Fraternal birth order and male sexual orientation
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. In several studies, the observation is that the more older brothers a man has from the same mother, the greater the probability is that he will have a homosexual orientation. It has sometimes been called the older brother effect. It has been estimated that 15% of the homosexual demographic is associated with fraternal birth order.
The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men." 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 also been observed amongst paedophiles. In one survey, gay and bisexual male paedophiles were typically younger than heterosexual male paedophiles, primarily the result of the homosexual/bisexual group being born later among their brothers, generally speaking.
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 (2002) 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 i.e., those who experienced a non-zero level of same-sex attraction were significantly less likely to have older sisters).
The failure of these studies to demonstrate the fraternal birth order effect has been attributed to their methodological limitations. Although they utilized large adolescent samples, the low base rates of same-sex attraction and behaviour in the population resulted in sample sizes that were too small for assessing the relation of birth order to sexual orientation. The fraternal birth order effect may also have been obscured in these studies due to their use of different methods of sexual orientation classification and their imprecise measures of sibships. Ray Blanchard explained that the demonstrability of the fraternal birth order effect depends partly on the adequate matching of the mean family size of the homosexual and heterosexual study groups and noted that in the two studies above, the mean family size of the homosexual groups was significantly smaller than that of the heterosexual comparison groups. Specifically, heterosexual males had larger numbers of siblings overall than the homosexual males which may have obscured the analyses of group differences in older brothers and prevented the demonstration of the fraternal birth order effect. Researchers have thus emphasized the necessity of comparing groups on measures of mean family size and have suggested that, in the two studies, an alternative birth order metric that controlled for sibship size could have produced findings consistent with the fraternal birth order effect. Since the publication of Bearman and Bruckner's study in 2002, studies that used representative national probability samples and direct reports on siblings' sexual orientation have found the fraternal birth order effect.
Theories on causation
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 stepbrothers.
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) 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. PMID 10051890.
- Valenzuela C. 2009. "Sexual Orientation, Handedness, Sex Ratio, and Fetomaternal Tolerance-Rejection". Biological Research 43(3), 347–356
- 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. doi:10.1073/pnas.0511152103. PMC 1502306. PMID 16807297.
- 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. doi:10.1017/S0021932098005112. PMID 9818557.
- 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. doi:10.1016/S0191-8869(00)00051-9.
- Blanchard R (September 2001). "Fraternal birth order and the maternal immune hypothesis of male homosexuality". Horm Behav 40 (2): 105–14. doi:10.1006/hbeh.2001.1681. PMID 11534970.
- Puts DA, Jordan CL, Breedlove SM (July 2006). "O brother, where art thou? The fraternal birth-order effect on male sexual orientation" (PDF). Proc. Natl. Acad. Sci. U.S.A. 103 (28): 10531–2. doi:10.1073/pnas.0604102103. PMC 1502267. PMID 16815969.
- Rahman Q., Clarke K., Morera T. (2009). "Hair whorl direction and sexual orientation in human males". Behavioral Neuroscience 123 (2): 252–256. doi:10.1037/a0014816. PMID 19331448.
- Cantor JM, Blanchard R, Paterson AD, Bogaert AF (February 2002). "How many gay men owe their sexual orientation to fraternal birth order?" (PDF). Arch Sex Behav 31 (1): 63–71. doi:10.1023/A:1014031201935. PMID 11910793.
- 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. doi:10.1007/s10508-005-1005-9. PMID 15772774.
- 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. doi:10.1007/s10508-006-9159-7. PMID 17345165.
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- Green R (July 2000). "Birth order and ratio of brothers to sisters in transsexuals". Psychol Med 30 (4): 789–95. doi:10.1017/S0033291799001932. PMID 11037086.
- 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. doi:10.1007/BF02437544. PMID 8899142.
- 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. doi:10.1080/00926230490247110. PMID 14660290.
- Bogaert, Anthony F., et al. "Pedophilia, sexual orientation, and birth order." Journal of Abnormal Psychology 106.2 (1997): 331.
- Blanchard R, Klassen P (April 1997). "H-Y antigen and homosexuality in men". J. Theor. Biol. 185 (3): 373–8. doi:10.1006/jtbi.1996.0315. PMID 9156085.
- 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. doi:10.1300/J082v51n04_09. PMID 17135133.
- Bearman, Peter; Brückner, Hannah (2002). "Opposite-sex twins and adolescent same-sex attraction" (PDF) 107. American Journal of Sociology. pp. 1179–1205.
- Francis, Andrew M. (2008). "Family and sexual orientation: the family-demographic correlates of homosexuality in men and women" (PDF) 45. Journal of Sex Research. pp. 371–377.
- VanderLaan DP; Blanchard R; Wood H; Garzon LC; Zucker KJ (2015). "Birth weight and two possible types of maternal effects on male sexual orientation: A clinical study of children and adolescents referred to a Gender Identity Service". Developmental Psychobiology 57 (1): 25–34. doi:10.1002/dev.21254. PMID 25345970.
- Doug P. VanderLaan; Ray Blanchard; Hayley Wood; Kenneth J. Zucker (2014). "Birth order and sibling sex ratio of children and adolescents referred to a Gender Identity Service.". PLoS One 9 (3): e90257. doi:10.1371/journal.pone.0090257. PMID 24651045.
- Blanchard R (2014). "Detecting and correcting for family size differences in the study of sexual orientation and fraternal birth order". Archives of sexual behaviour 43 (5): 845–52. doi:10.1007/s10508-013-0245-3. PMID 24578105.
The term “fraternal birth order effect” denotes a statistical relation most commonly expressed in one of two ways: Older brothers increase the odds of homosexuality in later born males or, alternatively, homosexual men tend to have more older brothers than do heterosexual men. The demonstrability of this effect depends partly on the adequate matching of the homosexual and heterosexual study groups with respect to mean family size. If the homosexual group has too many siblings, relative to the heterosexual group, the homosexual group will tend to show the expected excess of older brothers but may also show an excess of other sibling-types (most likely older sisters); if the homosexual group has too few siblings, it will tend not to show a difference in number of older brothers but instead may show a deficiency of other sibling-types (most likely younger brothers and younger sisters). In the first part of this article, these consequences are illustrated with deliberately mismatched groups selected from archived data sets. In the second part, two slightly different methods for transforming raw sibling data are presented. These are intended to produce family-size-corrected variables for each of the four original sibling parameters (older brothers, older sisters, younger brothers, and younger sisters). Both versions are shown to render the fraternal birth order effect observable in the deliberately mismatched groups. In the third part of the article, fraternal birth order studies published in the last 5 years were surveyed for failures to find a statistically significant excess of older brothers for the homosexual group. Two such studies were found in the nine examined. In both cases, the collective findings for older sisters, younger brothers, and younger sisters suggested that the mean family size of the homosexual groups was smaller than that of the heterosexual comparison groups. Furthermore, the individual findings for the four classes of siblings resembled those for the present experimentally mismatched groups in which the mean family size of the homosexual group was significantly smaller. This illustrates the necessity of comparing groups on measures of mean family size and removing this confound in some way when those means are markedly different.
- Blanchard R; Bogaert AF (2004). "Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples.". Am J Hum Biol. 16 (2): 151–7. PMID 14994314.
- Bogaert AF (2003). "Number of older brothers and sexual orientation: New tests and the attraction/behavior distinction in two national probability samples". J Pers Soc Psychol 84 (3): 644–52. PMID 12635923.
- Blanchard R, Bogaert AF (January 1996). "Homosexuality in men and number of older brothers". Am J Psychiatry 153 (1): 27–31. PMID 8540587.
- 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. doi:10.1016/j.jtbi.2004.04.021. PMID 15302549.
- 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. doi:10.1016/j.yhbeh.2005.09.002. PMID 16246335.
- Blanchard R (2007). "Sex ratio of older siblings in heterosexual and homosexual, right-handed and non-right-handed men". Archives of Sexual Behavior 37 (6): 977–81. doi:10.1007/s10508-006-9119-2. PMID 17186124.
- Blanchard, Ray (January 2008). "Review and theory of handedness, birth order, and homosexuality in men". Laterality 13 (1): 51–70. doi:10.1080/13576500701710432. PMID 18050001.