"There is a popular belief that people are born either gay or straight. Conservatives tend to believe that sexual orientation is actually sexual preference, which is chosen by the individual. This page represents a review of the scientific literature on the basis for homosexual orientation." -godandscience.org
Hypothalamus
Numerous studies of the brain have indicated particular regions of the Hypothalamus, called sexually-dimorphic nuclei, that are distinctly different in men and women. One in particular, the Interstitial Nuclei of the Anterior Hypothalamus (INAH) is known to be twice as large in men compared to women. Because the INAH influences sexual behavior in mammals, it is speculated that some parts of this nuclei may be atypically expressed in homosexual men, who are hypothesized to have an INAH volume more similar to women (McLaughlin 2018). In regards to the first of such experiments conducted by Simon LeVay in 1991, godandscience.org states,
"Postmortem examination of the brains of AIDS patients vs. control male subjects (presumed to be heterosexual) showed that the heterosexual men exhibited INAH3 that were twice the size of both females and presumably homosexual men who had died of AIDS (LeVay 1991). The study has been criticized for its uncertainty of sexual orientation of the subjects, and potential complications caused by the AIDS virus (HIV infects the human brain), and also by lowered testosterone levels found in AIDS patients.”
While this 1991 study did receive some criticism over experimental design, later follow-up studies attempted to addressed these confounding variables and confirm LeVay’s findings. One 2001 study measured INAH coronal sections for women, homosexual men, and heterosexual men like LeVay’s experiment, but uniquely recruited HIV+ and HIV- participants for each category (Bogaert et al. 2018).
The website claims that the result of this experiment was unsuccessful, stating that:
“IHAH3 volume was decreased in male heterosexual men who had contracted AIDS . . . suggesting that LeVays original finding was the result of HIV infection and not the result of sexual orientation."
However, the website was deliberately referencing the 0.015cc volume difference in heterosexual men that can be calculated from the publication’s data table, while ignoring the fact that this difference was determined to be statistically insignificant. In actuality, within-group analyses indicated that, contrary to the website’s claim, HIV status had no significant impact on INAH3. Furthermore, this experiment similarly replicated LeVay’s findings, and demonstrated that INAH3 tend to occupy a smaller volume in homosexual men (P, 0.10, two-tailed), suggesting a possible neural basis for same-sex attraction.(Bogaert et al. 2018).
In a similar, interesting experiment conducted in 2004, scientists tested this hypothesis by analyzing sexually-dimorphic nuclei in sheep, an animal in which ~8% of males are exclusively homosexual (androphilic). Scientists concluded that, just as in humans, the sexually-dimorphic nuclei of androphilic rams were significantly smaller, and more similar to that of females, than gynephilic rams (Roselli 2018).
Between LeVay’s findings, replicated results by Byne, and identical findings in another mammal species, scientific literature is much more supportive of a hypothalamic basis for sexual orientation than the website suggests.
However, an additional claim was made in this article that the differences observed in nuclei of homosexual men may be the result of and not the cause of their sexual orientation;
“A study that examined the sexually dimorphic nucleus of the hypothalamus showed a decrease in volume and cell number in females at 2-4 years postnatal (Swaab, Gooren, and Hofman 1992), suggesting that not only chemical and hormonal factors, but also social factors, might have influenced this process, casting doubt on the validity of LeVays findings.”
The website implies from this data that homosexuality is not biologically-based because the development of hypothalamic nuclei can theoretically be subject to environmental/social influences. However, given that such nuclei tend to mature by 1-2 years in males, and sexual attraction/maturity does not even begin to develop until ~10 years of age, it is virtually impossible for this difference to be the direct result of homosexuality (Swaab et al. 1992).
Instead, if any environmental factors are at play, they are likely confounding variables that affect both nuclei differentiation and homosexuality. However, even if the nuclei differences observed in these experiments were purely due to post-natal environmental factors, the result would still be a neurological basis/indication for sexual orientation.
Furthermore, multiple experiments have studied the activity of sexually-dimorphic nuclei in response to smelling certain sexual pheromones. Consistently, homosexual men are found to process pheromones congruently with heterosexual women, and homosexual women process pheromones via an entirely different pathway than heterosexual women.
Altogether, these findings paint a fairly convincing picture of a possible hypothalamic basis for homosexuality (Bogaert 2018). While some studies of this sort were found to be inconclusive, contrary to the website’s claim, they by no means constituted the majority of research on this subject. The majority of publications found some evidence of hypothalamic indicators of homosexuality, and many of them confirmed the findings of other, previous experiments.
Anterior Commissure
The website additionally claims:
“A study by Allen and Gorski examined the anterior commissure of the brain, finding that females and homosexual males exhibited a larger size than heterosexual males (Allen and Gorski 1992). However, later studies using more subjects found no such differences (Bishop and Wahlsten 1997).
This claim, however, is blatantly wrong, as the 1997 study by Bishop and Wahlsten only measured differences between men and women, and made no mention of sexual orientation (Bishop et al 1997). Furthermore, an additional study in 2008 replicated Allen and Gorski’s findings, concluding that the isthmal area of the corpus callosum is larger in homosexual men than heterosexual men (Manzouri et al. 2019).
Sex Affects Brain Structure
"Scientists originally hypothesized that brain structure affected sexual behavior. However, studies have shown that sexual experiences themselves can affect brain structure (Breedlove 1997). So, regardless of the brain differences found between those who practice homosexual vs. heterosexual behavior, it can never be certain if the brain affects the behavior or if the behavior affects the brain."
While epigenetic effects in humans are well-studied and documented8, it is inaccurate to assume that all of the aforementioned neural bases for sexual orientation are merely a result of homosexuality and not the cause of it. Many, if not all, of these neural structures largely finish developing before sexual attraction even begins, and therefore cannot be differentiated later in life by the individual’s sexual orientation/behavior (Bogaert 2018).
Furthermore, despite what the website claims, the experiment referenced in this passage did not study brain structure. It instead analyzed the motor neurons in the bulbocavernosus muscle which control some aspects of physical sexual function (Breedlove 1997).
Not only did this study say nothing about sexual attraction or the cognitive factors that lead to sexual behavior, but it was conducted in castrated mice that were artificially induced into a state of “much lower than normal concentrations of circulating androgens” and little to no applicability towards the realization of sexual orientation in healthy adolescents.
1. McLaughlin, Matthew. "Is There a Gay Brain? The Problems with Scientific Research of Sexual Orientation." The Great Lakes Journal of Undergraduate History 6.1 (2018): 45-60.
2. Deem, Richard. “Genetics and Homosexuality: Are People Born Gay? The Biological Basis for Sexual Orientation by Richard Deem, MSc.” Are People Born Gay? The Biological and Genetic Basis for Homosexuality, Godandscience.org, 2017, www.godandscience.org/evolution/homosexuals_born_gay.html#frisch.
3. Bogaert, Anthony F., and Malvina N. Skorska. "A short review of biological research on the development of sexual orientation." Hormones and Behavior 119 (2020): 104659.
4. Roselli, Charles E. "Neurobiology of gender identity and sexual orientation." Journal of neuroendocrinology 30.7 (2018): e12562.
5. Swaab, D. F., L. J. Gooren, and M. A. Hofman. 1992. "Gender and Sexual Orientation in Relation to Hypothalamic Structures." Horm Res 38 Suppl 2: 51-61. https://www.ncbi.nlm.nih.gov/pubmed/1292983.
6. Bishop, K. M., and D. Wahlsten. 1997. "Sex Differences in the Human Corpus Callosum: Myth or Reality?" Neurosci Biobehav Rev 21, no. 5 (Sep): 581-601. https://www.ncbi.nlm.nih.gov/pubmed/9353793.
7. Manzouri, Amirhossein, and Ivanka Savic. "Multimodal MRI suggests that male homosexuality may be linked to cerebral midline structures." PloS one 13.10 (2018).
8. Krippner, Stanley, and Deirdre Barrett. "Transgenerational Trauma: The Role of Epigenetics." Journal of Mind & Behavior 40.1 (2019).
9. Breedlove, S. M. 1997. "Sex on the Brain." Nature 389, no. 6653 (Oct 23): 801. http://dx.doi.org/10.1038/39764.
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