Sexual orientation in all its aspects

Boosted by his research and extensive reading on the influence of biology on sexual orientation, Jacques Balthazart summarises existing knowledge on the subject in a scientific article (1) in which he describes what is known based on animal models as well as on what is currently understood about the influence of hormones, genetics and epigenetics on sexual orientation in humans.

Jacques Balthazart’s book caused a stir when it was published in 2011 and has been the subject of much debate ever since. It is certainly true that “The Biology of Sexuality – We are born homosexual, we don’t choose to be so” (see article: Are we born homosexual?), clearly expresses the researcher’s message and it is one that does not find favour with everyone. Jacques Balthazart, emeritus head of the Behavioural Neuroendocrinology Research Group in the GIGA-neurosciences centre of the University of Liege, has been studying this question for many years, both on animal models in his laboratory and in the literature on the subject. “One of the main differences that exists in terms of the sexual behaviour of men and women is sexual orientation: a large majority of women are interested in men and a large majority of men are attracted by women”, explains Jacques Balthazart. “Therefore there is a huge difference between the sexes in terms of sexual attraction with the exception of a small section of the population of between 2% and 10% of individuals who are homosexual and who are therefore interested in having a same-sex partner." Struck by the fact that this sexual difference continued to be attributed to education or to deliberate choice, the scientist wanted to put down on paper, current biological knowledge on homosexuality, in a way that was accessible to all. “Repeated claims are made that homosexuality is caused by some deficiency in the mother’s role, the absence of a father or an unresolved Oedipus complex etc. But apart from this, there are thousands of scientific articles that show an influence of prenatal hormones, genetics or even epigenetics, continues Jacques Balthazart.

Oreintation sexuelle

Embryonic hormones at the heart of the controls of sexual orientation

In the context of a special volume of Philosophical Transactions of the Royal Society B (1) on the multiple origins of sexual differences in the brain, the coordinator of the volume asked Jacques Balthazart to carry out a review of the literature on the biological factors that influence sexual orientation. “In this article, I explain what we know about animals and that could have an impact on humans as well as what is actually known about humans”, says the scientist.  

In his laboratory, Jacques Balthazart studies how male (testosterone) and female (oestradiol) sex hormones act on the brain to control sexual behaviour. In order to do this, the researcher and his team used various animal models such as quail, canaries, rats or mice. “We realised that hormones act during very early life, that is to say, in utero/ovo, or during the first days of life, to irreversibly define how males and females will react to sex hormones as adults. This is what is known as the organising effects of hormones on the brain”, explains the scientist. Hormones will therefore influence the structure of the brain so that it will be organised in such a way as to produce male or female sexual behaviour. By means of several experiments, the researchers were able to demonstrate that it is possible to deliberately reverse sexual orientation in animals such as rats by manipulating sex hormones at birth or slightly before. The females then show male behaviour and males show female behaviour when they are at the reproductive age.  

In the west of the US, there is an animal model that is well known in terms of homosexuality. It involves a sheep population in which 8% to 9% of the rams are strictly homosexual. “This came to light after fertility problems were identified in these sheep” explains Jacques Balthazart. “Research on this sheep population shows that the homosexual rams do not present hormonal problems in adulthood but that their sexually dimorphic nucleus – responsible for sexual behaviour and located in the brain preoptic area – is smaller than in the other males and is of similar size to that present in the females”. Significantly, this nucleus is essentially regulated by embryonic hormones…If the embryo is exposed to an high level of testosterone, it will develop a male-sized nucleus.  If there is not much testosterone at this stage of development, the size of the sexually dimorphic nucleus will be smaller like those of the females.

(1) Balthazart J. 2016 Sex differences in partner preferences in humans and animals. Phil. Trans. R. Soc. B 371: 20150118.

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