Fraternal birth order and male sexual orientation

The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men". 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. In 1996, Ray Blanchard and Anthony Bogaert claimed that the later birth order of homosexual men was solely due to a number of older brothers and not older sisters. They also found that each older brother increased the odds of homosexuality in a later-born brother by 33%. In a study published in 1998, Blanchard called this phenomenon the fraternal birth order effect. According to several studies, each older brother increases a male child's naturally occurring odds of having a homosexual orientation by 28–48%. However, the numbers of older sisters, younger brothers, and younger sisters is more controversial. Some studies have found that siblings other than male older siblings have no effect on those odds. One 2002 study estimated that approximately one in seven homosexual males owes their sexual orientation to the fraternal birth order effect. For many years, research seemed to indicate there was no effect of birth order on sexual orientation in women. Secondly, some researchers believe that fraternal birth order effect operates through a biological mechanism during prenatal life, not during childhood or adolescence. Since handedness develops prenatally, this finding might indicate that prenatal mechanisms underlie the fraternal birth order effect. As birth weight is undeniably prenatally determined, it might indicate that a common developmental factor that operates before birth necessarily underlies the fraternal birth order effect and male sexual orientation. Thirdly, the fraternal birth order effect has been observed in diverse samples such as homosexual males from different ethnicities, different cultures, different historical eras, and widely separated geographic regions. The effect has also been observed in homosexual men from convenience and representative, national probability samples. 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. NLGN4Y is important in male brain development; the maternal immune reaction to it, in the form of anti-NLGN4Y antibodies, is thought to alter the brain structures underlying sexual orientation in the male fetus. The study found that women had significantly higher anti-NLGN4Y levels than men. The result also indicates that mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons. ==Empirical findings==

Biodemographics The fraternal birth order effect is a phenomenon that can be described in one of two ways: • A 2020 re-analysis that replicated the FBO effect but took family size and other previous constraints into account further confirmed Blanchard's findings. The fraternal birth order effect is independent of some potential confounding factors such as age, year of birth, and socioeconomic status. Additionally, to detect the fraternal birth order effect, it is necessary that family size of homosexual and heterosexual groups are not strongly affected by the various parental strategies (so-called 'stopping rules') of ceasing reproduction after one child, after one male child, or after a child of each sex, because in these particular situations, neither homosexual nor heterosexual males have enough older brothers to make comparisons meaningful. 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. 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. No type of sibling (i.e., older brothers, older sisters, younger brothers or younger sisters) is reliably related to women's sexual orientation. Blanchard and Ellis (2001) studied 3229 adult, homosexual and heterosexual, men and women (the probands) whose mothers knew the sex of every child (or fetus) that they were pregnant with prior to the proband. Information on birth weight, maternal gravidity, and other demographic variables was reported on questionnaires completed by the probands' mothers. Later, another study found the fraternal birth order effect may be limited to only moderately right-handed men, as extreme right-handers also did not display a fraternal birth order effect. 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. According to VanderLaan et al., cross-cultural similarities in childhood behavior support the idea that similar biological influences, which transcend cultural differences, play a role in the development of male homosexuality. This idea would be further supported if it could be demonstrated that causal biological factors, such as the fraternal birth order mechanism (which is biological in nature), are likely to influence the development of male homosexuality in non-Western cultures. Thus, establishing the existence of the fraternal birth order effect — a hypothesized outcome of the fraternal birth order mechanism — in a non-Western culture would further substantiate arguments that similar biological influences underlie the development of homosexuality across cultures. Studies in Western as well as non-Western cultures have demonstrated fraternal birth order effect (as well as fecundity effects) in relation to male homosexuality. The cross-cultural consistency with which these effects have been documented is consistent with the conclusion that culturally invariant, biological processes underlie the development of homosexuality in males. The Netherlands, Independent Samoa, Turkey, the United Kingdom, and the United States. The effect has additionally been observed in participants examined during childhood as well as adulthood The demonstration of the fraternal birth order effect in meta-analysis of studies by Blanchard and colleagues, Another study (Blanchard and Bogaert, 1998) did not confirm a later fraternal birth order for men with sexual offenses against prepubescent boys or girls, but did confirm it for men with offenses against pubescent boys or girls. 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. The first study was a retrospective study of sex offenders, which included only those subjects whose clinical charts happened to contain birth order data so the results of the study may have been affected by selection bias. The second study was a reanalysis of archived data from a classic study of sexual offenders from the year 1965. There was minimal recoverable information regarding the subjects' offense histories, and there is a possibility that the sexual preferences of the pedophiles in the study were not accurately classified from the available information. 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. Each type of pedophilic group was compared with a control group that consisted of gynephilic men (i.e., men attracted to adult females). The study found that homosexual pedophiles had more older brothers than the gynephilic control group while the bisexual and heterosexual pedophiles did not. 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). 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. Other findings 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". 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). According to VanderLaan et al., the failure of these studies to demonstrate the fraternal birth order effect is attributable to their methodological flaws. 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. 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. 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 placed 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 manual sex. 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. 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. Ray Blanchard performed a reanalysis of Frisch's data using procedures that have been used in prior studies of fraternal birth order. According to his analysis, "the only group whose data resembled data from previous studies was the homosexually married males", reaffirming the birth order effect. He further criticized drawing conclusions from data of married persons, since heterosexually married persons also have "markedly different older-sibling sex ratios" than heterosexually oriented persons. Effect in women Whether or not older brothers increase the probability of women having a non-heterosexual or homosexual orientation is less consistent. Some previous scientific research found older siblings have no impact on the sexuality of women, while others found that older siblings have the same effect on women as on men. Following updates in methodology, some recent studies in large samples have identified effects of older siblings on the sexuality of women quantitatively similar to that of men, as well as one meta-analysis. Most research on the fraternal birth order effect has been carried out on homosexual men. Sororal birth order effect Some research has sought to extend the idea of the 'fraternal birth order' effect to the effect of older sisters on the sexuality of younger siblings, calling it the "sororal birth order effect". ==Theories about the fraternal birth order mechanism==

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. This is because 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. It has been hypothesized that this is caused by an in-utero maternal alloimmune response, specifically, the mother's immune system becoming sensitized to H-Y antigens involved in the sexual differentiation of male vertebrates, Specifically, maternal antibodies to the neuroligin NLGN4Y protein, Y-chromosome protein important in male fetal brain development, have been found to be involved in the fraternal birth order effect. ==See also==