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A correlation between fraternal birth order and male sexual orientation has been suggested by 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 orientation. It has sometimes been called the older brother effect.
The fraternal birth order effect is the strongest known predictor of sexual orientation. According to several studies, each older brother increases a man's odds of having a homosexual orientation by 28–48%. The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men. There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters.
The fraternal birth order effect has 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, the United Kingdom, the Netherlands, and Polynesia.
The effect has been found even in males not raised with their biological brothers, suggesting an in-utero environmental causation. To explain this finding, a maternal immune response has been hypothesized. Male fetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates. Other studies have suggested the influence of birth order was not due to a biological, but a social process.
Bearman and Brückner (2008) argue that studies showing a fraternal birth order effect have used nonrepresentative samples and/or indirect reports on siblings’ sexual orientation. Their analysis, focusing on opposite-sex twins, did not find an association "between same-sex attraction and number of older siblings, older brothers, or older sisters". A study by Francis (2008), using the same Add Health survey but with broader analysis, saw a very weak correlation of male same-sex attraction with having multiple older brothers (but did find a significant negative correlation of male same-sex attraction with having older sisters.)
Theories on the cause
Anthony Bogaert's work involving adoptees concludes that the effect is not due to being raised with older brothers, but is hypothesized to have something to do with changes induced in the mother's body when gestating a boy that affects subsequent sons. An in-utero maternal immune response has been hypothesized for this effect. 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.
The fraternal birth order effect appears to interact with handedness, as the incidence of homosexuality correlated with an increase in older brothers is seen only in right-handed males.
Bogaert (2006) replicated the fraternal birth order effect on male sexual orientation, in a sample including both biological siblings and adopted siblings. Only the older biological brothers influenced sexual orientation; there was no effect of adopted siblings. Bogaert concluded that his finding strongly suggest a prenatal origin to the fraternal birth-order effect.
McConaghy (2006) published in a sociological journal that he found no relationship between the strength of the effect and degree of homosexual feelings, rather than homosexual identity or homosexual behavior, leading him to conclude that the influence of birth order on degree of homosexual feelings was not due to a biological, but a social process.
- Biology and sexual orientation
- Birth order
- Handedness and sexual orientation
- Prenatal hormones and sexual orientation
- ↑ Blanchard R (1997). Birth order and sibling sex ratio in homosexual versus heterosexual males and females. Annu Rev Sex Res 8: 27–67.
- ↑ 2.0 2.1 2.2 Bogaert AF (July 2006). Biological versus nonbiological older brothers and men's sexual orientation. Proc. Natl. Acad. Sci. U.S.A. 103 (28): 10771–4.
- ↑ Blanchard R, Zucker KJ, Siegelman M, Dickey R, Klassen P (October 1998). The relation of birth order to sexual orientation in men and women. J Biosoc Sci 30 (4): 511–9.
- ↑ 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.
- ↑ 5.0 5.1 Blanchard R (September 2001). Fraternal birth order and the maternal immune hypothesis of male homosexuality. Horm Behav 40 (2): 105–14.
- ↑ Puts DA, Jordan CL, Breedlove SM (July 2006). O brother, where art thou? The fraternal birth-order effect on male sexual orientation. Proc. Natl. Acad. Sci. U.S.A. 103 (28): 10531–2.
- ↑ Cantor JM, Blanchard R, Paterson AD, Bogaert AF (February 2002). How many gay men owe their sexual orientation to fraternal birth order?. Arch Sex Behav 31 (1): 63–71.
- ↑ Bogaert AF (February 2005). Sibling sex ratio and sexual orientation in men and women: new tests in two national probability samples. Arch Sex Behav 34 (1): 111–6.
- ↑ 9.0 9.1 Blanchard R, Lippa RA (April 2007). Birth order, sibling sex ratio, handedness, and sexual orientation of male and female participants in a BBC internet research project. Arch Sex Behav 36 (2): 163–76.
- ↑ Blanchard R, Sheridan PM (1992). Sibship size, sibling sex ratio, birth order, and parental age in homosexual and nonhomosexual gender dysphorics. Journal of Nervous and Mental Diseases 180: 40–7.
- ↑ Green R (July 2000). Birth order and ratio of brothers to sisters in transsexuals. Psychol Med 30 (4): 789–95.
- ↑ Blanchard R, Zucker KJ, Cohen-Kettenis PT, Gooren LJ, Bailey JM (October 1996). Birth order and sibling sex ratio in two samples of Dutch gender-dysphoric homosexual males. Arch Sex Behav 25 (5): 495–514.
- ↑ Poasa KH, Blanchard R, Zucker KJ (2004). Birth order in transgendered males from Polynesia: a quantitative study of Samoan fa'afāfine. J Sex Marital Ther 30 (1): 13–23.
- ↑ 14.0 14.1 14.2 Blanchard R, Klassen P (April 1997). H-Y antigen and homosexuality in men. J. Theor. Biol. 185 (3): 373–8.
- ↑ 15.0 15.1 McConaghy N, Hadzi-Pavlovic D, Stevens C, Manicavasagar V, Buhrich N, Vollmer-Conna U (2006). Fraternal birth order and ratio of heterosexual/homosexual feelings in women and men. J Homosex 51 (4): 161–74.
- ↑ Bearman, Peter (2002). Opposite-sex twins and adolescent same-sex attraction 107: 1179–1205.
- ↑ Francis, Andrew M. (2008). Family and sexual orientation: the family-demographic correlates of homosexuality in men and women 45: 371–377.
- ↑ Blanchard R, Bogaert AF (January 1996). Homosexuality in men and number of older brothers. Am J Psychiatry 153 (1): 27–31.
- ↑ Blanchard R (September 2004). Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men. J. Theor. Biol. 230 (2): 173–87.
- ↑ Blanchard R, Cantor JM, Bogaert AF, Breedlove SM, Ellis L (March 2006). Interaction of fraternal birth order and handedness in the development of male homosexuality. Horm Behav 49 (3): 405–14.
- ↑ Blanchard R (2007). Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men. Archives of Sexual Behavior.
- ↑ Blanchard R (January 2008). Review and theory of handedness, birth order, and homosexuality in men. Laterality 13 (1): 51–70.
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