It took me awhile to find this info in more recent articles.
"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."
"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.
"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 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."
http://brainblogger.com/2015/05/14/homosexuality-in-the-brain/
This was interesting.
"Given the results regarding both INAH 3 and AC correlation to sexual orientation, the understanding that no single brain structure correlates to sexual orientation carries weight. The apparent interconnected nature of varying brain regions suggests that factors operating early in development differentiate on the basis of gender and sexual orientation within sexually dimorphic structures and brain function in a cumulative manner2. Exploring which specific factors influence which kind of development requires an endocrinologic approach alongside traditional neurobiological studies."
"By extension, this demonstration could also suggest that steroid hormones affect the human hypothalamus to influence sexual orientation. Studies investigating this effect would further emphasize the critical biology of sexual orientation and eliminate the social argument of “choosing” one’s sexuality."
http://www.bu.edu/thenerve/archives/spring-2010/reviews-spring-2010/homosexuality/
Some reading on the original research.
1. Morris, J. A., Gobrogge, K. L., Jordan, C. L. & Breedlove, S. M. 2004. Brain aromatase: dyed-in-the-wool homosexuality, Endocrinology 145:475–477.
2. Breedlove, SM. 1992. Sexually dimorphism in the vertebrate nervous system, The Journal of Neuroscience 12: 4133-4142.
3. Levay, S. 1991. A Difference in Hypothalamic Structure between Heterosexual and Homosexual Men, Science 253:1034-1037.
4. Allen LS, Gorski RA. 1992. Sexual orientation and size of the anterior commissure in the human brain, Proc Natl Acad Sci USA 89:7199–7202.
5. Roselli CE, Larkin K, Resko JA, Stellflug JN, Stormshak F. 2004. The volume of a sexually dimorphic nucleus in the ovine medial preoptic area/anterior hypothalamus varies with sexual partner preference. Endocrinology 145:478-483
6. James, W. H. 2005. Biological and psychological determinants of male and female human sexual orientation, Journal of Biosocial Science 37:555–567.
7. Savic, H. Berglund and P. Lindstrom. 2005. Brain response to putative pheromones in homosexual men, Proc. Natl. Acad. Sci. USA 102:7356–7361.
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