early 1990s, researchers have shown that homosexuality is more common in brothers and relatives on the same maternal line, and a genetic factor is taken to be the cause. Also relevant - although in no way proof - is research identifying physical differences in the brains of adult straight and gay people, and a dizzying array of homosexual behaviour in animals.
Scientists don't know the answer to this Darwinian puzzle, but there are several theories. It's possible that different mechanisms may be at work in different people. Most of the theories relate to research on male homosexuality. The evolution of lesbianism is relatively understudied - it may work in a similar way or be completely different.
The genes that code for homosexuality do other things too
The evolutionary puzzle of homosexuality
In
the last two decades, dozens of scientific papers have been published
on the biological origins of homosexuality - another announcement was
made last week. It's becoming scientific orthodoxy. But how does it fit
with Darwin's theory of evolution?
It mocks those who "think it's a decision, and you can be cured with some treatment and religion - man-made rewiring of a predisposition". A minority of gay people disagree, maintaining that sexuality is a social construct, and they have made a conscious, proud choice to take same-sex partners.
But scientific opinion is with Macklemore. Since the early 1990s, researchers have shown that homosexuality is more common in brothers and relatives on the same maternal line, and a genetic factor is taken to be the cause. Also relevant - although in no way proof - is research identifying physical differences in the brains of adult straight and gay people, and a dizzying array of homosexual behaviour in animals.
Continue reading the main story
Qazi Rahman King's College London"We know that women tend to like more feminine behavioural features and facial features in their men”
But since gay and lesbian people have fewer children than straight people, a problem arises.
"This is a paradox from an evolutionary perspective," says
Paul Vasey from the University of Lethbridge in Canada. "How can a trait
like male homosexuality, which has a genetic component, persist over
evolutionary time if the individuals that carry the genes associated
with that trait are not reproducing?"Scientists don't know the answer to this Darwinian puzzle, but there are several theories. It's possible that different mechanisms may be at work in different people. Most of the theories relate to research on male homosexuality. The evolution of lesbianism is relatively understudied - it may work in a similar way or be completely different.
The allele - or group of genes - that sometimes codes for
homosexual orientation may at other times have a strong reproductive
benefit. This would compensate for gay people's lack of reproduction and
ensure the continuation of the trait, as non-gay carriers of the gene
pass it down.
There is some evidence for this second theory. Andrea Camperio-Ciani, at the University of Padova in Italy, found that maternal female relatives of gay men have more children than maternal female relatives of straight men. The implication is that there is an unknown mechanism in the X chromosome of men's genetic code which helps women in the family have more babies, but can lead to homosexuality in men. These results haven't been replicated in some ethnic groups - but that doesn't mean they are wrong with regards to the Italian population in Camperio-Ciani's study.
Gay people were 'helpers in the nest'
The fa'afafine of Samoa dislike being called "gay" or "homosexual"
Some researchers believe that to understand the evolution of
gay people, we need to look at how they fit into the wider culture.
Paul Vasey's research in Samoa has focused on a theory called kin selection or the "helper in the nest" hypothesis. The idea is that gay people compensate for their lack of children by promoting the reproductive fitness of brothers or sisters, contributing money or performing other uncle-like activities such as babysitting or tutoring. Some of the gay person's genetic code is shared with nieces and nephews and so, the theory goes, the genes which code for sexual orientation still get passed down.
Vasey believes that his Samoan result was different because the men he studied there were different. He studied the fa'afafine, who identify as a third gender, dressing as women and having sex with men who regard themselves as "straight". They are a transgender group who do not like to be called "gay" or "homosexual".
Vasey speculates that part of the reason the fa'afafine are more attentive to their nephews and nieces is their acceptance in Samoan culture compared to gay men in the West and Japan ("You can't help your kin if they've rejected you"). But he also believes that there is something about the fa'afafine way of life that means they are more likely to be nurturing towards nieces and nephews, and speculates that he would find similar results in other "third gender" groups around the world.
If this is true, then the helper in the nest theory may partly explain how a genetic trait for same-sex attraction hasn't been selected away. That hypothesis has led Vasey to speculate that the gay men who identify as men and have masculine traits - that is to say, most gay men in the West - are descended from men who had a cross-gendered sexuality.
Gay people do have children
Elton John and David Furnish have had two
children with a surrogate mother, although the identity of the
biological father is secret
In the US, around 37% of lesbian, gay, bisexual and transsexual people have a child, about 60% of which are biological. According to the Williams Institute, gay couples that have children have an average of two.
How many gay people have children also depends on how you
define being "gay". Many of the "straight" men who have sex with
fa'afafine in Samoa go on to get married and have children.
"The category of same-sex sexuality becomes very diffuse when you take a multicultural perspective," says Joan Roughgarden, an evolutionary biologist at the University of Hawaii. "If you go to India, you'll find that if someone says they are 'gay' or 'homosexual' then that immediately identifies them as Western. But that doesn't mean there's no homosexuality there."
Similarly in the West, there is evidence that many people go through a phase of homosexual activity. In the 1940s, US sex researcher Alfred Kinsey found that while just 4% of white men were exclusively gay after adolescence, 10% had a three-year period of gay activity and 37% had gay sex at some point in their lives.
A national survey of sexual attitudes in the UK last year came up with lower figures. Some 16% of women said they had had a sexual experience with another woman (8% had genital contact), and 7% of men said they had had a sexual experience with a man (with 5% having genital contact).
But most scientists researching gay evolution are interested in an ongoing, internal pattern of desire rather than whether people identify as gay or straight or how often people have gay sex. "Sexual identity and sexual behaviours are not good measures of sexual orientation," says Paul Vasey. "Sexual feelings are."
It's not all in the DNA
Continue reading the main story
Homosexual activity in animals
- Some 400 species engage in homosexual activity, including bonobos (male and female) which are closely related to humans
- In some cases there are reproductive reasons, eg male Goodeid fish mimic females to dupe rivals
- Long-term preference for same-sex mates is rare, but 6% of male bighorn sheep (pictured) are effectively "gay"
- Research on animal behaviour helped overturn Texan sodomy laws - though scientists caution that human homosexuality may be quite different
There are two or more ways this
might happen. One possibility is that the allele confers a psychological
trait that makes straight men more attractive to women, or straight
women more attractive to men. "We know that women tend to like more
feminine behavioural features and facial features in their men, and that
might be associated with things like good parenting skills or greater
empathy," says Qazi Rahman, co-author of Born Gay; The Psychobiology of
Sex Orientation. Therefore, the theory goes, a low "dose" of these
alleles enhances the carrier's chances of reproductive success. Every
now and then a family member receives a larger dose that affects his or
her sexual orientation, but the allele still has an overall reproductive
advantage.
Another way a "gay allele" might be able to compensate for a
reproductive deficit is by having the converse effect in the opposite
sex. For example, an allele which makes the bearer attracted to men has
an obvious reproductive advantage to women. If it appears in a man's
genetic code it will code for same-sex attraction, but so long as this
happens rarely the allele still has a net evolutionary benefit. There is some evidence for this second theory. Andrea Camperio-Ciani, at the University of Padova in Italy, found that maternal female relatives of gay men have more children than maternal female relatives of straight men. The implication is that there is an unknown mechanism in the X chromosome of men's genetic code which helps women in the family have more babies, but can lead to homosexuality in men. These results haven't been replicated in some ethnic groups - but that doesn't mean they are wrong with regards to the Italian population in Camperio-Ciani's study.
The fa'afafine of Samoa dislike being called "gay" or "homosexual"
Paul Vasey's research in Samoa has focused on a theory called kin selection or the "helper in the nest" hypothesis. The idea is that gay people compensate for their lack of children by promoting the reproductive fitness of brothers or sisters, contributing money or performing other uncle-like activities such as babysitting or tutoring. Some of the gay person's genetic code is shared with nieces and nephews and so, the theory goes, the genes which code for sexual orientation still get passed down.
Continue reading the main story
Find out more
Qazi Rahman appeared on the Why Factor on the BBC World Service
Sceptics have pointed out that
since on average people share just 25% of their genetic code with these
relatives, they would need to compensate for every child they don't have
themselves with two nieces or nephews that wouldn't otherwise have
existed. Vasey hasn't yet measured just how much having a homosexual
orientation boosts siblings' reproduction rate, but he has established
that in Samoa "gay" men spend more time on uncle-like activities than
"straight" men.
"No-one was more surprised than me," says Vasey about his
findings. His lab had previously shown that gay men in Japan were no
more attentive or generous towards their nieces and nephews than
straight, childless men and women. The same result has been found in the
UK, US and Canada. Vasey believes that his Samoan result was different because the men he studied there were different. He studied the fa'afafine, who identify as a third gender, dressing as women and having sex with men who regard themselves as "straight". They are a transgender group who do not like to be called "gay" or "homosexual".
Vasey speculates that part of the reason the fa'afafine are more attentive to their nephews and nieces is their acceptance in Samoan culture compared to gay men in the West and Japan ("You can't help your kin if they've rejected you"). But he also believes that there is something about the fa'afafine way of life that means they are more likely to be nurturing towards nieces and nephews, and speculates that he would find similar results in other "third gender" groups around the world.
If this is true, then the helper in the nest theory may partly explain how a genetic trait for same-sex attraction hasn't been selected away. That hypothesis has led Vasey to speculate that the gay men who identify as men and have masculine traits - that is to say, most gay men in the West - are descended from men who had a cross-gendered sexuality.
Elton John and David Furnish have had two
children with a surrogate mother, although the identity of the
biological father is secret
Continue reading the main story
“Start Quote
Joan Roughgarden University of HawaiiIf you go to India, you'll find that if someone says they are 'gay' or 'homosexual' then that immediately identifies them as Western”
These figures may not be high
enough to sustain genetic traits specific to this group, but the
evolutionary biologist Jeremy Yoder points out in a blog post
that for much of modern history gay people haven't been living openly
gay lives. Compelled by society to enter marriages and have children,
their reproduction rates may have been higher than they are now.
"The category of same-sex sexuality becomes very diffuse when you take a multicultural perspective," says Joan Roughgarden, an evolutionary biologist at the University of Hawaii. "If you go to India, you'll find that if someone says they are 'gay' or 'homosexual' then that immediately identifies them as Western. But that doesn't mean there's no homosexuality there."
Similarly in the West, there is evidence that many people go through a phase of homosexual activity. In the 1940s, US sex researcher Alfred Kinsey found that while just 4% of white men were exclusively gay after adolescence, 10% had a three-year period of gay activity and 37% had gay sex at some point in their lives.
A national survey of sexual attitudes in the UK last year came up with lower figures. Some 16% of women said they had had a sexual experience with another woman (8% had genital contact), and 7% of men said they had had a sexual experience with a man (with 5% having genital contact).
But most scientists researching gay evolution are interested in an ongoing, internal pattern of desire rather than whether people identify as gay or straight or how often people have gay sex. "Sexual identity and sexual behaviours are not good measures of sexual orientation," says Paul Vasey. "Sexual feelings are."
Qazi Rahman says that alleles coding for same sex attraction
only explain some of the variety in human sexuality. Other, naturally
varying biological factors come into play, with about one in seven gay
men, he says, owing their sexuality to the "big brother effect".
Brothers of a different kind - identical twins - also pose a tricky question. Research has found that if an identical twin is gay, there is about a 20% chance that the sibling will have the same sexual orientation. While that's a greater likelihood than random, it's lower than you might expect for two people with the same genetic code.
Dr William Byne, editor-in-chief of the journal LGBT Health,
believes sexuality may well be inborn, but thinks it could be more
complicated than some scientists believe. He notes that the heritability
of homosexuality is similar to that for divorce, but "social science
researchers have not… searched for 'divorce genes'. Instead they have
focused on heritable personality and temperamental traits that might
influence the likelihood of divorce."
For Qazi Rahman, it's the media that oversimplifies genetic theories of sexuality, with their reports of the discovery of "the gay gene". He believes that sexuality involves tens or perhaps hundreds of alleles that will probably take decades to uncover. And even if heterosexual sex is more advantageous in evolutionary terms than gay sex, it's not only gay people whose sexuality is determined by their genes, he says, but straight people too.
Qazi Rahman appeared on the Why Factor on the BBC World Service. Listen again to the programme on iPlayer or get the Why Factor podcast.
Follow @BBCNewsMagazine on Twitter and on Facebook
Continue reading the main story
The advantages of bisexuality
- The anthropologist RC Kirkpatrick has suggested that bisexuality may be linked to better survival rates
- Citing examples that include pre-colonial Hawaii, 15th Century Florence and 17th Century Japan (pictured) he says that a willingness to have gay sex helped individuals and families form alliances and gain advantages
- This meant people were able to live longer, have children and provide for those children
This has nothing to do with
George Orwell, but describes the observation that boys with older
brothers are significantly more likely to become gay - with every older
brother the chance of homosexuality increases by about a third. No-one
knows why this is, but one theory is that with each male pregnancy, a
woman's body forms an immune reaction to proteins that have a role in
the development of the male brain. Since this only comes into play after
several siblings have been born - most of whom are heterosexual and go
on to have children - this pre-natal quirk hasn't been selected away by
evolution.
Exposure to unusual levels of hormone before birth can also
affect sexuality. For example, female foetuses exposed to higher levels
of testosterone before birth show higher rates of lesbianism later on.
Studies show that "butch" lesbian women and men have a smaller
difference in length between their index and ring fingers - a marker of
pre-natal exposure to testosterone. In "femme" lesbians the difference
has been found to be less marked.Brothers of a different kind - identical twins - also pose a tricky question. Research has found that if an identical twin is gay, there is about a 20% chance that the sibling will have the same sexual orientation. While that's a greater likelihood than random, it's lower than you might expect for two people with the same genetic code.
Continue reading the main story
Qazi Rahman King's College London"I think straight people are the interesting ones in this, ironically”
William Rice, from the
University of California Santa Barbara, says that it may be possible to
explain this by looking not at our genetic code but at the way it is
processed. Rice and his colleagues refer to the emerging field of
epigenetics, which studies the "epimarks" that decide which parts of our
DNA get switched on or off. Epimarks get passed on to children, but
only sometimes. Rice believes that female foetuses employ an epimark
that makes them less sensitive to testosterone. Usually it's not
inherited, but occasionally it is, leading to same-sex preference in
boys.
For Qazi Rahman, it's the media that oversimplifies genetic theories of sexuality, with their reports of the discovery of "the gay gene". He believes that sexuality involves tens or perhaps hundreds of alleles that will probably take decades to uncover. And even if heterosexual sex is more advantageous in evolutionary terms than gay sex, it's not only gay people whose sexuality is determined by their genes, he says, but straight people too.
Qazi Rahman appeared on the Why Factor on the BBC World Service. Listen again to the programme on iPlayer or get the Why Factor podcast.
Follow @BBCNewsMagazine on Twitter and on Facebook
In today's Magazine
-
In 1998, Lebanese artist Akram Zaatari discovered Studio Shehrazade, the workplace of Hashem el Madani, and a treasure trove of photographs from a lost world.Guns, flared trousers and same-sex kisses
No comments:
Post a Comment
Please leave a comment-- or suggestions, particularly of topics and places you'd like to see covered