Editing Biology and sexual orientation (section)

===Birth order===
{{Main|Fraternal birth order and sexual orientation}}
Blanchard and Klassen (1997) reported that each additional older brother increases the odds of a man being gay by 33%.<ref name="Blanchard_1997">{{cite journal | vauthors = Blanchard R, Klassen P | title = H-Y antigen and homosexuality in men | journal = Journal of Theoretical Biology | volume = 185 | issue = 3 | pages = 373–8 | date = April 1997 | pmid = 9156085 | doi = 10.1006/jtbi.1996.0315 | bibcode = 1997JThBi.185..373B | url = http://faculty.bennington.edu/~sherman/sex/H-Y%20anitgen.pdf | url-status = dead | citeseerx = 10.1.1.602.8423 | archive-url = https://web.archive.org/web/20120915214732/http://faculty.bennington.edu/~sherman/sex/H-Y%20anitgen.pdf | archive-date = 2012-09-15 }}</ref><ref>{{cite news |url=https://www.nytimes.com/2007/04/10/health/10gene.html |title=Pas de Deux of Sexuality Is Written in the Genes |date=10 April 2007 |newspaper=The New York Times| vauthors = Wade N }}</ref> This is now "one of the most reliable epidemiological variables ever identified in the study of sexual orientation".<ref>{{cite journal | vauthors = Blanchard R | 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 | year = 1997 | doi = 10.1080/10532528.1997.10559918 | pmid = 10051890 }}</ref> To explain this finding, it has been proposed that male fetuses provoke a maternal immune reaction that becomes stronger with each successive male fetus. This maternal immunization hypothesis (MIH) begins when cells from a male fetus enter the mother's circulation during pregnancy or while giving birth.<ref name="Anthony_2011">{{cite journal |author-link=Anthony Bogaert |name-list-style=amp |vauthors=Bogaert AF, Skorska M |date=April 2011 |title=Sexual orientation, fraternal birth order, and the maternal immune hypothesis: a review |journal=Frontiers in Neuroendocrinology |volume=32 |issue=2 |pages=247–54 |doi=10.1016/j.yfrne.2011.02.004 |pmid=21315103 |s2cid=45446175}}</ref> Male fetuses produce H-Y antigens which are "almost certainly involved in the sexual differentiation of vertebrates". These Y-linked proteins would not be recognized in the mother's immune system because she is female, causing her to develop antibodies which would travel through the placental barrier into the fetal compartment. From here, the anti-male bodies would then cross the blood/brain barrier (BBB) of the developing fetal brain, altering sex-dimorphic brain structures relative to sexual orientation, increasing the likelihood that the exposed son will be more attracted to men than women.<ref name="Anthony_2011"/> It is this antigen which maternal H-Y antibodies are proposed to both react to and 'remember'. Successive male fetuses are then attacked by H-Y antibodies which somehow decrease the ability of H-Y antigens to perform their usual function in brain masculinization.<ref name="Blanchard_1997"/>

In 2017, researchers discovered a biological mechanism of gay people who tend to have older brothers. They believed the [[Neuroligin 4 Y-linked]] protein was responsible for a later son being gay. They found that women had significantly higher anti-NLGN4Y levels than men. In addition, mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons. The results suggest an association between a maternal immune response to NLGN4Y and subsequent sexual orientation in male offspring.<ref name="Bogaert_2018"/>

The fraternal birth order effect, however, does not apply to instances where a firstborn is homosexual.<ref name="Cantor_2002">{{cite journal | vauthors = Cantor JM, Blanchard R, Paterson AD, Bogaert AF | title = How many gay men owe their sexual orientation to fraternal birth order? | journal = Archives of Sexual Behavior | volume = 31 | issue = 1 | pages = 63–71 | date = February 2002 | pmid = 11910793 | doi = 10.1023/a:1014031201935 | s2cid = 203129 }}</ref><ref name="Blanchard_2004">{{cite journal | vauthors = Blanchard R, Bogaert AF | title = Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples | journal = American Journal of Human Biology | volume = 16 | issue = 2 | pages = 151–7 | date = 2004 | pmid = 14994314 | doi = 10.1002/ajhb.20006 | s2cid = 21108939 }}</ref>