Fraternal birth order and male sexual orientation

The fraternal birth order effect is an observation that the more older brothers a male has from the same mother, the greater probability he will have a homosexual orientation. It was identified by Ray Blanchard and Anthony Bogaert in the 1990s, who attributed the effect to a prenatal biological mechanism, as the association is only present in men with older biological brothers, and not present among men with older step-brothers. Hypothesized to be the result of a maternal immune response to male fetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development, it would leave some regions of the male brain in the 'female typical' arrangement – resulting in attraction to males.

Overview

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 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%. Later the same year, Blanchard and Bogaert reported the older brother effect in the Kinsey Interview Data, a "very large and historically significant data base". In a study published in 1998, Blanchard called this phenomenon the fraternal birth order effect.
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. 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. More recent studies report more equivocal results. 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. However, following a large population-based study of the phenomenon in the Netherlands revealed the effect was found in women as well.
Secondly, some researchers believe that fraternal birth order effect operates through a biological mechanism during prenatal life, not during childhood or adolescence. Some evidence for this is the fact that the fraternal birth order effect has been found even in males not raised with their biological brothers, and preliminary biochemical evidence was found in a lab study in 2017. 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.
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, as the incidence of homosexuality correlated with an increase in older brothers is seen only in right-handed males. Since handedness develops prenatally, this finding might indicate that prenatal mechanisms underlie the fraternal birth order effect. It has also been found that homosexual males with older brothers have significantly lower birth weights compared to heterosexual males with older brothers. 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 fraternal birth order effect has been observed in places such as Brazil, Canada, Finland, Iran, Italy, The Netherlands, Samoa, Spain, Turkey, the United Kingdom, and the United States. 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 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: Older brothers increase the odds of homosexuality in later-born males or, alternatively, homosexual men tend to have more older brothers than do heterosexual men. It has been found that the proportion of older brothers is 31% greater in the sibships of homosexual males than in the sibships of heterosexual males. Alternatively, the ratio of older brothers to other siblings is 47% greater for homosexual males than it is for heterosexual males.
After statistically controlling for number of older brothers, homosexual and heterosexual males do not differ in their mean number of older sisters, younger sisters or younger brothers. Older sisters, younger sisters, and younger brothers have no effect on the odds of homosexuality in later born males – they neither enhance nor counteract the fraternal birth order effect. Blanchard and Bogaert investigated whether homosexual men have a higher mean birth order than heterosexual men primarily because they have more older brothers or because they have more older siblings of both sexes. They confirmed that homosexuality was positively correlated with a man's number of older brothers, not older sisters, younger sisters, or younger brothers.
In a few studies, homosexual subjects have occasionally displayed both a larger number of older brothers and a larger number of older sisters in comparison to heterosexual men. This is because a person's number of older brothers and number of older sisters tend to be positively correlated. So, if Proband A has more older brothers than Proband B, chances are that Proband A also has more older sisters than Proband B. These findings of excess of older sisters are hence occasional by-products of homosexual men having an excess of older brothers, are not found as consistently as the excess of older brothers, and thus need not detract from the significance of the fraternal birth order effect. When samples are drawn from populations with relatively high fertility rates, the positive correlation between number of older brothers and older sisters may give the false impression that both the number of older brothers and the number of older sisters are associated with male sexual orientation. Indeed, two samples from the high fertility Samoan population displayed simultaneous fraternal and 'sororal' birth order effects. However, direct comparison of the magnitudes of these two effects showed that the fraternal birth order effect took precedence in the studies. Various studies and meta-analyses have confirmed that only the older brother effect is consistently associated with homosexuality:

A 1998 meta-analysis by Jones and Blanchard investigated whether older sisters have no effect on sexual orientation in later-born males, or simply a weaker effect than older brothers. To this end, they developed competing mathematical models of the two possibilities: they derived two theoretical equations; the first equation applies if sisters have no direct relation to a proband's sexual orientation but brothers do; the second applies if sisters have the same relation to a proband's sexual orientation as do brothers. They then compared the fit of these models to the empirical data available at that time and found that the first equation held for homosexual men and the second for heterosexual men. They also concluded that any tendency for homosexual males to be born later among their sisters is, in effect, a statistical artifact of their tendency to be born later among their brothers.
A 2004 meta-analysis involved 10,143 male subjects of which 3181 were homosexual and 6962 were heterosexual. Its results reinforced the conclusion that homosexual men tend to have more older brothers than do heterosexual men, and the conclusion that differences in all other sibship parameters are secondary consequences of the difference in older brothers. The same conclusions have been reached with analyses of individual rather than aggregate data, and by independent investigators.
A 2015 meta-analysis determined that only the older brother effect/fraternal birth order effect was reliably associated with male sexual orientation across previously published studies.
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. It has also been found that the fraternal birth order effect can be demonstrated whether the homosexual and heterosexual groups being compared on older brothers both have large or small family sizes, so long as both groups have the same family size. Family size differs per country but the implication is that the larger a family size becomes the greater the commonality for homosexuality among men becomes. 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 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. 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.
The relation between number of older brothers and male homosexuality is also not an artifact of birth interval. Blanchard and Bogaert conducted a study to investigate whether homosexual men are, on average, born a shorter time after their next-older siblings than are heterosexual men. They found that mean birth intervals preceding heterosexual and homosexual males were virtually identical.
No type of sibling is reliably related to women's sexual orientation.