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

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

Notes

References