Back ترتيب الولادة بين الأخوة والتوجه الجنسي للذكور Arabic ভাতৃবৎ জন্মৰ ক্ৰম আৰু পুৰুষৰ যৌন অভিমুখিতা Assamese Efeutu del orde de nacencia de los hermanos AST জন্মসূত্রে ভ্রাতৃসম্পর্ক এবং পুরুষ যৌন অভিমুখিতা Bengali/Bangla Older Brother Effect German Efecto del orden de nacimiento de los hermanos Spanish Efekt braterskiej kolejności urodzeń Polish Ordinea nașterii fraților și orientarea sexuală la bărbați Romanian Братский порядок рождения и мужская сексуальная ориентация Russian Братски ред рођења и мушка сексуална оријентација Serbian
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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.
The effect becomes stronger with each additional male pregnancy, with odds of the next son being homosexual increasing by 38–48%. This does not mean that all or most sons will be homosexual after several male pregnancies, but rather, the odds of having a homosexual son increase from approximately 2% for the first born son, to 3% for the second, 5% for the third and so on.[1][2] In a 2017 study; mothers with gay sons, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein compared to mothers with heterosexual sons.[1][3]
It has generally been thought that this maternal response would not apply to first-born gay sons and that they may owe their orientation to other mechanisms.[3] However, mothers with no sons also showed antibodies to male cells, which may be a result of male miscarriages or abortion.[3][4]
- ^ a b 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.
- ^ Blanchard R (1997). "Birth order and sibling sex ratio in homosexual versus heterosexual males and females". Annual Review of Sex Research. 8: 27–67. doi:10.1080/10532528.1997.10559918. PMID 10051890.
- ^ a b c 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.
- ^ Coghlan, Andy (December 2017). "We may know why younger brothers are more likely to be gay". New Scientist. Retrieved 2020-08-03.