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.[1] 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.[2]

Empirical basis

The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men."[3] According to several studies, each older brother increases a man's odds of having a homosexual orientation by 28–48%.[4][5][6][7][8] The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men.[9] There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters.[10][11]

In 1958, it was reported that homosexual men tend to have a greater number of older siblings (i.e., a 'later' or 'higher birth order') than comparable heterosexual men and in 1962, these findings were published in detail.[12] In 1996, Ray Blanchard and Anthony Bogaert demonstrated that the later birth order of homosexual men was solely due to an excess of older brothers and not older sisters.[13] They also showed that each older brother increased the odds of homosexuality in a later-born brother by 33%.[13] Later the same year, Blanchard and Bogaert demonstrated the older brother effect in the Kinsey Interview Data, a "very large and historically significant data base."[14][15] In a study published in 2004, Blanchard called this phenomenon the fraternal birth order effect as it was determined that male sexual orientation is related only to the number of older brothers, not related to other sibling types (i.e., younger brothers, older sisters, younger sisters), and that this relation between sexual orientation and older brothers is found only in males, not females.[16]

The fraternal birth order effect has been found even in males not raised with their biological brothers.[3] Non-biological siblings, such as adopted or step-brothers, have no effect on male sexual orientation.[3] This demonstrates that the fraternal birth order effect operates during prenatal life, not during childhood or adolescence.[15] To explain how the effect may operate prenatally, a maternal immune response has been hypothesized.[17]

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.[11][18][19][20] As handedness develops prenatally,[21] this finding suggests that prenatal mechanisms underlie the fraternal birth order effect.[15]

The fraternal birth order effect has been demonstrated in diverse samples such as homosexual men from different races,[22] different cultures,[23] and different historical eras.[24][25] The effect has also been demonstrated in homosexual men from convenience[13] and representative, national probability samples.[26] Two groups that are attracted to males but differ most strongly from typical homosexual men are androphilic MtF transsexuals (also called "homosexual transsexuals") and men who are sexually attracted to physically immature males.[27][28] The fraternal birth order effect has 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,[29] the United Kingdom,[30] the Netherlands,[31] and Polynesia.[32] Fraternal birth order has also been found to correlate with same-sex attraction in pedophilic men.[28] In one study, homosexual-bisexual male pedophiles had a later general birth order than heterosexual male pedophiles and this late birth order was primarily due to the homosexual-bisexual group being born later among their brothers than later among their sisters.[33] However, authors of this study have stated that the results may have been affected by selection bias.[28]

Contrary evidence

Bearman and Brückner (2002) argued 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."[34] 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).[35]

The failure of these studies to demonstrate the fraternal birth order effect has been attributed to their methodological limitations.[36] 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.[37] 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.[37] 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.[38][note 1] 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.[37] 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.[37][38] 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.[39][26]

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.[6][17][13][16] Male fetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates.[17] 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.

Bogaert (2006) replicated the fraternal birth order effect on male sexual orientation in a sample including both biological siblings and adopted siblings.[3] 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.

The interaction of the fraternal birth order effect with handedness also suggests that the effect operates prenatally, as handedness develops in utero.[15]

McConaghy (2006) investigated birth order in men and women who anonymously reported some homosexual feelings, few of whom identified as homosexual. He found a fraternal birth order in men who reported homosexual feelings and when comparing the strength of the effect and degree of homosexual feelings (rather than homosexual identity or homosexual behavior) he found no relationship between the two, 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 in the subjects studied.[40] However, multiple studies have shown that the fraternal birth order effect does not operate through social or postnatal (e.g. learning/rearing/environmental) mechanisms; rather it is biological in nature and operates prenatally.[3][15][18][41][42] The fraternal birth order effect on male sexual orientation is unaffected by family dynamics, living with older brothers or learning.[42]

See also

Notes

  1. ↑ According to Ray Blanchard, the demonstrability of the fraternal birth order effect partly depends upon proper matching of homosexual and heterosexual study groups with respect to mean family size. If the homosexual study group has too few siblings, in comparison to the heterosexual study group, the homosexual group will tend to show no difference in number of older brothers and may show a scarcity of other sibling types (most commonly younger brothers and younger sisters). If the homosexual study group has too many siblings, it will not only show the expected excess of older brothers but may also show an excess of other sibling types (most commonly older sisters). In his three-part article, Blanchard first demonstrated the aforementioned outcomes using purposely mismatched groups selected from archived data sets. In the second part, Blanchard presented two different methods for transforming raw sibling data. Both methods intended to "produce family-size-corrected variables for each of the four original sibling parameters (older brothers, older sisters, younger brothers, and younger sisters)."[38] Consequently, both methods were successful in demonstrating the fraternal birth order effect in the purposely mismatched groups. In the third part of the article, Blanchard surveyed studies about fraternal birth order and found two studies that did not demonstrate the fraternal birth order effect in their homosexual groups. In both studies, the collective findings for older sisters, younger brothers, and younger sisters as well as individual findings for the four sibling classes suggested that mean family size of the homosexual study groups was comparatively smaller to that of the heterosexual groups in both studies.[38]

References

  1. ↑ 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.
  2. ↑ Valenzuela C. 2009. "Sexual Orientation, Handedness, Sex Ratio, and Fetomaternal Tolerance-Rejection". Biological Research 43(3), 347–356
  3. 1 2 3 4 5 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.
  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. doi:10.1017/S0021932098005112. PMID 9818557.
  5. ↑ 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.
  6. 1 2 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.
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ 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.
  11. 1 2 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.
  12. ↑ Slater E (13 January 1962). "Birth order and maternal age of homosexuals". Lancet 1 (7220): 69–71. doi:10.1016/S0140-6736(62)91719-1. PMID 13913808. Retrieved 31 December 2015.
  13. 1 2 3 4 Blanchard R, Bogaert AF (January 1996). "Homosexuality in men and number of older brothers". Am J Psychiatry 153 (1): 27–31. doi:10.1176/ajp.153.1.27. PMID 8540587.
  14. ↑ Blanchard R; Bogaert AF (December 1996). "Biodemographic comparisons of homosexual and heterosexual men in the Kinsey interview data". Arch Sex Behav 25 (6): 551–79. doi:10.1007/bf02437839. PMID 8931880. Retrieved 31 December 2015.
  15. 1 2 3 4 5 Bogaert AF; Skorska M (2011). "Sexual orientation, fraternal birth order, and the maternal immune hypothesis: a review". Front Neuroendocrinol. 32 (2): 247–54. doi:10.1016/j.yfrne.2011.02.004. PMID 21315103.
  16. 1 2 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.
  17. 1 2 3 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.
  18. 1 2 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.
  19. ↑ 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.
  20. ↑ 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.
  21. ↑ Hepper PG; Shahidullah S; White R (1991). "Handedness in the human fetus". Neuropsychologia 29 (11): 1107–11. doi:10.1016/0028-3932(91)90080-R. PMID 1775228.
  22. ↑ Bogaert AF (October 1998). "Birth order and sibling sex ratio in homosexual and heterosexual non-white men". Arch Sex Behav 27 (5): 467–73. doi:10.1023/A:1018752513198. ISSN 1573-2800. PMID 9795727.
  23. ↑ VanderLaan DP; Vasey PL (June 2011). "Male sexual orientation in independent Samoa: evidence for fraternal birth order and maternal fecundity effects". Arch Sex Behav 40 (3): 495–503. doi:10.1007/s10508-009-9576-5. ISSN 1573-2800. PMID 20039114.
  24. ↑ Blanchard R; Bogaert AF (January 1996). "Homosexuality in men and number of older brothers". Am J Psychiatry 153 (1): 27–31. doi:10.1176/ajp.153.1.27. ISSN 1535-7228. PMID 8540587.
  25. ↑ Bogaert AF; Blanchard R; Crosthwait LE (October 2007). "Interaction of birth order, handedness, and sexual orientation in the Kinsey interview data". Behav Neurosci. 121 (5): 845–53. doi:10.1037/0735-7044.121.5.845. PMID 17907817.
  26. 1 2 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. doi:10.1037/0022-3514.84.3.644. PMID 12635923.
  27. ↑ Blanchard R; Bogaert AF (1998). "Birth order in homosexual versus heterosexual sex offenders against children, pubescents, and adults". Arch Sex Behav 27 (6): 595–603. PMID 9883307.
  28. 1 2 3 Blanchard R; Barbaree HE; Bogaert AF; Dickey R; Klassen P; Kuban ME; Zucker KJ (2000). "Fraternal birth order and sexual orientation in pedophiles". Arch Sex Behav 29 (5): 463–78. doi:10.1023/A:1001943719964. PMID 10983250.
  29. ↑ 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. doi:10.1097/00005053-199201000-00009.
  30. ↑ 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.
  31. ↑ 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.
  32. ↑ 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.
  33. ↑ Bogaert Anthony F.; et al. (1997). "Pedophilia, sexual orientation, and birth order". Journal of Abnormal Psychology 106 (2): 331–335. doi:10.1037/0021-843x.106.2.331.
  34. ↑ Bearman, Peter; Brückner, Hannah (2002). "Opposite-sex twins and adolescent same-sex attraction" (PDF). American Journal of Sociology 107: 1179–1205. doi:10.1086/341906.
  35. ↑ Francis, Andrew M. (2008). "Family and sexual orientation: the family-demographic correlates of homosexuality in men and women" (PDF). Journal of Sex Research 45: 371–377. doi:10.1080/00224490802398357.
  36. ↑ 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.
  37. 1 2 3 4 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. PMC 3961213. PMID 24651045.
  38. 1 2 3 4 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.
  39. ↑ 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. doi:10.1002/ajhb.20006. PMID 14994314.
  40. ↑ 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.
  41. ↑ Bogaert AF (April 2003). "Interaction of older brothers and sex-typing in the prediction of sexual orientation in men". Arch Sex Behav 32 (2): 129–34. doi:10.1023/A:1022827524721. ISSN 1573-2800. PMID 12710827.
  42. 1 2 Rahman Q; Wilson G (2005). Born Gay: The Psychobiology of Sex Orientation. London: Peter Owen Publishers. ISBN 0720613094.
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