W HAT IS THE S CIENCE BEHIND HOMOSEXUALITY ? A SOCIAL POLICY REPORT Homosexual behaviour is a nearly universal phenomenon in the animal kingdom. It is common across species, from worms to frogs to birds, concludes a recent literature review of the area. There is a strong possibility that homosexuality is normal in a biological sense, that is, it is a behaviour that evolved as an important element of early human social organisation. Homosexuals may be some genetic carriers of some of mankind’s rare attributes. Although homosexuality is mostly common in animals it finds its expression in the most intelligent of primates. Neurologists have known for decades that our genes largely shape the physical structure of the brain during prenatal development. Our genes determine the size, shape, connectivity of the lobes and nuclei, the basic plan of the cerebral cortex, and the quantity of grey matter in different regions that affect language and reasoning.
Physical brain geometry has effects on our thinking, feeling and
behaviour. People who are born with variations in geometry have variations in the way their minds work. Studies done on various subjects, determines both structural and functional brain differences to be highly correlated with sexual orientation. According to Swaab, this difference in structure and function develops during the foetal and neonatal phases.
LeVay [3], in his study done in 1991, established that the anterior
hypothalamus, the region controlling the secretion of sex hormones from pituitary gland, in non-primates is the cause for the typical male sexual behaviour. This was tested on male monkeys, where damage to this area resulted for the monkeys to have heterosexual behaviour impaired but still maintain their sexual drive.
When the equivalent of the human
hypothalamus was observed, a difference was found in two small groups of densely packed large cells that are related to sexual behaviour.
The two being the
interstitial nucleus of the anterior hypothalamus, INAH 2 and INAH 3 and it was found that these were larger in men than in females. Brain tissue samples, taken from 41 subjects showed a significant difference between heterosexual men and women as well as between heterosexual and homosexual men, where INAH 3 was found to be more than twice as large heterosexual men. Also, there was no significant difference between homosexual men and heterosexual women. This confirms that INAH 3 is dimorphic with sexual orientation.
However, the drawbacks of this study were that brain tissues of
homosexual women could not be obtained. Therefore, only sexual orientation in men was tested. In addition, the criticism that makes this data less reliable was that all the brain tissues observed were obtained from autopsies of those who died from AIDS. This was accounted by LeVay by stating that even when homosexual men were compared with heterosexual AIDS patients, the size in INAH 3 was still visible [2]. In another study done in 2001, there was no significant difference established in the INAH 3 size between heterosexual and homosexual men. However, the same neurons were observed to be more packed together in heterosexual men [3]. In 2008, MRI and PET scan studies were done by Savic and Linstrรถm [4] as an MRI is used to determine certain medical conditions, including abnormalities of the brain and spinal cord, while PET aids in determining the function of these organs. It was found that homosexual subjects were more common than heterosexual subjects to be left-handed or ambidextrous. Also heterosexual subjects had greater cerebral symmetry when compared to right-handed ones. Studies show that this link is related to a difference in the corpus callosum, as the connections between the left and the right hemisphere were found to vary in homosexual and heterosexual men.
The main difficulty
encountered was that the results obtained were not repeatable and results from other studies were contradicting [3].
Furthermore, PET analysis suggests that
amygdala connections tend to be similar between homosexual men and heterosexual females with greater connections than heterosexual men and homosexual females. This is the region of emotional response to external stimuli, anxiety and difference in moods [6]. PET studies done by Savic et al. [1] explored the effect of putative pheromones secreted during perspiration on sexual behaviour.
These are possible chemical
substances that get released by the body and affect the behaviour and physiology of other surrounding individuals.
It was concluded that the hypothalamus of
homosexual men and heterosexual women were stimulated in the same manner. Yet, heterosexual male and homosexual females were not stimulated by these pheromones. In fact the percentage of females falling in depression is larger than for males and this is also seen in homosexual subjects, where homosexual males are more prone to depression and suicide than homosexual females. Swaab explains that this due to the fact that the hypothalamus of homosexual men is less responsive to fluoxetine. Fluoxetine, an antidepressant, inhibits the neurotransmitter, serotonin, uptake by the brain hence resulting in a deficit of the serotonin, which lead to depression [1]. However, these factors cannot be viewed only from a biological point of view, as other influences as social pressure also take part. In addition, sensorimotor cortex, associated with fight and flight reactions, have stronger connections in men and, in fact, this is also observed in homosexual women. Hence, these patterns observed are sex-atypical [4]. A difference was also found in the biological time clock that keeps the body on a 24-hour schedule, known as the suprachiasmatic nucleus
(SCN). This was determined to be twice as large in homosexual men than in heterosexual men [5]. Another study done by Allen and Gorski concluded that the anterior commissure, in charge of the left-right temporal cortex connections, is also larger in homosexual men than in heterosexual men. Which in turn was found to be larger in heterosexual women than in heterosexual men.
Again this was
contradicted by a research done in 2002 by Lasco found no relationship between sexual orientation and the size of the anterior commissure [1]. Initially in the late 20th century, according to a study done by Money, it was assumed that it was society that shaped gender identity and that this influence starts to take place after the age of one. However, contradicting this is the clear case of John/Joan. Where even though this boy had his genitals removed and was raised as a girl, he had changed his gender identity to male by the age of fourteen. This example serves to show that gender identity is programmed in an irreversible manner [6]. With all that being said, it is of the utmost importance to understand that this information is not stating that homosexuality is caused solely by biological and chemical factors. The research being done is simply to understand the science behind homosexuality and by no means to try to alter or have control over the process. The biological aspects discussed are to be linked with other factors, which include environmental, social and physiological influences.
We would like to thank Chantelle Spiteri and Matthias Vassallo from the Social Policy subcommittee for their contribution to this report. References: [1] Bao, Ai-Min, Swaab, D. F. Sexual differentiation of the human
brain: Relation to gender identity, sexual orientation and neuropsychiatric disorders. Volume 32, Issue 2, April 2011, Pages 214–226 [2] LeVay, S. (1991). A difference in hypothalamic structure between heterosexual and homosexual men. Science, 253(5023): 1034-1037. [3] Malory, M. (2012). Is Homosexuality a Choice? Scientific American [4] Savic, I. Lindstrom, P. (2008) PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. PNAS vol. 105 no. 27 [5] Swaab, D. F. (2008). Sexual orientation and its basis in brain structure and function. Proc. Natl. Acad. Sci. USA, 105 (2008), pp. 10273–10274 [6] Swaab, D. F, Garcia-Falgueras, A. Sexual differentiation of the human brain in relation to gender identity and sexual orientation. (2009) Funct Neurol. 2009