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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%. 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. 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 mammals. 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.
- 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.
- 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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