Fraternal birth order

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In psychology the fraternal birth order effect is the name given to the observation that the more older brothers a man has, the greater the probability is that he will have a homosexual sexual orientation. The fraternal birth order effect is the strongest known predictor of sexual orientation, each older brother increases a man's chances of being gay by about 33%^[1][2]. Even so, the fraternal birth order effect only accounts for a maximum of one seventh of the prevalence of homosexuality in men^[3]. There seems to be no effect on sexual orientation in women, and no effect of 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 HY antigen with each son. The HY antigen (histocompatibility Y-antigen) is found on the surface of the cells of male, but not female, mammals^[4]. 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 brother is seen only in right handed males ^[5][6][7] (see also handedness and sexual orientation).

[edit] References

1. ^ Blanchard, R. (2001). Fraternal birth order and the maternal immune hypothesis of male homosexuality. Hormones and Behavior, 40:105-114. ([1]
2. ^ Puts, D.A., Jordan, C. L. and Breedlove, S. M. (2006) O brother, where art thou? The fraternal birth-order effect on male sexual orientation. Proc. Nat. Acad. Sci. USA. 103:10531-10532. [2]
3. ^ 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
4. ^ Wachtel, S. S. (1983) H–Y Antigen and the Biology of Sex Determination. Grune & Stratton, New York.
5. ^ Blanchard R, Lippa R. "Birth Order, Sibling Sex Ratio, Handedness, and Sexual Orientation of Male and Female Participants in a BBC Internet Research Project". Arch Sex Behav. PMID 17345165. 
6. ^ 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.
7. ^ Blanchard, R. 2007. Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men, Archives of Sexual Behavior. in press.

• Blanchard, R., Zucker, K. J., Bradley, S. J., & Hume, C. S. (1995). Birth order and sibling sex ratio in homosexual male adolescents and probably pre-homosexual feminine boys. Developmental Psychology, 31:22–30.
• Blanchard, R. & Bogaert, A.F. (1996) Homosexuality in men and number of older brothers. American Journal of Psychiatry, 153:27-­31.
• Blanchard, R. (1997). Birth Order and Sibling Sex Ratio in Homosexual Versus Heterosexual Males and Females. Annual Review of Sex Research, 8:27-67
• Blanchard, R., Zucker, K.J., Siegelman, M., Dickey, R. & Klassen, P. (1998). The relation of birth order to sexual orientation in men and women. Journal of Biosocial Science, 30:511-­519.
• Blanchard, R. (2004). Quantitative and theoretical analyses of the relation between older brother and homosexuality in men. Journal of Theoretical Biology, 230:173-187.
• Bogaert, A.F. (2005) Sibling Sex Ratio and Sexual Orientation in Men and Women: New Tests in Two National Probability Samples. Archives of Sexual Behavior, 34:111-116.
• Bogaert, A.F. (2006). Biological versus nonbiological older brothers and men’s sexual orientation. Proc. Nat. Acad. Sci. USA. 103