The Science Of Sexuality

by / 15/08/11

It’s pretty well established that queer people are ‘born this way’, but why is this so? What happens before birth that makes us attracted to different genders?

Are there environmental factors that can influence sexuality after birth? Here comes the science you’ve been waiting for!
It seems pretty safe to say that if there was conclusive evidence that a single factor causes homosexuality, you would have heard about it. A while back the media got quite excited about a so-called ‘gay gene’, but this makes as much sense as referring to a ‘height gene’ or a ‘skin colour gene’. This is because these traits are influenced by more than one gene.

There are real, physical differences in the brains of men—there are far fewer studies done on women’s sexuality, but that doesn’t mean that there aren’t differences in the brains of women either—which change the way they react to pheromones. These are chemicals which trigger a response in other individuals. For instance, it has been found that gay men, when exposed to a testosterone-related pheromone in male sweat, had a region of their brain—their hypothalamus, if you’re interested—activated, and heterosexual men had the same region activated when exposed to a pheromone similar to oestrogen found in female urine. The hypothalamus doesn’t react this way to normal smells, only to sexual ones, so this is pretty compelling evidence that the gays aren’t just pretending.

In fact, there are many physiological differences between heterosexuals and non-heterosexuals such as differing sizes of brain hemispheres, responses to certain chemicals and activity of different brain regions. Homosexuals of both genders are significantly more likely to be left-handed or ambidextrous than heterosexuals, and this phenomenon appears to be linked to birth order—see next paragraph—In short, there are many physical differences that clearly illustrate that queer people are different from heterosexuals in more ways than which gender we are sexually attracted to. Wikipedia (so shoot me) has a list of these in its article entitled Biology and Sexuality.

One of the most significant statistical factors influencing sexuality in males is birth order. Sometimes called the older brother effect, this phenomenon has been identified from studies which show that the more older brothers a male has, the more likely it is he will be a homosexual adult. This effect is stronger than you might expect; each older brother increases a man’s odds of being homosexual by 28-48 per cent.

A proposed explanation for the older brother effect is the maternal immune hypothesis which suggests that the mother grows more immune to male antigens (more technical shit: antigens are chemicals related to the immune system) with each pregnancy, and so produces more ‘anti-male’ antibodies. This phenomenon seems linked to handedness too; it looks as though it only applies to right-handed men.
An oversimplified evolutionary crash course: if you have more children, you have more grandchildren, more great-grandchildren etc., so if you have a combination of genes which allows you to reproduce to a greater extent, then those super-reproductive genes will be passed on to future generations and those offspring will be more evolutionary successful too—evolutionary successfulness is essentially how many offspring you have, or more accurately, how much your genes proliferate.

So what about genes which contribute to homosexuality? If you only sleep with people of the same gender, you don’t have babies, so your gay genes don’t get anywhere and die out, right? At first glance homosexuality seems to be something which would be selected against, a reproductive disadvantage which natural selection would come down on like a (metric) tonne of bricks and crush before it even gets going.
Well, to a certain extent, it does, but it’s not as simple as that.

Hold on, this shit’s about to get technical.

It is thought that while (currently unidentified) genes which predispose to homosexuality reduce reproductive success if an individual with those genes is homosexual, the same genes could confer a reproductive advantage when carried by heterosexual people. This means that while these genes might sometimes reduce reproductive rates (by making gays), they can also increase them in other situations, so they remain at their relatively high proportions in the gene pool. This idea runs along the lines that a straight man with some ‘gay genes’ might be more sensitive to a woman’s perspective, get on with her better and consequently have a better chance of getting it on (this is very similar to an explanation for bisexuality). Similarly, it’s been found that female relatives of gay men on their mother’s side tend to have more offspring, which could indicate that women carrying some combination of ‘gay man genes’ are more fertile, which may well more than offset the lack of reproduction from gay males. This also indicates that some of these genes could be carried on the X chromosome.

Another possible explanation is the ‘gay uncle’ hypothesis which suggests that people who do not reproduce may still confer an advantage on their close relatives by helping them in the raising of their children, providing resources such as food, shelter, defence etc. The siblings of the ‘gay uncle’ will always share a high number of genes with him, so even though the gay uncle has no offspring directly, he is helping many of his own genes survive in his nephews’ and nieces’ bodies.

Those are some possible explanations for why homosexuality exists, but what about the how?

A foetus is by default female, but early on in pregnancy, if the child will be male (has a Y chromosome), sexual organs are differentiated—the first such event happens at seven weeks—that is to say, if the foetus will be born a male, its sexual organs will begin developing as male ones rather than the default female ones. This is initiated by hormone (signalling chemicals such as testosterone) doses from the mother. We can all see that men and women have their brains organised differently, and in fact many of these differences are hard-wired before birth. Brain differentiation occurs later in development than sexual organ differentiation, and so the two processes can sometimes move in different directions. Many changes brought about during brain differentiation are permanent and the brain’s ‘circuitry’ is laid down, but a lot of the differences don’t become apparent until later in life. During puberty in males, when sexuality emerges, the raised levels of testosterone activate these pre-programmed circuits and behavioural patterns which were built in during brain development.

These differences in brain structure between the sexes, which are caused not just by hormones but also by their interaction between genes and developing brain cells, are thought to be the basis of differences in sexual behaviours, such as sexual orientation and gender identity. It’s pretty easy to see how transsexualism could result from this process: a male body, with a Y chromosome, has male sexual organs determined early on in development. However, for some reason the second dose of testosterone which would normally set the brain along a male development path never arrives. This causes the brain to develop along a female path instead, resulting in a female person in a male body, or vice versa.

If you found this interesting I would encourage you to read Wikipedia’s articles on Biology and Sexuality, Prenatal Hormones and Sexual Orientation (a more technical one), Bisexuality, and Homosexual behaviour in animals. There’s a lot I haven’t covered here, especially bisexuality which deserves a lot more space than I was able to give it. In the meantime, here is what some paediatric endocrinologists had to say in a report from 2006:

“The foetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed or organized into our brain structures when we are still in the womb. There is no indication that social environment after birth has an effect on gender identity or sexual orientation.”
Boom. Science. It works, bitches.

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