Synonyms
Definition
Fraternal birth order (FBO) refers to the birth order of sons within a family – whether, for example, a boy is the oldest, youngest, or middle child among his brothers. The number of sisters is irrelevant to fraternal birth order. The fraternal birth order (FBO) effect refers to a consistent finding in men’s sexual orientation, such that gay men have a greater number of older brothers compared to heterosexual men.
Introduction
In the search for clues about the development of sexual orientation, psychologists Ray Blanchard and Anthony Bogaert published a seminal paper in 1996 on the association between birth order and sexual orientation in men. They found that in a sample of 302 gay men, matched on year of birth with 302 heterosexual men, number of older brothers was associated with being gay. Older sisters, younger brothers, younger sisters, and age of parent at time of birth of the men were not related to sexual orientation. Also, each additional older brother...
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References
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Blanchard, R., & Bogaert, A. F. (1996). Homosexuality in men and number of older brothers. The American Journal of Psychiatry, 153, 27–31. doi:10.1176/ajp.153.1.27.
Blanchard, R., & Bogaert, A. F. (2004). Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples. American Journal of Human Biology, 16, 151–157. doi:10.1002/ajhb.20006.
Blanchard, R., & Klassen, P. (1997). H-Y antigen and homosexuality in men. Journal of Theoretical Biology, 185, 373–378. doi:10.1006/jtbi.1996.0315.
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Bogaert, A. F., & Skorska, M. N. (2011). Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review. Frontiers in Neuroendocrinology, 32, 247–254. doi:10.1016/j.yfrne.2011.02.004.
VanderLaan, D. P., & Vasey, P. L. (2011). Male sexual orientation in independent Samoa: Evidence for fraternal birth order and maternal fecundity effects. Archives of Sexual Behavior, 40, 495–503. doi:10.1007/s10508-009-9576-5.
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Skorska, M.N., Bogaert, A.F. (2016). Fraternal Birth Order, The. In: Weekes-Shackelford, V., Shackelford, T., Weekes-Shackelford, V. (eds) Encyclopedia of Evolutionary Psychological Science. Springer, Cham. https://doi.org/10.1007/978-3-319-16999-6_3373-1
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DOI: https://doi.org/10.1007/978-3-319-16999-6_3373-1
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