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] In 1958, it was reported that homosexual men tend to have a greater number of older siblings (i.e., a 'later/higher birth order') than comparable heterosexual men and in 1962, these findings were published in detail.[4] 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.[5] They also showed that each older brother increased the odds of homosexuality in a later-born brother by 33%.[5] 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."[6][7] In a study published in 2004, Blanchard called this phenomenon the fraternal birth order effect.[8]

Research over the years has established several facts. First, homosexual men do tend to have a higher birth order than heterosexual men, and this higher birth order is attributed to homosexual men having greater number of older brothers.[9] According to several studies, each older brother increases a male child's naturally occurring odds of having a homosexual orientation by 28–48%.[10][11][12][13][14][note 1] However, the numbers of older sisters, younger brothers,and younger sisters have no effect on those odds.[9] It has been estimated that approximately one in seven homosexual males owe their sexual orientation to the fraternal birth order effect.[17] There seems to be no effect of birth order on sexual orientation in women.[18][19]

Secondly, the fraternal birth order effect has been found even in males not raised with their biological brothers – it has been determined that biological brothers increase the odds of homosexuality in later-born males even if they were reared in different households, whereas non-biological siblings, such as step-brothers or adopted brothers, have no effect on male sexual orientation.[3] This directly demonstrates that the fraternal birth order effect operates during prenatal life, not during childhood or adolescence.[7] Indirect evidence also indicates that the fraternal birth order effect is prenatal and biological in nature rather than postnatal and psychosocial: The fraternal birth order effect seems to interact with handedness, as the incidence of homosexuality correlated with an increase in older brothers is seen only in right-handed males.[19][20][21][22] Since handedness develops prenatally,[23] this finding suggests that prenatal mechanisms underlie the fraternal birth order effect.[7] It has also been found that homosexual males with older brothers have significantly lower birth weights compared to heterosexual males with older brothers.[24][25] As birth weight is undeniably prenatally determined, it is known that a common developmental factor that operates before birth necessarily underlies the fraternal birth order effect and male sexual orientation.[26]

Thirdly, the fraternal birth order effect has been demonstrated in diverse samples such as homosexual males from different races,[27] different cultures,[28] different historical eras,[5][29] and widely separated geographic regions.[9] The fraternal birth order effect has been demonstrated in Brazil, Canada, Iran, Italy, The Netherlands, Independent Samoa, Spain, Turkey, the United kingdom, and the United States.[9] The effect has also been demonstrated in homosexual men from convenience[5][30] and representative, national probability samples.[31] 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.[32][33] The fraternal birth order effect has been observed among androphilic 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 (also called "heterosexual transsexuals"). This has been reported in samples from Canada,[34] the United Kingdom,[35] the Netherlands,[36] and Polynesia.[37] Fraternal birth order has also been found to correlate with same-sex attraction in pedophilic men.[33] 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.[38] However, authors of this particular study have stated that the results may have been affected by selection bias.[33]

Non-conforming studies

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."[39] 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).[40]

The failure of these studies to demonstrate the fraternal birth order effect has been attributed to their methodological flaws.[41] 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.[42] 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.[42] 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.[43][note 2] 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.[42] 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 would have produced findings consistent with the fraternal birth order effect.[42][43] 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.[31][44]

A study involving 43 androphilic male-to-female (MtF) transsexuals and 49 heterosexual men from South Korea showed that while the Korean androphilic MtF transsexuals had a significantly late birth order, so did the Korean heterosexual men. It was found that (unlike Western androphilic MtF transsexuals) the Korean transsexuals had a significant excess of sisters, not brothers. Similarly, Korean heterosexual men also had a significant excess of sisters but not brothers. In both groups, the excess of sisters to brothers was largely due to both groups having a higher number of older sisters. The fraternal birth order effect was prevented from being demonstrated because sibling sex ratios had been distorted by the 'male-preference stopping rule' governing parental reproductive behavior whereby, as most Korean parents have a marked preference for sons, they continue to have children (i.e., daughters) until a male baby is born, after which they stop having children.[45]

Currin et al. (2015) carried out a study investigating the existence of the fraternal birth order effect in a variety of sexual orientation dimensions — namely, identity, attraction, fantasies, and behavior — whereas previous research studied only one such dimension (identity). Participants in the study were split into two groups: a "heterosexual group" and a "non-heterosexual" group. To determine what label participants used to identify their sexual orientation, participants were asked, "How would you classify your sexual orientation?" and selected from one of five options (heterosexual, mostly heterosexual, bisexual, mostly gay/lesbian, gay/lesbian). People who selected "heterosexual" were place in the heterosexual group, whereas people selecting "gay/lesbian" were placed in the non-heterosexual group. To assess sexual attraction, participants were asked, "How sexually attracted are you to men?" and selected from a 7 point Likert scale ranging from 1 (not at all) to 7 (very much). Participants were also asked "How sexually attracted are you to women?" and were presented with the same Likert scale. People who selected 1 (not at all) when answering the same sex sexual attraction question were placed in the heterosexual group. People who selected 2 or greater were placed in the non-heterosexual group. To assess sexual behavior, participants were asked, "What is the total number of male sexual partners you have had?" and "What is the total number of female sexual partners you have had?" Sexual partner was defined as someone with whom the participant had penile–vaginal penetration, oral sex, anal sex, and/or mutual masturbation. Individuals who identified having any same-sex sexual partner (i.e., 1 or more) were placed in the non-heterosexual group while individuals who did not identify having a same-sex sexual partner were placed in the heterosexual group.[16]

To assess sexual fantasy, participants were asked "What percent of your sexual fantasies during masturbation involve men?" and rated the question from 0% to 100%. Each participant was also asked "What percent of your sexual fantasies during masturbation involve women?" and rated the question from 0% to 100%. If an individual endorsed having any same-sex sexual fantasies at all (i.e., 1% or greater), they were placed in the non-heterosexual group, otherwise they were placed in the heterosexual group. Using these criteria for sexual orientation identity, attraction, fantasies, and behavior, Currin et al. (2015) were unable to demonstrate the effect for any dimensions in their sample of 722 right-handed men (of which 500 were classified as heterosexual and 122 were classified as non-heterosexual), although the study did adjust for family size differences between the two groups.[16] A 2017 meta-analysis analyzed studies about the fraternal birth order effect including Currin et al. (2015). The meta-analysis had a total sample of 7140 homosexual males and 12,837 heterosexual males. The results of the meta-analysis confirmed the reliability of the fraternal birth order effect.[9]

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.[12][5][8][46] Male fetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates.[46] 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 and with birth weight also suggests that the effect operates prenatally, as handedness and birth weight both develop in utero.[7][26]

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.[47] However, multiple studies have demonstrated that the fraternal birth order effect does not operate through social or postnatal (e.g., learning or rearing or environmental) mechanisms; rather it is biological in nature and operates prenatally.[3][7][20][48][49] It is also known that the fraternal birth order effect on male sexual orientation is unaffected by family dynamics, living with older brothers or learning.[49]

See also

Notes

  1. ↑ The naturally occurring odds of a male child (with no older brothers) being homosexual are estimated to be 2%. Due to the fraternal birth order effect, those naturally occurring odds are increased to 2.6% in a male child with one older brother; a male child with two older brothers will have a 3.5% chance of being homosexual; with three, and four older brothers, the chances are increased to 4.6%, and 6.0%, respectively.[15][16][12]
  2. ↑ 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)."[43] 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.[43]

References

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  2. ↑ Valenzuela C (2009). "Sexual Orientation, Handedness, Sex Ratio, and Fetomaternal Tolerance-Rejection". Biological Research. 43 (3): 347–356. doi:10.4067/s0716-97602010000300012.
  3. 1 2 3 4 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. PMC 1502306 Freely accessible. PMID 16807297. doi:10.1073/pnas.0511152103.
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  5. 1 2 3 4 5 Blanchard R, Bogaert AF (January 1996). "Homosexuality in men and number of older brothers". Am J Psychiatry. 153 (1): 27–31. PMID 8540587. doi:10.1176/ajp.153.1.27.
  6. ↑ 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. PMID 8931880. doi:10.1007/bf02437839.
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  17. ↑ 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. PMID 11910793. doi:10.1023/A:1014031201935.
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  28. ↑ 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. ISSN 1573-2800. PMID 20039114. doi:10.1007/s10508-009-9576-5.
  29. ↑ 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. PMID 17907817. doi:10.1037/0735-7044.121.5.845.
  30. ↑ Williams TJ; Pepitone ME; Christensen SE; Cooke BM; Huberman AD; Breedlove NJ; Breedlove TJ; Jordan CL; Breedlove SM (Mar 2000). "Finger-length ratios and sexual orientation" (PDF). Nature. 404 (6777): 455–6. PMID 10761903. doi:10.1038/35006555. Retrieved 17 August 2016.
  31. 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. PMID 12635923. doi:10.1037/0022-3514.84.3.644.
  32. ↑ 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.
  33. 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. PMID 10983250. doi:10.1023/A:1001943719964.
  34. ↑ 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.
  35. ↑ Green R (July 2000). "Birth order and ratio of brothers to sisters in transsexuals". Psychol Med. 30 (4): 789–95. PMID 11037086. doi:10.1017/S0033291799001932.
  36. ↑ 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. PMID 8899142. doi:10.1007/BF02437544.
  37. ↑ 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. PMID 14660290. doi:10.1080/00926230490247110.
  38. ↑ 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.
  39. ↑ 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.
  40. ↑ 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.
  41. ↑ 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. PMID 25345970. doi:10.1002/dev.21254.
  42. 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. PMC 3961213 Freely accessible. PMID 24651045. doi:10.1371/journal.pone.0090257.
  43. 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 Behavior. 43 (5): 845–52. PMID 24578105. doi:10.1007/s10508-013-0245-3.
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  45. ↑ Kenneth J. Zucker; et al. (2007). "Birth order and sibling sex ratio in homosexual transsexual South Korean men: Effects of the male-preference stopping rule". Psychiatry and Clinical Neurosciences. 61: 529–533. PMID 17875032. doi:10.1111/j.1440-1819.2007.01703.x.
  46. 1 2
  47. ↑ 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. PMID 17135133. doi:10.1300/J082v51n04_09.
  48. ↑ 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. ISSN 1573-2800. PMID 12710827. doi:10.1023/A:1022827524721.
  49. 1 2 Rahman Q; Wilson G (2005). Born Gay: The Psychobiology of Sex Orientation. London: Peter Owen Publishers. ISBN 0720613094.
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