To a certain extent, homosexuals are biologically different from heterosexuals as evident from the structural or anatomical and functional difference between their brains. This fact further establishes the biological underpinning of sexual orientation and homosexuality.
Acclaimed Dutch neurologist Dick F. Swaab mentioned in his review of literatures that sexual differentiation of the human brain occurs during the fetal and neonatal development that subsequently programs sexual orientation and gender identity. This biological development process involving sexual differentiation is accompanied by many structural and functional brain differences among heterosexual, homosexual, and even bisexual groups.
Swaab made the first breakthrough study in 1990 that revealed a structural difference between the brain of homosexual men and heterosexual men. This difference was in the brain area called suprachiasmatic nucleus that is responsible for regulating the circadian rhythm or biological clock. Accordingly, homosexual men have larger suprachiasmatic nucleus, about twice as large, than heterosexual men.
In 1991, British-American neuroscientist Simon LeVay made another significant discovery. His study revealed that the volume of an area of the brain called INAH-3 was twice larger in heterosexual men than in homosexual men. INAH-3 is part of the anterior hypothalamus and homologues have been observed to play a direct role in the sexual behaviours of several animals including the rhesus monkey and sheep. It is also important to note that INAH-3 is significantly larger in males than in females irrespective of age.
The study of anatomists Laura Allen and Roger Gorski in 1992 also revealed another difference. Accordingly, homosexual men have larger anterior commissure than heterosexual men. This brain structure contains nerve fibres that connect the left and the right temporal cortex. While it does not influence sexual behaviour, the anterior commissure is involved in the sex differences related to cognitive abilities and language.
I. Savic, H. Berglund, and P. Lindström studies the effect of scent, particularly a pheromone derived from progesterone and excreted in perspiration in concentrations 10 times higher in men than in women. Although this pheromone influences the sexual behaviours and stimulate the activation of hypothalamus of heterosexual women and homosexual men, Savic et al found out that it did not elicit a response in the hypothalamus of heterosexual men.
Savic and Lindström also performed a PET and MRI on the brains of homosexual and heterosexual male subjects. Result revealed that there are differences in the cerebral asymmetry and functional connectivity between homosexuals and heterosexuals. It is also interesting to note that the brains of homosexual men more closely resemble the brains of heterosexual women. For example, the number of nerves connecting the two hemispheres of the brain in homosexual men is also more similar to the number in heterosexual women.
The same study of Savic and Lindström also revealed that the brains of homosexual women and heterosexual men are also similar in structure and functions. For instance, the scans revealed that the right brain hemispheres of lesbians are slightly larger—just like the right brain hemispheres of heterosexual men. In comparison, homosexual men and heterosexual men have symmetrical hemispheres. In terms of functionality, both homosexual women and heterosexual men are wired for a greater fight or flight response while homosexual men and heterosexual women have lesser response.
Handedness has also been associated with differences in the structure and function of the brains between right-handed and left-handed people. Particularly, left-handed individuals have a larger region of the posterior corpus callosum, a thick ban of nerve fibres connecting the two brain hemispheres. An earlier research has also revealed that there is a higher proportion of left-handers in the homosexual population.
To determine a possible correlation between handedness, brains structure, and homosexuality, researchers Sandra F. Witelson et al determined whether the anatomy of the brain of the sub-group of right-handed homosexual men is similar to that of left-handers. They found out that the posterior part of the corpus callosum is larger in homosexual than heterosexual men.
It is also worth mentioning that sexual differentiation of the human brain occurs during the fetal and neonatal development according to Swaab. Nonetheless, there is a pressing theory that might explain the emergence of homosexual offspring. R. Blanchard was the first to introduce the Fraternal Birth Order Theory in his 1997 study. This theory suggests that the more older brothers a man has from the same mother, the greater the probability is that he will have a homosexual orientation.
Blanchard expanded this hypothesis by including the Maternal Immune Hypothesis of Male Homosexuality in 2000. Accordingly, the fraternal birth order effect demonstrates the progressive immunisation of some mothers to Y-linked minor histocompatibility antigens by each succeeding male fetus and the concomitantly increasing effects of such maternal immunisation on the future sexual orientation of each succeeding male fetus. This immunisation to a particular antigen affects the sexual differentiation of a subsequent male fetus. In other words, a mother who have conceived several male offspring develop immunity toward an antigen that is responsible for triggering the development of a male-oriented brain. The result is that the later male offspring develop a brain that is less male-oriented.
Of course, both the Fraternal Birth Order Theory and the Maternal Immune Hypothesis of Male Homosexuality do not explain why homosexuality also emerges in a smaller number of offspring. They do not also explain why homosexuality emerges in women.
Nonetheless, these differences in the structure and the functionality of the brain between homosexuals and heterosexuals do not directly explain the cause of homosexuality. Rather, these differences suggest that homosexuality has a biological or more appropriately, neurological underpinning. Furthermore, the aforementioned studies support the hypothesis of a neurobiological basis in the origin of sexual orientation. It is important to note that the results of the studies are not conclusive but rather suggestive.
Further details of the review of Dick F. Swaab are in the article “Sexual orientation and its basis in brain structure and brain function” published in 2008 in the journal Proceedings of the National Academy of Sciences.
Further details of the study of Savic and Lindström are in the article “PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects” published in 2008 in the Proceedings of the National Academy of Sciences. More details of the study of Witelson et al are in the article “Corpus callosum anatomy in right-handed homosexual and heterosexual men” published in 2007 in the journal Archives of Sexual Behaviour.
Details of the 1997 study of Blanchard are in the article “Birth order and sibling sex ration in homosexual versus heterosexual males and females” published in the journal Annual Review of Sex Research. Further details of the 2001 follow-up study are in the article “Fraternal birth order and the maternal immune hypothesis of male homosexuality” published in 2001 in the journal Hormones and Behavior.