The Older Brother Effect

The Older Brother Effect, also known as the fraternal birth order (FBO) effect, is the finding that androphilic males (homosexual/ bisexual males) have a greater number of older brothers than gynephilic males (heterosexual males) (VanderLaan 977). In other words, homosexuality increases 33% with each successive male birth (Bogaert 248).

There is strong evidence that FBO is an effect of an immunological response some mothers develop while pregnant with a male child. Anthony F. Bogaert and Malvina Skorska describe how it works in their article, "Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review". Basically, the process begins when cells from the male fetus cross over into the mother’s bloodstream, either during pregnancy or child birth. The mother’s immune system would recognize these male cells as foreign and produce anti-male antibodies. These antibodies then crossover into the placenta, where the fetus is developing. These antigens may then enter into the brain, possibly altering areas responsible for sexual orientation (such as the anterior hypothalamus) and resulting in homosexual desires. The level of brain alteration would depend on the number and binding strength of the antibodies that invade the brain. Since both the number and binding strength of the antibodies would increase in each successive pregnancy, the chance of brain alteration would therefore increase with each successive birth (248). Due to the prenatal cause of FBO, it only applies to biologically related brothers (VanderLaan 495).

Interestingly, FBO affects have a more limited target than one might expect. It makes sense that FBO only affects males, and not females, since it is an effect of anti-male antigens, however; it make be less clear why FBO is limited to right-handed males only, especially since left-handedness and ambidexterity is often linked with homosexuality. Studies have found that both handedness and sexual orientation develop prenatally. This may be explained by MHC or RH genes, which are both linked to immunological response and handedness (Bogaert 251). Ray Blanchard, in his article, “Sex Ratio of Older Sibilings in Heterosexual and Homosexual, Right-Handed and Non-Right-Handed Men” explains three theories. The first of which hypothesizes, “Older brothers have opposite effects on sexual orientation in right-handed and non-right-handed males, that is, they increase the odds of homosexuality in right-handers and decrease the odds in non-right-handers”.  The second theory, which is ties into the first, is “that older brothers increase the odds of non-right-handedness in heterosexual men”. The third theory is “that non-right-handedness prevents the development of homosexuality in men who have older brother.

Some additional physical differences include birth weight and adult stature. “This research shows that later born gay men tend to have a lower birth weight and be shorter as adults than earlier born gay men and heterosexual men” (Bogaert 251). One theory to explain this is perhaps the antigen that’s promotes homosexual orientation works against a Y-linked antigen also de-masculinizing the fetus (Bogaert 251).

According to Bogaert, “FBO is likely the most reliable epidemiological finding in almost a century of research on sexual orientation.” It has been prevalent in samples of both homosexual and transsexual men, men from different parts of the world, and Children who exhibit GID (Bogaert 248).

Sources:

Bogaert, Anthony F., Skorska, Malvina. "Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review". 17 February 2011. Frontiers in Neuroendocrinology. http://corwin.emich.edu/sfx_local/?ctx_ver=Z39.88-2004&ctx_enc=info%3Aofi%2Fenc%3AUTF-8&rfr_id=info:sid/summon.serialssolutions.com&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&rft.genre=article&rft.atitle=Sexual+orientation%2C+fraternal+birth+order%2C+and+the+maternal+immune+hypothesis%3A+A+review&rft.jtitle=Frontiers+in+Neuroendocrinology&rft.au=Bogaert%2C+Anthony+F&rft.au=Skorska%2C+Malvina&rft.date=2011-04-01&rft.issn=0091-3022&rft.volume=32&rft.issue=2&rft.spage=247&rft.epage=254&rft_id=info:doi/10.1016%2Fj.yfrne.2011.02.004&rft.externalDBID=n%2Fa&rft.externalDocID=10_1016_j_yfrne_2011_02_004

Blanchard, Ray. "Sex Ratio of Older Siblings in Heterosexual and Homosexual, Right-Handed and Non-Right-Handed Men". 21 December 2006. Springer Science+Business Media. http://www.springerlink.com/content/p6637137p0n10857/

VanderLaan, Doug P., Vasey Paul L. "Male Sexual Orientation in Independent Samoa: Evidence for Fraternal Birth Order and Maternal Fecundity Effects". 29 December 2009 Springer Science+Business Media. 
http://www.springerlink.com/content/j0k00168j1u8k460/?MUD=MP

-Amanda Ranusch