I can still remember the commotion which arose in 1990 when Dutch neuroscientist Dick Swaab published an article saying that homosexual men had an area in their hypothalamus which was twice the size of the same area in heterosexual men. Many found the idea – that homosexuality might have a biological basis – quite alarming! At the time I worked at the Department of Psychiatry of what is now the Erasmus Medical Centre in Rotterdam. Psychiatrists asked me what I made of Swaab’s article. I could see them thinking – or sometimes even saying – that it could not be true. My answer was that although I did not understand the implications of the findings, I could confirm that it was reliable research. Which meant that they could accept the results to be sound.
Homosexuality has a biological basis
Much has changed in the meantime. Homosexuality undoubtedly has a hereditary component, and there are many who are aware of their sexual orientation from their early years, in some cases long before puberty. Although I have no way of knowing if every homosexual is aware of this from an early age. The question I concern myself with here is how to understand the biological basis. Differences in the hypothalamus have meanwhile been found by other researchers than Swaab. And if something is hereditary then it has a biological basis. Till recently I was unable to understand how that could be. I assume that homosexuals usually have heterosexual parents. Do heterosexual parents then have homosexual genes? It is now clear to me that this is the wrong question to ask.
The role of DNA
DNA, which you inherit from your parents, plays a role in every human trait1. A human being has 23 pairs of chromosomes, and each of these is a long chain of DNA. Part of the DNA chain consists of genes, but there are also sections of DNA which regulate the activity of genes, switching genes on or off, or making them more or less active. Complex interplays of genes and those other sections of DNA play a role in the development of all human traits.
For each complex trait there are hundreds, and often thousands, of sections of DNA involved. Furthermore, there is no single section of DNA which is exclusively involved in the development of one single trait. One particular section of DNA which differs between homosexual and heterosexual men also plays a role in the working of the thyroid gland. Knowing this inclines us to think that the thyroid and homosexuality have something to do with each other. But that is unfounded, because each section of DNA plays a role in many different functions in humans, and these functions are not necessarily related to each other. Although some sections of DNA have been identified which differ between homo- and heterosexuals, their role is as yet unclear.
Male – female differences
There are obvious differences in homosexuality between men and women. Women for instance are more often bisexual. Perhaps it is better to say that men are more commonly either hetero- or homosexual whereas this boundary is much more diffuse in women. These differences between men and women are also for a part based on differences in DNA. It appears that both biological and psychological aspects of homosexuality are different in the male and female.
The physical differentiation between men and women is based on the fact that men have a Y chromosome and women do not. This results not only in different primary sexual characteristics but also in differences in brain structure, some of which are mediated by hormones. The process of sexual differentiation in the brain is complicated. This process involves not only the X and Y chromosome, but also other chromosomes which work together with the sex chromosome in a complex dynamic system.
The sexual orientation however does not directly relate to the physical differentiation. It seems that sexual orientation develops towards a man or woman; in the first case leading to homosexuality and the second to heterosexuality if the subject is male. And if the orientation towards the male develops in a female, then she is heterosexual; and in the second case homosexual.
How does homosexuality develop?
Several factors other than DNA play a role in the development of traits. What are these factors which play a role in the development of same sex orientation? This is not quite clear at present. Surprisingly, the circumstances in which you grow up – including upbringing, culture, religion – play little or no part in this. There are cultural differences in prevalence of homosexuality, but they probably have less to do with the actual prevalence than with the ease of coming out in a given culture.
Homosexuality is biologically determined
It appears that the inherent tendency to sexual orientation is entirely or almost entirely biologically determined. How can that be when DNA only partially determines this characteristic? The development of a human being starts with DNA, but DNA is not a blueprint for the creation of a human being. Even perfectly identical DNA will never lead to the development of two identical human beings. This is shown by the fact that even homozygotic twins are not identical2. The development of one attribute – such as sexual orientation- in a complex dynamic system like that of the human being and the brain is not predictable. Within a large group of people it is possible to predict approximately how many will turn out to be homosexual, but not which particular individuals.
Sexual orientation is a biological phenomenon just as is left- handedness. Parents, society and church do not play any significant role in this. Going against your sexual orientation is unhelpful. And attempts, sometimes forcibly, to help people to change their orientation only leads to unhappiness. That applies to sexual orientation just as much as for left-handedness. The DNA that you inherit from your parents plays an important role in these traits, but genes which directly cause homosexuality do not exist.
Bailey JM, Rieger G, Krishnappa RS, Kolundzija AB, Dawood K, Sanders AR (2020): Familiality of Gender Nonconformity Among Homosexual Men. Arch Sex Behav 49:2461–2468.
Ganna A, Verweij KJH, Nivard MG, Maier R, Wedow R, Busch AS, et al. (2019): Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior. Science 365. DOI: 10.1126/science.aat7693
Sanders AR, Beecham GW, Guo S, Dawood K, Rieger G, Badner JA, et al. (2017): Genome-Wide Association Study of Male Sexual Orientation. Sci Rep 7:16950.
Bailey JM, Vasey PL, Diamond LM, Breedlove SM, Vilain E, Epprecht M (2016): Sexual Orientation, Controversy, and Science. Psychol Sci Public Interest 17:45–101.
Mitchell K: Gay genes? Yeah, but no, well kind of… but, so what? http://www.wiringthebrain.com/2014/03/gay-genes-yeah-but-no-well-kind-of-but.html. March 24, 2014.
Mitchell K: Sexual orientation – in the genes? http://www.wiringthebrain.com/2010/05/sexual-orientation-in-genes.html. May 31, 2010.
Fisher SE (2006): Tangled webs: Tracing the connections between genes and cognition. Cognition 101:270–297.
Swaab DF, Hofman MA (1990): An enlarged suprachiasmatic nucleus in homosexual men. Brain Res 537:141–148.