Editing Biology and sexual orientation (section)

====Sexually dimorphic nuclei in the anterior hypothalamus====
LeVay also conducted some of these early researches. He studied four groups of [[neuron]]s in the [[hypothalamus]] called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the brain to study, because of evidence that it played a role in the regulation of [[sexual behaviour in animals]], and because INAH2 and INAH3 had previously been reported to differ in size between men and women.<ref name="LeVay_1991">{{cite journal | vauthors = LeVay S | title = A difference in hypothalamic structure between heterosexual and homosexual men | journal = Science | volume = 253 | issue = 5023 | pages = 1034–7 | date = August 1991 | pmid = 1887219 | doi = 10.1126/science.1887219 | s2cid = 1674111 | bibcode = 1991Sci...253.1034L }}</ref>

He obtained brains from 41 deceased hospital patients. The subjects were classified into three groups. The first group comprised 19 gay men who had died of [[AIDS]]-related illnesses. The second group comprised 16 men whose sexual orientation was unknown, but whom the researchers presumed to be heterosexual. Six of these men had died of AIDS-related illnesses. The third group was of six women whom the researchers presumed to be heterosexual. One of the women had died of an AIDS-related illness.<ref name="LeVay_1991"/>

The [[HIV-positive people]] in the presumably heterosexual patient groups were all identified from medical records as either [[intravenous drug abuse]]rs or recipients of [[blood transfusion]]s. Two of the men who identified as heterosexual specifically denied ever engaging in a homosexual sex act. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been primarily or exclusively heterosexual "on the basis of the numerical preponderance of heterosexual men in the population".<ref name="LeVay_1991"/>

LeVay found no evidence for a difference between the groups in the size of INAH1, INAH2 or INAH4. However, the INAH3 group appeared to be twice as big in the heterosexual male group as in the gay male group; the difference was highly significant, and remained significant when only the six AIDS patients were included in the heterosexual group. The size of INAH3 in the homosexual men's brains was comparable to the size of INAH3 in the heterosexual women's brains.{{Citation needed|date=March 2019}}

William Byne and colleagues attempted to identify the size differences reported in INAH 1–4 by replicating the experiment using brain sample from other subjects: 14 HIV-positive homosexual males, 34 presumed heterosexual males (10 HIV-positive), and 34 presumed heterosexual females (9 HIV-positive). The researchers found a significant difference in INAH3 size between heterosexual men and heterosexual women. The INAH3 size of the homosexual men was apparently smaller than that of the heterosexual men, and larger than that of the heterosexual women, though neither difference quite reached statistical significance.<ref name="Byne_2001">{{cite journal | vauthors = Byne W, Tobet S, Mattiace LA, Lasco MS, Kemether E, Edgar MA, Morgello S, Buchsbaum MS, Jones LB | display-authors = 6 | title = The interstitial nuclei of the human anterior hypothalamus: an investigation of variation with sex, sexual orientation, and HIV status | journal = Hormones and Behavior | volume = 40 | issue = 2 | pages = 86–92 | date = September 2001 | pmid = 11534967 | doi = 10.1006/hbeh.2001.1680 | s2cid = 3175414 }} {{dead link|date=March 2020 |bot=InternetArchiveBot |fix-attempted=yes}}</ref>

Byne and colleagues also weighed and counted numbers of neurons in INAH3 tests not carried out by LeVay. The results for INAH3 weight were similar to those for INAH3 size; that is, the INAH3 weight for the heterosexual male brains was significantly larger than for the heterosexual female brains, while the results for the gay male group were between those of the other two groups but not quite significantly different from either. The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation.<ref name="Byne_2001" />

LeVay has said that Byne replicated his work, but that he employed a two-tailed statistical analysis, which is typically reserved for when no previous findings had employed the difference. LeVay has said that "given that my study had already reported a INAH3 to be smaller in gay men, a one tailed approach would have been more appropriate, and it would have yielded a significant difference [between heterosexual and homosexual men]".<ref name="LeVay_2016" />{{rp|110}}

[[J. Michael Bailey]] has criticized LeVay's critics—describing the claim that the INAH-3 difference could be attributable to AIDS as "aggravating", since the "INAH-3 did not differ between the brains of straight men who died of AIDS and those who did not have the disease".<ref name="Bailey_2003">{{cite book |url=https://www.researchgate.net/publication/281747420 |title=The Man Who Would Be Queen |vauthors=Bailey J |date=2003-03-10 |publisher=Joseph Henry Press |isbn=978-0-309-08418-5 |format=PDF}}</ref>{{rp|120}} Bailey has further criticized the second objection that was raised, that being gay might have somehow caused the difference in INAH-3, and not vice-versa, saying "the problem with this idea is that the hypothalamus appears to develop early. Not a single expert I have ever asked about LeVay's study thought it was plausible that sexual behavior caused the INAH-3 differences."<ref name="Bailey_2003" />{{rp|120}}

The SCN of homosexual males has been demonstrated to be larger (both the volume and the number of neurons are twice as many as in heterosexual males). These areas of the hypothalamus have not yet been explored in homosexual females nor bisexual males nor females. Although the functional implications of such findings still have not been examined in detail, they cast serious doubt over the widely accepted Dörner hypothesis that homosexual males have a "female hypothalamus" and that the key mechanism of differentiating the "male brain from originally female brain" is the epigenetic influence of testosterone during prenatal development.<ref>{{cite journal | vauthors = Swaab DF, Gooren LJ, Hofman MA | title = Gender and sexual orientation in relation to hypothalamic structures | journal = Hormone Research | volume = 38 | issue = Suppl 2 | pages = 51–61 | year = 1992 | pmid = 1292983 | doi = 10.1159/000182597 | doi-broken-date = 1 November 2024 | url = https://pure.knaw.nl/portal/en/publications/gender-and-sexual-orientation-in-relation-to-hypothalamic-structures(7cb8b769-4329-407a-b0ee-13e011017f68).html | hdl-access = free | hdl = 20.500.11755/7cb8b769-4329-407a-b0ee-13e011017f68 | type = Submitted manuscript }}</ref>

A 2010 study by Garcia-Falgueras and Swaab stated that "the fetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed or organized into our brain structures when we are still in the womb. There is no indication that social environment after birth has an effect on gender identity or sexual orientation."<ref>{{Citation |last1=Garcia-Falgueras |first1=Alicia |title=Sexual Hormones and the Brain: An Essential Alliance for Sexual Identity and Sexual Orientation |date=2010 |journal=Endocrine Development |volume=17 |pages=22–35 |editor-last=Loche |editor-first=S. |url=https://karger.com/books/book/2680/chapter/5758416 |access-date=2024-06-13 |publisher=S. Karger AG |language=en |doi=10.1159/000262525 |isbn=978-3-8055-9302-1 |last2=Swaab |first2=Dick F. |pmid=19955753 |editor2-last=Cappa |editor2-first=M. |editor3-last=Ghizzoni |editor3-first=L. |editor4-last=Maghnie |editor4-first=M. |url-access=subscription}}</ref>