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. Ray Blanchard identified the association and referred to it 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.[1] It has sometimes been called the older brother effect.

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[edit] Empirical basis

The fraternal birth order effect is the strongest known predictor of sexual orientation.[2] According to several studies, each older brother increases a man's odds of developing a homosexual sexual orientation by 28%–48%.[3][4][5][6] The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men.[7] There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters.[8][9]

The fraternal birth order effect also been observed among male-to-female transsexuals: MtF transsexuals who are sexually interested in men have a greater number of older brothers than MtF transsexuals who are sexually interested in women. This has been reported in samples from Canada,[10] from the United Kingdom,[11] and from the Netherlands.[12]

[edit] Theories of what causes the effect

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.[13] The presence of this foreign protein when bearing a son could trigger the mother's immune response, which may then lead to different brain development patterns in later male children.[14][15][5][16]

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.[9][17][18][19]

Bogaert (2006) replicated the fraternal birth order effect on male sexual orientation, in a sample including both biological siblings and adopted siblings.[2] Only the older biological brothers influenced sexual orientation; there was no effect of adopted siblings. Bogaert concluded that his finding supports only the biological theories of what causes the birth order effect.

[edit] See also

[edit] References

  1. ^ Blanchard, R. (1997). Birth order and sibling sex ratio in homosexual versus heterosexual males and females. Annual Review of Sex Research, 8, 27-67.
  2. ^ a b Bogaert, A. F. (2006). Biological versus nonbiological older brothers and men's sexual orientation. Proceedings of the National Academy of Sciences, 103, 10777-10774.
  3. ^ 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.
  4. ^ Ellis, L. & Blanchard, R. (2001). Birth order, sibling sex ratio, and maternal miscarriages in homosexual and heterosexual men and women. Personality and Individual Differences, 30, 543-552.
  5. ^ a b Blanchard, R. (2001). Fraternal birth order and the maternal immune hypothesis of male homosexuality. Hormones and Behavior, 40, 105-114.
  6. ^ Puts, D. A., Jordan, C. L., Breedlove, M. (2006). O brother, where art thou? The fraternal birth-order effect on male sexual orientation. Proceedings of the National Academy of Sciences USA, 103, 10531-10532.
  7. ^ 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.
  8. ^ 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.
  9. ^ a b Blanchard, R., & Lippa, R. (2007). Birth order, sibling sex ratio, handedness, and sexual orientation of male and female participants in a BBC Internet research project. Archives of Sexual Behavior, 36, 163-176.
  10. ^ Blanchard, R., & Sheridan, P. M. (1992). Sibship size, sibling sex ratio, birth order, and parental age in homosexual and nonhomosexual gender dysphorics. Journal of Nervous and Mental Diseases, 180, 40–47.
  11. ^ Green, R. (2000). Birth order and ratio of brothers to sisters in transsexuals. Psychological Medicine, 30, 789–795.
  12. ^ Blanchard, R., Zucker, K., Cohen-Kettenis, P., Gooren, L., & Bailey, J. (1996). Birth order and sibling sex ratio in two samples of Dutch gender-dysphoric homosexual males. Archives of Sexual Behavior, 25, 495–514.
  13. ^ Wachtel, S. S. (1983) H–Y antigen and the biology of sex determination. New Yokr: Grune & Stratton.
  14. ^ Blanchard, R. & Bogaert, A. F. (1996) Homosexuality in men and number of older brothers. American Journal of Psychiatry, 153, 27­-31.
  15. ^ Blanchard, R. & Klassen, P. (1997). H-Y antigen and homosexuality in men. Journal of Theoretical Biology, 185, 373-378.
  16. ^ Blanchard, R. (2004). Quantitative and theoretical analyses of the relation between older brother and homosexuality in men. Journal of Theoretical Biology, 230, 173-187.
  17. ^ 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.
  18. ^ Blanchard, R. (2007). Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men. Archives of Sexual Behavior.
  19. ^ Blanchard, R. (2008). Review and theory of handedness, birth order, and homosexuality in men. Laterality, 13, 51-70.