Fraternal birth order and sexual orientation

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 orientation. [Blanchard, R. (1997). Birth order and sibling sex ratio in homosexual versus heterosexual males and females. "Annual Review of Sex Research, 8, 27-67."] It has sometimes been called the "older brother effect".

Empirical basis

The fraternal birth order effect is the strongest known predictor of sexual orientation.Bogaert, A. F. (2006). [http://www.pnas.org/content/103/28/10771.full.pdf Biological versus nonbiological older brothers and men's sexual orientation] . "Proceedings of the National Academy of Sciences, 103," 10777-10774.] According to several studies, each older brother increases a man's odds of developing a homosexual sexual orientation by 28%–48%. [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.] [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.] Blanchard, R. (2001). Fraternal birth order and the maternal immune hypothesis of male homosexuality. "Hormones and Behavior, 40," 105-114.] [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.] The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men. [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.] There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters. [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.]

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, [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.] the United Kingdom, [Green, R. (2000). Birth order and ratio of brothers to sisters in transsexuals. "Psychological Medicine, 30," 789–795.] the Netherlands, [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.] and Polynesia. [Poasa, K. H., Blanchard, R., Zucker, K. J. (2004). Birth order in transgendered males from Polynesia: A quantitative study of Samoan fa'afafine. "Journal of Sex and Marital Therapy, 30," 13–23.]

Criticism

Peter Bearman (2002) questions the sampling method of Blanchard and other scientists who report a link between fraternal birth order and sexual orientation. He says that the studies work with nonrepresentative samples, and/or indirect reports on siblings’ sexual orientation. After repeating the experiment done by Blanchard he found "no association between same-sex attraction and number of older siblings, older brothers, or older sisters".cite journal
last=Bearman
first=Peter
coauthor=Brückner, Hannah
year=2002
title=Opposite-sex twins and adolescent same-sex attraction
publisher=American Journal of Sociology
volume=107
url=http://www.soc.duke.edu/~jmoody77/205a/ecp/bearman_bruckner_ajs.pdf
format=PDF
pages=1179–1205]

Theories of what causes the effect

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. 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. An in-utero maternal immune response has been hypothesized for this effect. [Blanchard, R. & Bogaert, A. F. (1996) Homosexuality in men and number of older brothers. "American Journal of Psychiatry, 153," 27­-31.] [Blanchard, R. & Klassen, P. (1997). H-Y antigen and homosexuality in men. "Journal of Theoretical Biology, 185," 373-378.] [Blanchard, R. (2004). Quantitative and theoretical analyses of the relation between older brother and homosexuality in men. "Journal of Theoretical Biology, 230," 173-187.]

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.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.] [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.] [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. (2008). Review and theory of handedness, birth order, and homosexuality in men. "Laterality, 13," 51-70.]

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, 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. [McConaghy N, Hadzi-Pavlovic D, Stevens C, Manicavasagar V, Buhrich N, Vollmer-Conna U. Fraternal birth order and ratio of heterosexual/homosexual feelings in women and men. J Homosex. 2006;51(4):161-74.]

ee also

*Biology and sexual orientation
*Birth order
*Handedness and sexual orientation
*Prenatal hormones and sexual orientation

References