Fraternal birth order and sexual orientation
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A correlation between fraternal birth order and sexual orientation has been suggested by recent research. Referred to in psychology as the fraternal birth order effect, the observation is that the more older brothers a man has, the greater the probability is that he will have a homosexual sexual orientation.
Overview
The fraternal birth order effect is the strongest known predictor of sexual orientation; according to research, each older brother increases a man's chances of being gay by about 33%.Blanchard, R. (2001). "Fraternal birth order and the maternal immune hypothesis of male homosexuality." Hormones and Behavior, 40:105-114.David A. Puts, Cynthia L. Jordan, and S. Marc Breedlove (2006) "O brother, where art thou? The fraternal birth-order effect on male sexual orientation." Proc. Nat. Acad. Sci. USA. 103:10531-10532. [http://www.pnas.org/cgi/content/extract/103/28/10531] Even so, the fraternal birth order effect only accounts for a maximum of one seventh of the prevalence of homosexuality in men.Cantor, J. M., Blanchard, R., Paterson, A. D. & Bogaert, A. F. (2002). How many gay men owe their sexual orientation to fraternal birth order. Archives of Sexual Behavior, 31:63-71 There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters.Large studies involving adoptees demonstrate that the effect is not due to being raised with older brothers, but appears to have something to do with changes induced in the uterus when gestating a boy that affects subsequent sons. The effect is present regardless of whether or not the older brothers are raised in the same family environment with the boy. There is no effect when the number of older brothers is increased by adopted brothers or step brothers. It is hypothesized that the fraternal birth order effect may be caused by increasing levels of antibodies produced by the mother to the histocompatibility Y-antigen with each son. The HY antigen is found on the surface of the cells of male, but not female mammals.Wachtel, S. S. (1983) H–Y Antigen and the Biology of Sex Determination. Grune & Stratton, New York. The presence of this foreign chemical when bearing a son could trigger the mother's immune response, which may then lead to different brain development patterns in later male children.The fraternal birth order effect appears to have the opposite effect in right-handed individuals than non-right-handed individuals, however the incidence of homosexuality correlated with an increase in older brothers is seen only in right handed males.Blanchard, R., Cantor, J. M., Bogaert, A. F., Breedlove, S. M., & Ellis, L. (2006). "Interaction of fraternal birth order and handedness in the development of male homosexuality." Hormones and Behavior, 49, 405–414.Blanchard, R. 2007. "Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men." Archives of Sexual Behavior.
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