Editing Sexual orientation (section)

==== Birth order ====

{{Main|Fraternal birth order and sexual orientation}}

A significant volume of research has demonstrated that the probability of a male growing up to be gay increases with each older brother he has from the same mother. Known as the ''fraternal birth order'' (FBO) effect, scientists attribute this to a [[Prenatal development|prenatal]] biological mechanism – specifically a maternal immune response to male fetuses – since the effect is only present in men with older biological brothers, and not present among men with older step-brothers and adoptive brothers. This process, known as the ''maternal immunization hypothesis'' (MIH), would begin when cells from a male fetus enter the mother's circulation during pregnancy. These cells carry Y-proteins, which are thought to play a role in brain masculinisation (sex-differentiation) during fetal development. The mothers immune system builds antibodies to these Y-proteins. These antibodies are later released on future male fetuses and interfere with the masculinization role of Y-proteins, leaving regions of the brain responsible for sexual orientation in the 'default' female-typical arrangement, causing the exposed son to be more attracted to men over women. Biochemical evidence for this hypothesis was identified in 2017, finding that mothers with a gay son, especially those with older brothers, had significantly higher levels of anti-bodies to the NLGN4Y Y-protein than mothers with heterosexual sons.<ref name=":22">{{Cite journal|last=Balthazart|first=Jacques|date=2018-01-09|title=Fraternal birth order effect on sexual orientation explained|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=115|issue=2|pages=234–236|doi=10.1073/pnas.1719534115|issn=0027-8424|pmc=5777082|pmid=29259109|doi-access=free}}</ref><ref name=":02">{{Cite journal|last1=Bogaert|first1=Anthony F.|last2=Skorska|first2=Malvina N.|last3=Wang|first3=Chao|last4=Gabrie|first4=José|last5=MacNeil|first5=Adam J.|last6=Hoffarth|first6=Mark R.|last7=VanderLaan|first7=Doug P.|last8=Zucker|first8=Kenneth J.|last9=Blanchard|first9=Ray|date=2018-01-09|title=Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=115|issue=2|pages=302–306|doi=10.1073/pnas.1705895114|issn=0027-8424|pmc=5777026|pmid=29229842|doi-access=free}}</ref>

The effect becomes stronger with each successive male pregnancy, meaning the odds of the next son being gay increase by 38–48%. This 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 first born son, to 4% for the second, 6% for the third and so on.<ref name=":22" /><ref name="Blanchard199722">{{cite journal|author=Blanchard R|year=1997|title=Birth order and sibling sex ratio in homosexual versus heterosexual males and females|journal=Annual Review of Sex Research|volume=8|pages=27–67|pmid=10051890}}</ref> Scientists have estimated between 15% and 29% of gay men may 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. The fraternal birth order effect would not likely apply to first born gay sons; instead, scientists say they may owe their orientation to genes, prenatal hormones and other maternal immune responses which also influence brain development.<ref name=":02" /> This effect is nullified if the man is left-handed.<ref>{{Cite journal|last1=Blanchard|first1=R.|last2=Cantor|first2=J.M.|last3=Bogaert|first3=A.F.|last4=Breedlove|first4=S.M.|last5=Ellis|first5=L.|year=2006|title=Interaction of fraternal birth order and handedness in the development of male homosexuality|url=http://msu.edu/~breedsm/pdf/Blanchard2006IntFratHandHBFinal.pdf|journal=Hormones and Behavior|volume=49|issue=3|pages=405–14|doi=10.1016/j.yhbeh.2005.09.002|pmid=16246335|s2cid=16151756|access-date=2007-05-18|archive-date=2020-11-29|archive-url=https://web.archive.org/web/20201129162835/https://msu.edu/~breedsm/pdf/Blanchard2006IntFratHandHBFinal.pdf|url-status=live}}</ref> [[Ray Blanchard]] and [[Anthony Bogaert]] are credited with discovering the effect in the 1990s.<ref>{{Cite journal|last1=Bogaert|first1=Anthony F.|last2=Skorska|first2=Malvina|date=2011-04-01|title=Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review|url=http://www.sciencedirect.com/science/article/pii/S0091302211000227|journal=Frontiers in Neuroendocrinology|series=Sexual Differentiation of Sexual Behavior and Its Orientation|language=en|volume=32|issue=2|pages=247–254|doi=10.1016/j.yfrne.2011.02.004|pmid=21315103|s2cid=45446175|issn=0091-3022|access-date=2020-07-09|archive-date=2020-07-09|archive-url=https://web.archive.org/web/20200709203424/https://www.sciencedirect.com/science/article/pii/S0091302211000227|url-status=live}}</ref> [[J. Michael Bailey]] and [[Jacques Balthazart]] say the FBO effect demonstrates that sexual orientation is heavily influenced by prenatal biological mechanisms rather than unidentified factors in socialization.<ref name=":1">{{Cite journal|last=Bailey|first=J. Michael|date=2018-01-01|title=The Fraternal Birth Order Effect Is Robust and Important|journal=Archives of Sexual Behavior|language=en|volume=47|issue=1|pages=18|doi=10.1007/s10508-017-1115-1|pmid=29159754|s2cid=35597467|issn=1573-2800}}</ref><ref name=":22"/>