The INAH 3 is the third interstitial nucleus of the anterior hypothalamus. The INAH has been implicated in sexual behavior because of known sexual dimorphism in this area in humans and because it corresponds to an area of the hypothalamus that when lesioned, impairs heterosexual behavior in non-human primates without affecting sex drive. It has been reported to be smaller on average in homosexual men than in heterosexual men, and in fact has approximately the same size as INAH 3 in heterosexual women.
In 1991, LeVay showed that part of the sexually dimorphic nucleus (SDN), the interstitial nuclei of the anterior hypothalamus (INAH) 3, is twice as large in heterosexual men as and homosexual women, in terms of volume but not number of neurons.
SDN and its homologues widely exist in human, mammal and some other animal brains, including the third interstitial nucleus of the anterior hypothalamus (INAH3) in humans, ovine sexually dimorphic nucleus (oSDN) in the medial preoptic area/anterior hypothalamus (MPOA/AH) in sheep, sexually dimorphic nucleus in the preoptic area (SDN-POA) in rats, anterior hypothalamic nucleus (AHdc) in macaques, specific area in medial preoptic nucleus (POM) in quails, etc..
In 1991, LeVay  published a study of 41 autopsies on 16 heterosexual men (HeM), 19 homosexual men (HoM), and 6 heterosexual women (HeW) that targeted the interstitial nuclei of the anterior hypothalamus (INAH) 1, 2, 3, and 4. He showed that there were no significant differences between the three group's INAH 1, 2, and 4, which prior research already showed. However, INAH 3 was 1.7 as large and had twice the neurons in HeM as HoM and HeW. This research linked an established sexually dimorphic area of the brain to hetero- and homosexual behavior in men.
From Causes of transsexualism
In 2008, a new region with properties similar to that of BSTc in regards to transsexualism was found by Garcia-Falgueras and Swaab: the interstitial nucleus of the anterior hypothalamus (INAH3), part of the hypothalamic uncinate nucleus. The same method of controlling for hormone usage was used as in Zhou et al (1995) and Kruijver et al (2000). The differences were even more pronounced than with BSTc; control males averaged 1.9 times the volume and 2.3 times the neurons as control females, yet once again, regardless of hormone exposure, MTF transsexuals lay within the female range and the FTM transsexual within the male range.
While MRI images cannot resolve as fine details as structures such as BSTc and INAH3, they can much more easily allow the study of larger brain structures. In Luders et al (2009), 24 MTF transsexuals not-yet treated with cross-sex hormones were studied via MRI. While regional gray matter concentrations were more similar to men than women, there was a significantly larger volume of gray matter in the right putamen compared to men. As with many earlier studies, they concluded that transsexualism is associated with a distinct cerebral pattern.
An additional feature was studied in a group of FTM transsexuals who had not yet received cross-sex hormones: fractional anisotropy values for white matter in the medial and posterior parts of the right superior longitudinal fasciculus (SLF), the forceps minor, and the corticospinal tract. Rametti et al (2010) discovered that, "Compared to control females, FtM showed higher FA values in posterior part of the right SLF, the forceps minor and corticospinal tract. Compared to control males, FtM showed only lower FA values in the corticospinal tract."
A cautionary note is sounded in Hulshoff Pol et al (2006), which studied the changes in transsexual brains on hormone administration over four months via MRI. They discovered that whole brain volume for subjects, as well as hypothalamic volume for androgen treatment, and potentially more sexually dimorphic structures in that region, change significantly toward the size of the opposite gender during hormone treatment. The study does not criticize the controls used to account for this in the BSTc or INAH3 studies. The study additionally concluded that before hormonal treatment, total brain volumes of the subjects were equivalent according to birth sex.
From Simon LeVay
In 1991, LeVay published "A difference in hypothalamic structure between heterosexual and homosexual men" in Science. This article reported a difference in average size between the third Interstitial Nucleus of the Anterior Hypothalamus (INAH3) in the brains of heterosexual men and homosexual men: INAH3 was more than twice as large in heterosexual men as in homosexual men. The INAH3 size of homosexual men was the same as that of women. LeVay wrote that "This finding indicates that INAH is dimorphic with sexual orientation, at least in men, and suggests that sexual orientation has a biological substrate." LeVay added, "The existence of 'exceptions' in the present sample (that is, presumed heterosexual men with small INAH 3 nuclei, and homosexual men with large ones), hints at the possibility that sexual orientation, although an important variable, may not be the sole determinant of INAH 3 size. It is also possible, however, that these exceptions are due to technical shortcomings or to misassignment of subjects to their subject groups."
LeVay's finding was widely reported in the media. LeVay cautioned against misinterpreting his findings in a 1994 interview: "It’s important to stress what I didn’t find. I did not prove that homosexuality is genetic, or find a genetic cause for being gay. I didn’t show that gay men are born that way, the most common mistake people make in interpreting my work. Nor did I locate a gay center in the brain. The INAH3 is less likely to be the sole gay nucleus of the brain than a part of a chain of nuclei engaged in men and women's sexual behavior." Some critics of LeVay questioned the accuracy and appropriateness of his measurements, saying that the structures are difficult to see in tissue slices and that he measured in volume rather than cell count. Nancy Ordover wrote in her 2003 book American Eugenics that LeVay has been criticized for "his small sample size and for compiling inadequate sexual histories."
- "Central Nervous System Dimorphisms Related to Reproductive Behaviors" Dale Purves ed., Neuroscience, 2:ed (2001) Online http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=neurosci.section.2127
- LeVay S 1991 A difference in hypothalamic structure between heterosexual and homosexual men. Science 253: 1034–7
- Balthazart J, Ball G (2007). "Topography in the preoptic region: Differential regulation of appetitive and consummatory male sexual behaviors". Frontiers in Neuroendocrinology. 28 (4): 161–178. doi:10.1016/j.yfrne.2007.05.003. PMC . PMID 17624413.
- Vasey P, Pfaus J (2005). "A sexually dimorphic hypothalamic nucleus in a macaque species with frequent female=female mounting and same-sex sexual partner preference". Behavioural Brain Research. 157 (2): 265–272. doi:10.1016/j.bbr.2004.07.005. PMID 15639177.
- LeVay S (1991). "A difference in hypothalamic structure between heterosexual and homosexual men". Science. 253 (5023): 1034–1037. doi:10.1126/science.1887219. PMID 1887219.
- Garcia-Falgueras, A.; Swaab, D. F. (2008). "A sex difference in the hypothalamic uncinate nucleus: relationship to gender identity". Brain. 131 (Pt 12): 3132–46. doi:10.1093/brain/awn276. PMID 18980961.
- Luders, Eileen; Sánchez, Francisco J.; Gaser, Christian; Toga, Arthur W.; Narr, Katherine L.; Hamilton, Liberty S.; Vilain, Eric (2009). "Regional gray matter variation in male-to-female transsexualism". NeuroImage. 46 (4): 904–7. doi:10.1016/j.neuroimage.2009.03.048. PMC . PMID 19341803.
- Rametti, G; Carrillo, B; Gómez-Gil, E; Junque, C; Segovia, S; Gomez, A; Guillamon, A (2010). "White matter microstructure in female to male transsexuals before cross-sex hormonal treatment. A diffusion tensor imaging study". Journal of psychiatric research. 45 (2): 199–204. doi:10.1016/j.jpsychires.2010.05.006. PMID 20562024.
- Pol, H. E H.; Cohen-Kettenis, P. T; Van Haren, N. E M; Peper, J. S; Brans, R. G H; Cahn, W.; Schnack, H. G; Gooren, L. J G; Kahn, R. S (2006). "Changing your sex changes your brain: influences of testosterone and estrogen on adult human brain structure". European Journal of Endocrinology. 155: S107–S114. doi:10.1530/eje.1.02248.
- LeVay S (1991). A difference in hypothalamic structure between homosexual and heterosexual men. Science, 253, 1034-1037.
- Angier, Natalie (August 30, 1991). Zone of Brain Linked to Men's Sexual Orientation. New York Times
- Barinaga, Marcia (August 30, 1991). Is homosexuality biological? Science
- Ordover, Nancy (2003). American Eugenics: Race, Queer Anatomy, and the Science of Nationalism. University of Minnesota Press, ISBN 0-8166-3559-5