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Revision as of 05:53, 14 January 2006 by Pete.Hurd (talk | contribs)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)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%. Even so, the fraternal birth order effect only accounts for a maximum of one seventh of the prevalence of homosexuality in men. 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.
A chemical called the histocompatibility Y-antigen (the "HY antigen") is found on the surface of the cells of male mammals. It is hypothesized that the "older brother effect" may be related to increasing levels of antibodies produced by the mother in response to the presence of this chemical during pregnancy with the oldest son. These antibodies could then somehow trigger different brain development patterns in later male children, either in the uterus or in early childhood through breast milk. Later sons would then more likely to have a homosexual orientation as adults.
References
- Blanchard, R. & Bogaert, A.F. (1996) Homosexuality in men and number of older brothers. Am. J. Psychiat. 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. J. Biosoc. Sci. 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.
- 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.
- Wachtel, S. S. (1983) H–Y Antigen and the Biology of Sex Determination. Grune & Stratton, New York.
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