Men and women who have identified themselves as homosexuals are somewhat biologically different from those who have identified themselves as heterosexuals as evident from the structural or anatomical and functional difference between their brains. This fact establishes the biological underpinning of sexual orientation and homosexuality.
Homosexuality and the Brain: A Rundown of Studies Examining the Difference in Homosexual and Heterosexual Brain Structure and Function
Earlier Research on Homosexual Men and Women
In his literature review, acclaimed neurologist Dick F. Swaab mentioned that sexual differentiation of the human brain occurs during fetal and neonatal development that subsequently programs sexual orientation and gender identity. The biological development process involving sexual differentiation results in 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 the suprachiasmatic nucleus that is responsible for regulating the circadian rhythm or biological clock. Accordingly, homosexual men have a 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 behaviors 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 commissures than heterosexual men. This brain structure contains nerve fibers that connect the left and the right temporal cortex. While it does not influence sexual behavior, the anterior commissure is involved in the sex differences related to cognitive abilities and language.
Researchers 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 sexual behaviors and stimulates the activation of the 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. Results 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, on the other hand, homosexual men and heterosexual women have a 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 band of nerve fibers connecting the two brain hemispheres. Earlier research has also revealed that there is a higher proportion of left-handers in the homosexual population.
Handedness, Birth Order, and Maternal Immune Hypothesis
To determine a possible correlation between handedness, brains structure, and homosexuality, the team of 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.
Note that sexual differentiation of the human brain occurs during 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. The 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 immunization of some mothers to Y-linked minor histocompatibility antigens by each succeeding male fetus and the concomitantly increasing effects of such maternal immunization on the future sexual orientation of each succeeding male fetus.
The immunization to a particular antigen affects the sexual differentiation of a subsequent male fetus. In other words, a mother who has conceived several male offspring develops 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.
A Note on Homosexuality and Brain Structure and Function: Important Points for Considerations and Reconsiderations
Nonetheless, the aforementioned 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 also important to note that there is also a sizeable body of studies that have studied the similarities between homosexual and heterosexual individuals, particularly when it comes to the structure of their brains. The causes of homosexuality or the reason for its emergence in the human population remains unknown, although there are theories that have attempted to provide explanations, including one that comes from an evolutionary perspective. Nevertheless, the results of the studies are not conclusive but rather suggestive.
FURTHER READINGS AND REFERENCES
- Blanchard, R. 1997. “Birth Order and Sibling Sex Ratio in Homosexual Versus Heterosexual Males and Females.” Annual Review of Sex Research. 8: 27-67. PMID: 10051890
- Blanchard, R. 2001. “Fraternal Birth Order and the Maternal Immune Hypothesis of Male Homosexuality.” Hormones and Behavior. 40(2): 105-114. DOI: 1006/hbeh.2001.1681
- Asymmetry and Functional Connectivity Between Homosexual and Heterosexual Subjects.” Proceedings of the National Academy of Sciences. 105(27): 9403-9408. DOI: 1073/pnas.0801566105
- Swaab, D. F. 2008. “Sexual Orientation and Its Basis in Brain Structure and Function.” Proceedings of the National Academy of Sciences. 105(3): 10273-10274. DOI: 1073/pnas.0805542105
- Witelson, S. F., Kigar, D. L., Scamvougeras, A., Kideckel, D. M., Buck, B., Stanchev, P. L., Bronskill, M., and Black, S. 2007. “Corpus Callosum Anatomy in Right-Handed Homosexual and Heterosexual Men.” Archives of Sexual Behavior. 37(6): 857-863. DOI: 1007/s10508-007-9276-y
