Neuroscience and sexual orientation

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Sexual orientation is an enduring pattern of romantic or sexual attraction (or a combination of these) to persons of the opposite sex or gender, the same sex or gender, or to both sexes or more than one gender, or none of the aforementioned at all. [1] [2] The ultimate causes and mechanisms of sexual orientation development in humans remain unclear and many theories are speculative and controversial. However, advances in neuroscience explain and illustrate characteristics linked to sexual orientation. Studies have explored structural neural-correlates, functional and/or cognitive relationships, and developmental theories relating to sexual orientation in humans.

Contents

Developmental neurobiology

Many theories concerning the development of sexual orientation involve fetal neural development, with proposed models illustrating prenatal hormone exposure, maternal immunity, and developmental instability. Other proposed factors include genetic control of sexual orientation. No conclusive evidence has been shown that environmental or learned effects are responsible for the development of non-heterosexual orientation. [3]

As of 2005, sexual dimorphisms in the brain and behavior among vertebrates were accounted for by the influence of gonadal steroidal androgens as demonstrated in animal models over the prior few decades. The prenatal androgen model of homosexuality describes the neuro-developmental effects of fetal exposure to these hormones. [3] In 1985, Geschwind and Galaburda proposed that homosexual men are exposed to high androgen levels early in development and proposed that temporal and local variations in androgen exposure to a fetus's developing brain is a factor in the pathways determining homosexuality. [3] This led scientists to look for somatic markers for prenatal hormonal exposure that could be easily, and non-invasively, explored in otherwise endocrinologically normal populations. Various somatic markers (including 2D:4D finger ratios, auditory evoked potentials, fingerprint patterns and eye-blink patterns) have since been found to show variation based on sexual orientation in healthy adult individuals. [3]

Other evidence supporting the role of testosterone and prenatal hormones in sexual orientation development include observations of male subjects with cloacal exstrophy who were sex-assigned as female during birth only later to declare themselves male. This supports the theory that the prenatal testosterone surge is crucial for gender identity development. Additionally, females whose mothers were exposed to diethylstilbestrol (DES) during pregnancy show higher rates of bi- and homosexuality. [4]

Variations in the hypothalamus may have some influence on sexual orientation. Studies show that factors such as cell number and size of various nuclei in the hypothalamus may impact one's sexual orientation. [5]

Brain structure

There are multiple areas of the brain which have been found to display differences based on sexual orientation. Several of these can be found in the hypothalamus, including the sexually dimorphic nucleus of the preoptic area (SDN-POA) present in several mammalian species. Researchers have shown that the SDN-POA aides in sex-dimorphic mating behavior in some mammals, which is representative of human sexual orientation. [6] The human equivalent to the SDN-POA is the interstitial nucleus of the anterior hypothalamus, which is also sexually dimorphic and has demonstrated dissimilar sizes between sexualities. [6] [7] There are also other POA-like brain structures in the human brain which differ between sexual orientations, such as the suprachiasmatic nucleus and the anterior hypothalamus. [6] Using meta-analysis of neuroimaging, researchers have concluded that these areas are linked to sexual preferences in humans, which would explain why they may differ based on sexual orientation. [7]

Another area of the brain which demonstrates sexual orientation differentiation is the thalamus, which is a structure involved in sexual arousal and reward. The thalamus of heterosexual individuals was found to be bigger than that of homosexual individuals. [8] The placement of connections in the amygdala have been demonstrated to differ between heterosexual and homosexual individuals. [9] The posterior cingulate cortex, a part of the occipital lobe, the region of the brain that processes visual information, has also been demonstrated to have differences based on sexual orientation. [9]

Research has shown that a couple of the areas of connection between the hemispheres of the brain have differences in their size depending on sexual orientation. The front commission was found to be wider in homosexual men than heterosexual men, and the corpus callosum was found to be larger in homosexual men than heterosexual men. [9]

Some areas of the brain which researchers looked at but did not find differences in structure between sexualities are the temporal cortex, hippocampus and putamen. [9]

Fraternal birth order effect

Neuroscience has been implicated in the study of birth order and male sexual orientation. A significant volume of research has found that the more older brothers a man has from the same mother, the greater the probability he will have a homosexual orientation. Estimates indicate that there is a 33–48% increase in chances of homosexuality in a male child with each older brother, and the effect is not observed in those with older adoptive or step-brothers, indicative of a prenatal biological mechanism. [3] Ray Blanchard and Anthony Bogaert discovered the association in the 1990s, and named it the fraternal birth order (FBO) effect. The mechanism by which the effect is believed to operate states that a mother develops an immune response against a substance important in male fetal development during pregnancy, and that this immune effect becomes increasingly likely with each male fetus gestated by the mother. This immune effect is thought to cause an alteration in (some) later born males' prenatal brain development. The target of the immune response are molecules (specifically Y-linked proteins, which are thought to play a role in fetal brain sex-differentiation) on the surface of male fetal brain cells, including in sites of the anterior hypothalamus (which has been linked to sexual orientation in other research). Antibodies produced during the immune response are thought to cross the placental barrier and enter the fetal compartment where they bind to the Y-linked molecules and thus alter their role in sexual differentiation, leading some males to be attracted to men as opposed to women. Biochemical evidence to support this hypothesis was identified in 2017, finding mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than other samples of women, including mothers of heterosexual sons. [10] [11]

The effect does not mean that all or most sons will be gay after several male pregnancies, but rather, the odds of having a gay son increase from approximately 2% for the firstborn son, to 4% for the second, 6% for the third and so on. [10] [12] Scientists have estimated that 15–29% of gay men owe their sexual orientation to this effect, but the number may be higher, as prior miscarriages and terminations of male pregnancies may have exposed their mothers to Y-linked antigens. In addition, the effect is nullified in left-handed men. As it is contingent on handedness and handedness is a prenatally determined trait, it further attributes the effect to be biological, rather than psychosocial. [13] The fraternal birth order effect does not apply to the development of female homosexuality. [13] Blanchard does not believe the same antibody response would cause homosexuality in firstborn gay sons – instead, they may owe their orientation to genes, prenatal hormones and other maternal immune responses which also influence fetal brain development. [11]

The few studies which have not observed a correlation between gay men and birth order have generally been criticized for methodological errors and sampling methods. [14] J. Michael Bailey has said that no plausible hypothesis other than a maternal immune response has been identified. [14]

Research directions

As of 2005, research directions included: [3]

See also

Related Research Articles

<span class="mw-page-title-main">Heterosexuality</span> Attraction between people of the opposite sex or gender

Heterosexuality is romantic attraction, sexual attraction or sexual behavior between people of the opposite sex or gender. As a sexual orientation, heterosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" to people of the opposite sex; it "also refers to a person's sense of identity based on those attractions, related behaviors, and membership in a community of others who share those attractions." Someone who is heterosexual is commonly referred to as straight.

<span class="mw-page-title-main">Sexual orientation</span> Pattern of romantic or sexual attraction

Sexual orientation is an enduring personal pattern of romantic attraction or sexual attraction to persons of the opposite sex or gender, the same sex or gender, or to both sexes or more than one gender. Patterns are generally categorized under heterosexuality, homosexuality, and bisexuality, while asexuality is sometimes identified as the fourth category.

<span class="mw-page-title-main">Biology and sexual orientation</span> Field of sexual orientation research

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Birth order refers to the order a child is born in their family; first-born and second-born are examples. Birth order is often believed to have a profound and lasting effect on psychological development. This assertion has been repeatedly challenged. Recent research has consistently found that earlier born children score slightly higher on average on measures of intelligence, but has found zero, or almost zero, robust effect of birth order on personality. Nevertheless, the notion that birth-order significantly influences personality continues to have a strong presence in pop psychology and popular culture.

<span class="mw-page-title-main">Fraternal birth order and male sexual orientation</span> Theory of sexual orientation

Fraternal birth order, also known as the older brother effect, has been correlated with male sexual orientation, with a significant volume of research finding that the more older brothers a male has from the same mother, the greater the probability he will have a homosexual orientation. Ray Blanchard and Anthony Bogaert first identified the association in the 1990s and named it the fraternal birth order effect. Scientists have attributed the effect to a prenatal biological mechanism, since the association is only present in men with older biological brothers, and not present among men with older step-brothers and adoptive brothers. The mechanism is thought to be a maternal immune response to male fetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development. This would leave some regions of the brain associated with sexual orientation in the 'female typical' arrangement – or attracted to men. Biochemical evidence for this hypothesis was identified in 2017, finding mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons.

The sexually dimorphic nucleus (SDN) is an ovoid, densely packed cluster of large cells located in the medial preoptic area (POA) of the hypothalamus which is believed to be related to sexual behavior in animals. Thus far, for all species of mammals investigated, the SDN has been repeatedly found to be considerably larger in males than in females. In humans, the volume of the SDN has been found to be 2.2 times as large in males as in females and to contain 2.1 times as many cells. The human SDN is elongated in females and more spherical in males. No sex differences have been observed in the human SDN in either cell density or mean diameter of the cell nuclei. The volume and cell number of the human SDN considerably decreases with age, although the decrease in cell number is both sex and age-specific. In males, a substantial decrease in the cell number of the human SDN was observed between the age of 50–60 years. Cell death was more common in females than males, especially among those older than 70 years of age. The SDN cell number in females can drop to 10-15% of that found in early childhood.

<span class="mw-page-title-main">Androphilia and gynephilia</span> Sexual orientation to men or women

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References

  1. ↑ "Sexual orientation, homosexuality and bisexuality". American Psychological Association. Archived from the original on August 8, 2013. Retrieved August 10, 2013.
  2. ↑ "Sexual Orientation". American Psychiatric Association. Archived from the original on July 22, 2011. Retrieved January 1, 2013.
  3. 1 2 3 4 5 6 Rahman, Q (2005). "The neurodevelopment of human sexual orientation". Neuroscience & Biobehavioral Reviews. 29 (7): 1057–66. doi:10.1016/j.neubiorev.2005.03.002. PMID   16143171. S2CID   15481010.
  4. ↑ Swaab DF (December 2004). "Sexual differentiation of the human brain: relevance for gender identity, transsexualism and sexual orientation". Gynecological Endocrinology. 19 (6): 301–12. doi:10.1080/09513590400018231. PMID   15724806. S2CID   1410435.
  5. ↑ Swaab, DF, Gooren LJ, Hofman, MA (October 2010). "Brain research, gender and sexual orientation,Pub Med.
  6. 1 2 3 Bogaert, Anthony F.; Skorska, Malvina N. (March 2020). "A short review of biological research on the development of sexual orientation". Hormones and Behavior. 119: 104659. doi: 10.1016/j.yhbeh.2019.104659 . PMID   31911036. S2CID   210055292.
  7. 1 2 Lau, Holning (2018-08-01), "Sexual Orientation and Gender Identity Discrimination", BRILL, pp. 1–52, doi: 10.1163/9789004345492_002 , ISBN   9789004345492, S2CID   240352057 {{citation}}: Missing or empty |title= (help)
  8. ↑ Votinov, Mikhail; Goerlich, Katharina S.; Puiu, Andrei A.; Smith, Elke; Nickl-Jockschat, Thomas; Derntl, Birgit; Habel, Ute (2021). "Brain structure changes associated with sexual orientation". Scientific Reports. 11 (1): 5078. Bibcode:2021NatSR..11.5078V. doi:10.1038/s41598-021-84496-z. PMC   7930173 . PMID   33658542.
  9. 1 2 3 4 Frigerio, Alberto; Ballerini, Lucia; Valdés Hernández, Maria (2021-05-06). "Structural, Functional, and Metabolic Brain Differences as a Function of Gender Identity or Sexual Orientation: A Systematic Review of the Human Neuroimaging Literature". Archives of Sexual Behavior. 50 (8): 3329–3352. doi:10.1007/s10508-021-02005-9. ISSN   0004-0002. PMC   8604863 . PMID   33956296.
  10. 1 2 Balthazart, Jacques (2018-01-09). "Fraternal birth order effect on sexual orientation explained". Proceedings of the National Academy of Sciences of the United States of America. 115 (2): 234–236. Bibcode:2018PNAS..115..234B. doi: 10.1073/pnas.1719534115 . ISSN   0027-8424. PMC   5777082 . PMID   29259109.
  11. 1 2 Bogaert, Anthony F.; Skorska, Malvina N.; Wang, Chao; Gabrie, José; MacNeil, Adam J.; Hoffarth, Mark R.; VanderLaan, Doug P.; Zucker, Kenneth J.; Blanchard, Ray (2018-01-09). "Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y". Proceedings of the National Academy of Sciences of the United States of America. 115 (2): 302–306. Bibcode:2018PNAS..115..302B. doi: 10.1073/pnas.1705895114 . ISSN   0027-8424. PMC   5777026 . PMID   29229842.
  12. ↑ Blanchard R (1997). "Birth order and sibling sex ratio in homosexual versus heterosexual males and females". Annual Review of Sex Research. 8: 27–67. PMID   10051890.
  13. 1 2 Bogaert AF; Skorska M (2011). "Sexual orientation, fraternal birth order, and the maternal immune hypothesis: a review". Front Neuroendocrinol. 32 (2): 247–54. doi:10.1016/j.yfrne.2011.02.004. PMID   21315103. S2CID   45446175.
  14. 1 2 Bailey, J. Michael (2018-01-01). "The Fraternal Birth Order Effect Is Robust and Important". Archives of Sexual Behavior. 47 (1): 18. doi:10.1007/s10508-017-1115-1. ISSN   1573-2800. PMID   29159754. S2CID   35597467.
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