Fraternal birth order and male sexual orientation
Ray Blanchard and Anthony Bogaert first identified the association in the 1990s and named it the fraternal birth order effect.
[1][3] It has generally been thought that this maternal response would not apply to first-born gay sons and that they may owe their orientation to other mechanisms.
[3][4] The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men".
[11][12] According to several studies, each older brother increases a male child's naturally occurring odds of having a homosexual orientation by 28–48%.
[11] It has been estimated that approximately one in seven homosexual males owes their sexual orientation to the fraternal birth order effect.
[20][21] Secondly, the fraternal birth order effect operates through a biological mechanism during prenatal life, not during childhood or adolescence.
[5] 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 has been confirmed to interact with handedness,[22][9] as the incidence of homosexuality correlated with an increase in older brothers is seen only in right-handed males.
[34][35][36] In a 2017 study, researchers found an association between a maternal immune response to neuroligin 4 Y-linked protein (NLGN4Y) and subsequent sexual orientation in their sons.
Indeed, two samples from the high fertility Samoan population displayed simultaneous fraternal and 'sororal' birth order effects.
For example, a study done in mainland China did not find any fraternal birth order effect, which the authors attributed to the one-child policy.
[11][21][44][45][46] The relation between number of older brothers and male homosexuality is not an artifact of higher maternal or paternal age at the time of the proband's birth.
[14][39] This implies that the birth order phenomenon cannot be explained by increased mutation rates in the ova or sperm cells of aging mothers or fathers, respectively.
[17] Estimates of the proportion of homosexual men who owe their sexual orientation to the fraternal birth order effect have ranged from 15.1%[19] to 28.6%.
Information on birth weight, maternal gravidity, and other demographic variables was reported on questionnaires completed by the probands' mothers.
[9] In a 2017 study, researchers found an association between a maternal immune response to neuroligin 4 Y-linked protein (NLGN4Y) and subsequent sexual orientation in their sons.
[57] Bogaert (2006) provided a direct test pitting prenatal against postnatal (e.g., social/rearing) mechanisms and sought to determine which of the two account for the fraternal birth order effect.
The study found that the amount of time reared with older brothers, either biological or non-biological, neither predicted sexual orientation nor affected the relationship between older brothers and sexual orientation, thereby pointing to a prenatal origin of the fraternal birth order effect.
[46][45][40] The fraternal birth order effect has been found in homosexual males from different ethnicities,[29] including White, Black, Hispanic, East Indian, Asian, Middle Eastern and Polynesian.
[58] Retrospective studies conducted in Brazil, Guatemala, Independent Samoa, the Philippines, Thailand, and Turkey have found that the same is true of homosexual males raised in these non-Western cultures.
[74] Three studies have investigated whether sexual orientation also correlates with fraternal birth order in men attracted to physically immature males.
[76] The inconsistency of these findings regarding the correlation of sexual orientation and fraternal birth order in pedophiles may be related to methodological problems in the two studies.
[71] Blanchard et al. (2000) therefore conducted a study in which data were collected with the specific purpose of examining the relation of fraternal birth order to sexual orientation in homosexual, bisexual and heterosexual pedophiles.
These results confirm that fraternal birth order correlates with sexual orientation in pedophiles, as it does in teleiophiles (i.e., people exclusively attracted to adults).
[71][77][78] To prevent misunderstanding or misuse of their studies on fraternal birth order in pedophiles, researchers have stressed that any conclusion that homosexual pedophilia shares an etiological factor with androphilia does not imply that ordinary homosexual men (androphiles) are likely to molest boys, any more than the conclusion that heterosexual pedophilia shares an etiological factor with gynephilia would imply that ordinary heterosexual men (gynephiles) are likely to molest girls.
[71] A 2014 Internet study attempted to analyze the relationship between self-identification as asexual, birth order and other biological markers, in comparison to individuals of other sexual orientation groups.
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.
[55] 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.
[11] Frisch et al. (2006) did not find a correlation between older brothers and same-sex unions between men in a sample of over 2 million Danes.
[85] Ray Blanchard performed a reanalysis of Frisch's data using procedures that have been used in prior studies of fraternal birth order.
[86] 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.
