From an evolutionary point of view, homosexuality is rather difficult to explain. Any genetic trait that reduces the chances of producing the offspring usually gets eliminated very quickly in populations. Nonetheless, homosexuality appears to persist in humans throughout the history of our species. There are multiple hypotheses attempting to explain this phenomenon but no hard proofs confirming any of them.

Many scientists believe that sexual orientation is determined by peculiarities in the structure of the brain. Although at present time we cannot say what makes people gay, we can study their brain to see how their sexual orientation is reflected in the way it functions.

The Masculine and the Feminine Brain

Gender identity, which is the feeling of being like a man or a woman, influences various aspects of human behavior. These include choice of toys as a child and gender-specific cognitive, motor, and personality characteristics. Gender identity also has a role to play in determining sexuality. And all these developments are triggered by testosterone. Male and female fetuses vary in the level of this sex hormone.

According to one study, the human brain can show “masculine” or “feminine” traits, irrespective of physical sexual characteristics. When the fetus develops, gender identity and the sexual differentiation of the genitals may develop independently of one another. The former takes place during the second half of pregnancy while the latter starts much earlier, within the first 8 weeks of gestation. Incidentally, incongruent development in these two regions usually leads to transsexuality.

Sexual Orientation is in the Brain

Several other studies indicate that sexual orientation — heterosexuality, bisexuality, and homosexuality — is determined by peculiarities of the brain structure and differences in brain chemistry. Cultural or societal factors, upbringing, moral leanings, and educational attainments do not determine sexual orientation as greatly as neural mechanisms do. In fact, scientists have identified several areas of the brain that they believe determine sexuality in an individual. These areas include the hypothalamus and the amygdala. Inter-hemispheric neural connectivity has also been found to contribute significantly to sexual orientation.

A landmark study by Savic and Lindström indicates that there are cerebral differences in homosexual and heterosexual individuals. There are differences in brain anatomy, activities, and neurological connections. Brain scan images of the subjects who participated in this study show that the brains of homosexual individuals exhibit similar structure and functionality as that of heterosexual individuals of the opposite gender.

According to the study, lesbians and straight men have similar brain structures and functionalities while gay men and straight women share neural characteristics. For instance, MRI findings prove that the right hemispheres of the brains of the lesbians and heterosexual men have slightly greater volumes than their left hemispheres. But the left and right hemispheres of gay men and heterosexual women are symmetrical.

Another study on the brains of male-to-female (MTF) transsexual individuals, men, and women found that the volumes of gray matter across the different regions of the brains of MTF transsexuals are more or less similar to that in the brains of the men. However, the MTF transsexuals exhibited slightly greater volume of gray matter in the right putamen region of their brains compared to the men in the study. MTF transsexual individuals may have the same physical sexual characteristics as men, but they report feeling like women. This study bolsters the claims of the Savic-Lindström study that brain structure and functionality play a significant role in determining gender identity.

According to the findings of the Savic-Lindström study, the number of neural connections also varied between hetero- and homosexual subjects. For instance, gay men and straight women showed greater neural connectivity in the cingulate cortex and contralateral amygdala regions than straight men and lesbians respectively. On the other hand, straight men and lesbian women exhibited significantly more neural connections in the frontal lobe cortex and the parietal cortex regions compared to gay men and straight women respectively.

Incidentally, the Savic-Lindström study is the first to establish that differences in neural connections can influence sexual orientation. The objective of the study was also to establish that sexual orientation is largely determined by factors that are congenital.

Neurological Underpinnings of Sexual Attraction

Neurological differences determine human behavior to a large extent. So it is likely that the neural differences in hetero- and homosexual individuals influence their sexual preferences as well. Hetero- and homosexual women have different responses to specific odors, particularly those emitted by 4,16-androstadien-3-one (AND), which is a testosterone-derivative that is found primarily in male sweat, and estra-1,3,5(10),16-tetraen-3-ol (EST), which is an estrogen-like substance that is found in the urine of women. These substances are believed to be similar in nature and function to pheromones that are emitted by individual members of a species to elicit specific responses, sexual in many cases, from members of the same species.

The above study was conducted on straight men, gay men, and straight women. It was found that homosexual men and heterosexual women displayed similar brain activation patterns when they inhaled the AND chemical. The hypothalamic area of their brains were activated when they inhaled AND. In contrast, the straight women exhibited activity only in the olfactory region of their brains when they were exposed to the EST chemical. The hypothalamus is known to be active in processing sexual signals while the olfactory region only processes smells. During the same study, the subjects were exposed to commonly-occurring smells, and it was found that their brains had similar responses.

So, are people born gay or lesbian? Scientists hesitate to provide a conclusive answer.

A slew of studies indicate that neurological factors greatly influence sexual orientation. The functionalities of regions in the brain like the amygdala and the hypothalamus have been proven to be determined genetically and are influenced by hormones. Developments in these regions kick in even before an individual learns cognitive skills or is exposed to environmental and educational settings. But scientists still do not negate the role of environmental factors.

References

Bao, A., & Swaab, D. (2011). Sexual differentiation of the human brain: Relation to gender identity, sexual orientation and neuropsychiatric disorders Frontiers in Neuroendocrinology, 32 (2), 214-226 DOI: 10.1016/j.yfrne.2011.02.007

Hines, M. (2010). Sex-related variation in human behavior and the brain Trends in Cognitive Sciences, 14 (10), 448-456 DOI: 10.1016/j.tics.2010.07.005

Luders, E., Sánchez, F., Gaser, C., Toga, A., Narr, K., Hamilton, L., & Vilain, E. (2009). Regional gray matter variation in male-to-female transsexualism NeuroImage, 46 (4), 904-907 DOI: 10.1016/j.neuroimage.2009.03.048

Nugent, B., Wright, C., Shetty, A., Hodes, G., Lenz, K., Mahurkar, A., Russo, S., Devine, S., & McCarthy, M. (2015). Brain feminization requires active repression of masculinization via DNA methylation Nature Neuroscience, 18 (5), 690-697 DOI: 10.1038/nn.3988

Savic, I., Berglund, H., & Lindstrom, P. (2005). Brain response to putative pheromones in homosexual men Proceedings of the National Academy of Sciences, 102 (20), 7356-7361 DOI: 10.1073/pnas.0407998102

Savic I, & Lindström P (2008). PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. Proceedings of the National Academy of Sciences of the United States of America, 105 (27), 9403-8 PMID: 18559854

Swaab, D. (2007). Sexual differentiation of the brain and behavior Best Practice & Research Clinical Endocrinology & Metabolism, 21 (3), 431-444 DOI: 10.1016/j.beem.2007.04.003

Swaab, D. (2008). Sexual orientation and its basis in brain structure and function Proceedings of the National Academy of Sciences, 105 (30), 10273-10274 DOI: 10.1073/pnas.0805542105

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