Gender differences and sexual preferences

When it comes to gender and sexual orientation, most of you would probably agree that it has always been a controversial topic. Not only are people’s opinions mixed about a variety of aspects related to gender and sex (the differences between males and females; the “gay problem”; the transgender and transsexual “trend” etc.), but it is also the complexity of these aspects that makes them difficult to be understood and accepted.

Do women’s brain differ to that of male’s? If so, what are the factors involved in the process of differentiation and when and how does all of this occur? Is homosexuality a choice or a genetic determination? Why do some people attempt to change their gender? These are some of the most common questions that have been raised throughout time and that I have come across. Nevertheless, one question that really got me thinking is: Are homosexuals, bisexuals, polysexuals, demisexuals, transgenders, transsexuals and so on NORMAL?

Hopefully, by the end of this article you will find some of these questions at least partially answered. Given the fact that the article is intended to cover such a diverse subject, I have decided to divide it into chapters. Having said that, let’s dig in!

Why do boys like blue and girls like pink?

It might come as a surprise that the behaviour of people according to their gender is not as much influenced by the society as one would expect. It is true that parents guide their children to behave in a certain way depending on whether they are boys or girls: the type of toys they are offered (action toys versus dolls); the kind of sports/activities they are encouraged to perform; the colours of their cloths and/or rooms etc. Moreover, it seems that this distinction between girls and boys is perpetuated in school by teachers: boys are expected to perform better tasks that involve mathematical and spatial reasoning, while girls are said to outperform boys in word comprehension and writing. Some have argued that all the previously mentioned are myths and that society should stop differentiating between genders when it comes to brain and intellect.

It turns out that these differences exit with or without the social stereotypes, or more precisely these stereotypes are based on real facts. Studies have shown that female monkeys prefer dolls, while male of the same species would rather play with toy cars and balls. This is not surprising if we consider the evolutionary roles of males and females inside the family and community: males have evolved specific abilities for more dynamic activities such as hunting and protecting their territory from enemies, thus their native spatial skills; females are structurally designed for more domestic activities, such as motherhood and housekeeping, therefore are more sociable and more inclined to have a better verbal memory than men.

So how are our brains programmed to develop certain male/female behavioural characteristics in us? It all comes down to genes. As we all know, at a chromosome level females differ from males in the heterosome pairs: XX for women and XY for men. Even though the X chromosome is larger and contains the majority of genes (including those coding for some masculinity traits), the Y chromosome has a crucial role in sex determination. The presence or absence of a specific gene called the sex-determining region of the Y chromosome (SRY) “decides” whether the foetus develops into a male or female. This gene codes for the protein testis-determining factor (TDF) which is responsible for the differentiation of the foetus’ genitals into testes.

The hormones produced by the testes (androgens), primarily the testosterone, have a very important role in the male development, as well as the oestrogen (like estradiol) influence the female features. During the first half of pregnancy, between the sixth and twelfth week, testosterone (produced at first by the Y chromosome) differentiates the sex organs into testes or ovaries. During the second half of pregnancy, the brain differences occur, due to a peak production of testosterone. The role of hormones in gender differentiation is important not only during the intrauterine period, but throughout the whole life of an individual. At puberty, the release of sex hormones induce the secondary female/male characteristics, such as facial hair, breasts, voice change etc.

It is important to note that the release of hormones is highly regulated by the nervous system and the endocrine system. Having said that, the pituitary gland secretes luteinising hormone (LH) and follicle-stimulating hormone (FSH) also known as gonadotropins which stimulate the gonads (testes and ovaries) to produce hormones. In turn, the pituitary gland’s hormone production is controlled by the hypothalamus through another hormone – gonadotropin-releasing hormone, which is influenced by the circadian cycle (more on this in a future article or if you have any questions, please address them to me). The adrenal glands also secrete a small amount of androgens.

Brain sexual dimorphisms

Yes! There are structural differences between the female and male brains and they are called sexual dimorphisms. The hypothalamus is a key region of the brain to sexual behaviour. As expected, a striking dimorphism can be observed here, more specifically within the preoptic area of the anterior hypothalamus. Here, the sexually dimorphic nucleus of the mammalian hypothalamus is significantly larger in males than in females. In humans, the preoptic area contains four clusters of neurons out of which at least one – INAH-3 (interstitial nuclei of the anterior hypothalamus-3)- was shown to be bigger in men. Also, the corpus callosum (the major neural pathway that connects the two cerebral hemispheres) and the bed nucleus of the stria terminalis (BST) are larger in men.

The abnormal (?!) 

We finally got to the point where we discuss the “anything else other than heterosexuality” which is often classified as abnormal. Heterosexuality is known to predominate and is thought to be the only form of sexual orientation in all the other species apart from humans. Nevertheless, this couldn’t be further from the truth. Fish, birds, reptiles and even some mammalian species shown homosexual behaviour. It appears to be more common in birds than in mammals, although the examples are quite sufficient to prove that mammals are inclined to develop homosexual behaviour too. However, homosexuality among mammals is often temporary and is due to certain situations, such as better protection of the offspring, defence mechanisms and seeking help from other animals against enemies.

Having read the chapter about gender differences, we are entitled to assume that some sort of chemical and structural modifications generate gender identity and sexual orientation. Many of these changes happen in the womb. As explained above, genes and hormones play a very important role in the development of a foetus into a male or a female. But if these two factors don’t “agree” with each other, the individual will experience sexual and/or gender changes. The genetic and hormonal influences can be easily observed in twins: monozygotic twins have an incidence of 50% of both being homosexuals, while the percentage in dizygotic twins in 25%. At the same time, in the case of opposite sex twins, the female twin is more likely to develop congenital adrenal hyperplasia due to being exposed to her brother’s testosterone.

Before we go deeper into the subject, I would like to point out a few interesting things. The principal female sex hormone estradiol is actually synthesised from testosterone by the action of an enzyme –  aromatase. At the same time, the androgen receptor gene is located on the X, not on the Y chromosome, so males have only one copy of this gene. 

The fact that males have a single copy of the androgen receptor gene makes them prone to androgen-insensitivity, if the gene is not functional. The androgen-insensitive genetic males develop normal testes and produce testosterone, but they look and behave like genetic females. They are also attracted to men instead of women. The female version of this is represented by congenital adrenal hyperplasia. Women with this condition have been exposed to an abnormally large amount of testosterone and they develop a man-like behaviour, being more inclined to choose women as their sexual partners.

Even though the up-bringing of children and the social environment might have some influence on their sexual preferences and gender identity, the hormonal and central nervous system structural and functional changes have been demonstrated to be the cause. In most cases, if too much or too little amount of a specific hormone is released (or the receptors for that hormone are inactive or hyperactive) during pregnancy, this leads to changes in brain development. Previous studies have indicated that some structures in the hypothalamus are larger in homosexual men than in the heterosexual ones. Other differences have been observed in the brains of transsexual people. The bed nucleus of the stria terminalis, for example, is smaller in male-to-female transsexuals than in males, being more similar to the women’s BST.

Also, brain circuits appear to function differently according to sexual orientation. Usually, the way some brain areas respond to specific pheromones (see previous article about olfactory memory) and other stimuli is similar in heterosexual men and homosexual women and consequently, in heterosexual women and homosexual men. These functional and structural differences appear early in development and cannot be changed after birth by any social and environmental means. 

I believe we can all agree now that there are many forms of “normal” in the world and that nature is a lot more open-minded than humans. We should learn to think outside the box and accept those who are not abnormal, but only different from us just as we are different from our parents, family, friends in terms of eye colour, food preferences, fashion style etc. Nevertheless, there is something I would like to place emphasis on: it’s one thing to be in a certain way and a completely different thing to choose something just because it’s cool or a lot of people do it. If you are one of those people who think being gay for instance is cool, but do not identify with it at all, my advice is: Don’t jump on the bandwagon! Be who you are and accept the others for who they are!

For further information:

Bear et al., 2006. Neuroscience – Exploring the Brain. s.l.:Lippincott Williams & Wilknins pp. 534-560

Swaab, 2014. We are our brains – From the womb to Alzheimer’s. Penguin Books, pp. 55-71

Kandel, 2005. Psychiatry, psychoanalisis and the new biology of mind. Trei, pp. 122-126

Photo taken by myself and edited by Isuru Priyaranga

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