The science of human sexuality, as with anything to do with our intriguing and sometimes infuriating species, is complex. Undisputed answers as to the exact biological reasons why individuals can be sexually attracted to people of the same sex as themselves, to persons of the other sex, or to people regardless of their sex, are few and far between.

What does emerge clearly throughout the research is the extreme interdisciplinary nature of trying to understand human sexuality and maybe here, science needs to take a leaf out of sexuality’s book: to catch up and accept the evidence that people sit in a much broader spectrum than given credit for.

What we do know beyond a shadow of doubt, however, is that our preferences are often hard-wired into our bodies – they are not purely psychological and, far less, a “lifestyle choice” – a deeply misguided meme for which Sigmund Freud has much to answer.

The spectrum of human sexual experience is vast, and its incredible diversity is not always acknowledged.

In this review of the current science we have tended to generalise (and for that we apologise), referring to “gay”, “straight” and, less frequently “bisexual”. The science itself often speaks in these simplistic terms, but people do not form such well defined blocs in their gender traits or their attraction to others – nor indeed in the gender they self-identify as.

As geneticist Sophia Frentz says: “What matters is that we do our best to minimise the impact our previously held values have on the science that is being done, and to ensure that that research is not only being done for good reasons, but reflects or at the very least acknowledges the community that currently exists.”

There are those who believe that science should take a back seat in discussions of sexuality. Some fear that the isolation of a “gay gene” (or genes) could lead to selection against foetuses carrying it, while some scientists question whether time and effort would not be better spent elsewhere.

“Homosexuality is not a disease, it’s part of natural human variation. I think we’ve reached the point that we have enough evidence that there’s a biological basis for sexual orientation,” Margaret McCarthy, a neuroscientist at the University of Maryland told The Scientist.

That may be true, but we should also never underestimate the power of science to guide wider social trends. In 2007, during the heated debates in Singapore over whether or not to repeal the law making sex between men a crime, the country’s conservative leader Lee Kuan Yew, never previously known for his sympathy for the gay community, stepped into the fray with his unexpected support for decriminalisation.

“If in fact it is true, and I have asked doctors this, that you are genetically born a homosexual…you can’t help it. So why should we criminalise it?

“They are born that way and that’s that. So if two men or two women are that way, just leave them alone.”

What the biologists say

Homosexual behaviour is a natural biological feature and is common among non-human animals. In at least one species – sheep – individual animals have been known to form lasting preferences for same-sex partners.

In fact, recently, carers discovered that the partner of the world’s oldest animal, a 186-year-old tortoise, was a male, not female as they had previously assumed. It suddenly explained why their continuous mating over the past 25 or more years had failed to ever produce any offspring.

Animals have also been often seen displaying bisexual behaviour – interacting with both opposite and same sex. This suggests a more biological purpose, in which it’s proposed that this may increase an animal’s chance of successful breeding later with the opposite sex. That may sound counter-intuitive, but one University of Frankfurt study, published by Royal Society Biology Letters, found that homosexual behaviour increases male attractiveness to females.

The researchers pointed out that females regularly use social information to choose a mate and male attractiveness increases after he has interacted sexually with a female – a phenomenon known as “mate choice copying”. More unexpectedly, the same appears to be true, at least for the the tropical freshwater fish Poecilia mexicana, with same-sex interaction. The female fish found the males more attractive after the male had a same-sex interaction in the same way they did with opposite sex interaction.

“Hence, direct benefits for males of exhibiting homosexual behaviour may help explain its occurrence and persistence in species in which females rely on mate choice copying as one component of mate quality assessment,” the paper said.

While these studies are useful, it is important to note that most animals can only be classified through sexual orientation rather than sexual preference.

“Sexual preferences require a higher cognitive involvement, something that only primates show,” says Dr Angelo Tedoldi, a neuroscientist at University of Queensland’s Queensland Brain Institute.

“Indeed, primates have been shown to have a more complex sexual behaviour that other animals in the animal kingdom. Bonobos are well known to have sexual intercourse for pleasure other than just for reproduction, both with different or same sex partners.”

Other studies have found evidence that sex hormones influence the development of very different gendered traits between gay and straight people; that there are differences in brain organisation between gay and straight people; and that birth order influences sexual orientation in men.

In 1993, a landmark paper was published in the academic journal, Science, that showed a linkage between DNA markers on the X chromosome and male sexual orientation. This spearheaded discussion on the possibility of a male “gay gene” and heritability of the trait.

For the study, Geneticist Dean Hamer at the National Cancer Institute in the US and his team analysed 114 families of homosexual men. They found increased rates of same-sex orientation in the maternal uncles and male cousins of the subjects, but not in their fathers or paternal relatives. The research revealed that a patch of DNA in the X chromosome – labelled Xq28 – was likely to be shared by brothers who were both gay.

While the media leapt on the catchy concept of the “gay gene”, subsequent studies to replicate this study have met varying levels of success.For more than 20 years Hamer’s research was called into question by subsequent studies. Then, in 2014, a Northwestern University study of 409 pairs of twins found a significant linkage for male sexual orientation to the same stretch of the X chromosome that Hamer had identified – Xq28.

The X-linked gene could explain how the trait persists in the population, even though gay men have fewer offspring, and studies such as Hamer’s and the later Northwestern one suggest there is a heritable component to homosexuality. However, few today believe only one gene or set of genes determines anyone’s sexual orientation.

“It’s very rare that a behavioural phenotype, or complex trait comes from a single gene. Most things don’t,” says Dr Hannah Brown, a reproductive epigeneticist at the University of Adelaide. “Many of our traits are from the combination of many genes working, or linking together.”

Is epigenetics the missing piece of the ‘gay gene’ puzzle?

With this thinking in mind, researchers William Rice, an evolutionary geneticist at the University of California, Santa Barbara; Urban Friberg, an evolutionary biologist at the University of Uppsala and Sergey Gavrilets, a mathematician at the University of Tennessee suggested that the search for a “gay gene” had been misguided from the start. The answer, they proposed in a 2013 paper published in The Quarterly Review of Biology, may instead lie in epigenetics – the study of the chemical changes that can alter how the genetic code is expressed, without changing the genetic code itself.

Rice and his colleagues proposed a model in which the epigenetic markers which steer sexual development in males could promote homosexual orientation in females, and vice versa. The elusiveness of the “gay gene” along with the tendency of homosexuality to run in families, supported this model, they said.

While there was undeniable heritability, only 20 per cent of identical twins are both gay, Rice noted, and the apparent absence of any “major” homosexual genes “made us suspicious that something besides genes produces heritability that isn’t genetic”.

The model could, they say, explain why not all identical twins share sexual orientation.

While it should be noted that the hypothesis has not yet been tested with real data, many researchers believe something of the kind could be in play.

Thomas Conway, a bioinformatics researcher at the Peter MacCallum Cancer Centre, believes that “multiple underlying mechanisms that contribute to sexuality”.

“It is very likely that while there may be a modest number of ‘gateway’ genes, there will be a number of multi-gene interactions combined with developmental factors, including maternal hormone levels at crucial times during gestation, for example, that affect the development of a person’s sexuality and gender identity.

“Height is a complex trait and we can measure it unambiguously, and it’s one dimensional.

“The last time I looked, a big GWAS study on height found that some 200 genes were involved, and surprisingly nutrition, childhood illness and the like play a relatively small role. So there is no height gene, even though height is very strongly genetically determined, with some environmental modulation.

“No one argues that being tall is a choice.”

Even same sex studies can be sexist

In all of this, there’s an obvious omission of women and other genders, and other sexual orientations including but not exclusive to lesbians and bisexuals.

These oversights have been made consistently, even in an Australian study in 2000 that reported on the heritability of homosexuality in 25,000 pairs of twins. It used the Kinsey scale, which is a seven-point scale for measuring sexual orientation (zero for exclusively heterosexual to six for exclusively homosexual). Controversially, it classified anyone with a Kinsey score of at least two as homosexual.

“Realistically, I understand the temptation to look at something like sexuality as a spectrum and deal with this in a scientific study by taking each end,” says Frentz.

“However, by doing this, we run the risk of missing key genetic links, and with most recent studies suggesting a genetic basis to sexuality being linked to epigenetic markers, it may be that this would have been more readily picked up.”

“This misses out huge chunks of the LGBTQI+ community.”

And that remains a problem along with the scarcity of funding for studies into sexuality. Private enterprise may take up some of that slack. Commercial genetics company 23andMe are trying the study the genetics of sexual orientation, in the “largest genome-wide association study of sexual orientation ever done”, as announced back in 2012. However, search results reveal that nothing has arisen in recent years from this study.

How neuroscience interprets same-sex attraction

Could brain structure and development account for differences in sexual orientation?

As far back as 1991, British-American neuroscientist Simon LeVay published a study looking at the structural differences between gay men and straight men.

Studying three groups (women, men assumed to be heterosexual, and homosexual men) he looked at their post-mortem brain tissues. He found four cell groups from the interstitial nuclei of the anterior hypothalamus (INAH), an area in the brain associated with the regulation of male-typical sexual behaviour. It was found that INAH 3 was more than twice as large in heterosexual men compared to women and homosexual men.

The conclusion he drew from this was that there is hypothalamic structure difference for sexual orientation in men.

Many studies have built on this work, such as in 2008 by the Stockholm Brain Institute that found striking similarities between the brain structures of gay men and straight women on the one hand, and of lesbians and straight men on the other.

Brain imaging showed that in homosexual men and heterosexual women the right and left hemispheres of are the brain almost exactly the same size, whereas lesbians and straight men have asymmetrical brains, with the right hemisphere significantly larger than the left. Before jumping to conclusions, it’s important to point out that identifying regions is not entirely conclusive. However they at least suggest there is a strong neurological basis in human sexuality. In neuroscience, it’s not enough to identify regions of the brain, functionality of these regions needs further clarification.

Dr Angelo Tedoldi, a neuroscientist, points to some of the difficulties with this sort of study.

“The major problems with studies about sexual preferences and differences in the brain of heterosexual and homosexual people are related to our understanding of sexuality. We do not have extensive information about the brain processes that drive sexuality in either males or females.

“We have an understanding of brain region differences and how some of these differences might be playing a role in sexual behaviour. However, we do not understand the driving forces behind sexual orientation or sexual preferences.”

The role your hormones play

Other factors are undoubtedly at play in determining the sexual orientation of individuals.

There’s evidence, for example, of developmental factors from the order of birth in boys to the sex hormones one is exposed to during development in the womb.

One effect dubbed the “fraternal birth order effect” or sometimes the “older brother effect” derives from studies which show that, the greater the number of older brothers a man has, the more likely he is to be gay. By some accounts, for every older brother a male child has, there is a 33 per cent greater chance of the male child being homosexual. That assumes that the progression is linear and it should be noted that other studies suggest that the first one or two brothers only has a small effect, growing rapidly with three or more brothers.

But why should this be? As it doesn’t appear to apply in the case of adopted siblings, it seems unlikely to be due to the social influence of the older brothers.

Hormonal processes in the womb have been suggested as one possible explanation.

Canadian researcher Ray Blanchard, the leader of the team which identified the effect, has hypothesised that the older brothers have an effect on the younger sibling, not directly, but through influencing the mother’s immune system. This effect, Blanchard and his colleagues suggest, sees the mother immunised against male-specific antigens in earlier pregnancies, leading to an effect on later ones.

But there are significant question marks over the research. Other studies have not been in complete agreement with the findings of the Toronto group. LeVay, in his comprehensive 2016 review of the literature, Gay Straight and the Reasons Why, points to one Northwestern University study of 1,700 gay and straight subjects that did find a strong correlation between the number of older brothers and sexual orientation. But it also found that the gay men also had more older and younger sisters – finding replicated in a later British study.

LeVay makes the further point that family sizes have changed dramatically over the years, which should have an impact on the overall percentage of the population that is same-sex oriented. But that rate has remained remarkable stable. He acknowledges the effect, which he says needs further research, suggesting the mechanism may work through a more specific effect on the brain circuitry that is responsible for sexual orientation.

There is some crossover into endocrinology in terms of sex hormones. There is strong evidence that prenatal exposure to different levels of testosterone can masculinise the foetal brain and influence male-typical behaviour. Regarding INAH 3 differences noted from the earlier mentioned neuroscience study, there is a possibility that it would be a marker of a downstream effect from sex hormones during development.

Sexual differentiation in the brain and the genitals occur during different stages of development. It is therefore possible for each to be influenced independently.

“Sex differences in cognition, gender identity (an individual’s perception of their own sexual identity), sexual orientation (heterosexuality, homosexuality or bisexuality), and the risks of developing neuropsychiatric disorders are programmed into our brain during early development,” state the authors of one review paper about sexual differentiation of the brain.

This brings up the important area of gender identity within sexuality. This further shows the complexity between sexuality, that it’s more of a spectrum of varying degrees rather than defined categories.

“Once we stop viewing sexuality through the lens of preconceived categories, it’s pretty clear that sexuality is a very complex trait,” clarifies Dr Thomas Conway.

An evolutionary view of the ‘gay gene’

Evolutionary geneticists bring their own perspective to the investigation, seeking to explain the prevalence of the trait of homosexuality across cultures and history.

This is often used by those who would argue against the biological nature of same sex attraction, by treating biological factors as immutable, a fixed point in human development. The science suggests that this is not the case.

Jenny Graves, professor of genetics at LaTrobe University in Melbourne, has no problem with the concept of gay genes.

“The idea that a person’s genetic makeup affects their mating preference is unsurprising,” she writes in The Conversation. “We see it in the animal world all the time. There are probably many genes that affect human sexual orientation.

But rather than thinking of them as “gay genes”, perhaps, she says, we should consider them “male-loving genes”.

“They may be common because these variant genes, in a female, predispose her to mate earlier and more often, and to have more children.”

She also notes that it would be surprising if there were not “female-loving genes” in lesbian women that, in a male, predispose him to mate earlier and have more children. It’s important to acknowledge that these possibilities are limited to binary genders and it doesn’t include those who see themselves as non-binary or otherwise.

Regardless, it leads to one of the more remarkable findings across the investigations into the origins of sexual orientation – the “fertile female” hypothesis, one possible explanation as to why, if gay men have fewer children on average, the “gay gene” variants don’t disappear.

Graves cites an Italian study that shows female relatives of gay men having 1.3 times as many children as the female relatives of straight men. One possible explanation is that “male-loving” alleles – our gene variants – in a female they predispose her to mate earlier and have more children, so making up for the fewer children of gay males.

That way, there could be thousands of male-loving and female-loving alleles in the population, with everyone inheriting a mixture of different variants.

“Combined with environmental influences, it will be hard to detect individual genes,” Graves writes.

She compares it to height, which is influenced by variants in thousands of genes, as well as the environment, and produces a distribution of people of different heights between the two extremes of very tall and the very short.

“In the same way, at each end of a continuous distribution of human mating preference, we would expect the ‘very male-loving’ and the ‘very female-loving’ in both sexes.”

Sigmund Freud has a lot to answer for

Historically, many people believed (and some still do) that homosexuality was a state of mind. Some argued that it was a lifestyle choice, or even a psychological disorder.

Psychological studies in sexuality often relate to sexual behaviour and sexual preference. More evident is the psychological effects and the mental wellbeing of those in LGBTQI+ communities due to stigma and discrimination.

There are worrying statistics that show the state of mental health of LGBQTI+ people from the National LGBTI Health Alliance, such as being twice as likely to suffer from mental health disorders than the general population.

There are fears around the same-sex families and their children as an excuse for hate towards these communities. However, the myths of poorer health and wellbeing of children in sex-same families is just that, a myth, as research published last month by the Murdoch Children’s Research Institute in the Medical Journal of Australia once again demonstrates.

The psychology of sexuality also includes gender identity, gender dysphoria and being comfortable (or not) with a birth-assigned gender.

“What matters is that we do our best to minimise the impact our previously held values have on the science that is being done, and to ensure that that research is not only being done for good reasons, but reflects or at the very least acknowledges the community that currently exists,” says Sophia Frentz.

At the end of the day, it isn’t just about the science of sexuality. It isn’t even science against bigotry, discrimination, prejudice and hate. It shouldn’t be political. Equality and equity for all is the end game.

It’s about what best can we do to support and foster the differences of individuals for a more diverse and inclusive society. As while science is about understanding the world around us, it’s also about accepting the world around us.

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This article was first published on Australia’s Science Channel, the original news platform of The Royal Institution of Australia.

The Royal Institution of Australia has an Education resource based on this article. You can access it here.