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.
The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men". 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.
Secondly, the fraternal birth order effect operates through a biological mechanism during prenatal life, not during childhood or adolescence. 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. Thirdly, the fraternal birth order effect has been demonstrated in diverse samples such as homosexual males from Fraternal birth order effect homosexuality in japan races,  different cultures,  different historical eras,   and widely separated geographic regions.
The fraternal birth order effect is a phenomenon that can be described in one of two ways: 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.
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.
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.
The fraternal birth order effect is independent of potential confounds such as age, year of birth, and socioeconomic status. The relation between number of older brothers and male homosexuality Fraternal birth order effect homosexuality in japan not an artifact of higher maternal or paternal age at the time of the proband's birth.
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 i. Estimates of the proportion of homosexual men who owe their sexual orientation to the fraternal birth order effect have ranged from Both direct and indirect evidence have demonstrated that the mechanism by which the fraternal birth order effect operates is prenatal in timing and biological in nature, rather than postnatal in timing or psychosocial in nature.
Indirect evidence was found first when
Fraternal birth order effect homosexuality in japan was discovered that the fraternal birth order effect interacts with birth weight. Then, Bogaert produced direct evidence that the fraternal birth order mechanism is prenatal.
Subsequent research related to handedness has further reinforced these findings. The finding that the fraternal birth order mechanism operates during a male's prenatal development in the mother's womb arose unexpectedly, in a study of sexual orientation, birth order, and birth weight.
Blanchard and Ellis studied adult, homosexual and heterosexual, men and women Fraternal birth order effect homosexuality in japan 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. Each of these three findings has since Fraternal birth order effect homosexuality in japan replicated in other studies     and the general finding that boys with older brothers have smaller birth weights than boys with older sisters is in line with earlier studies.
The interaction of fraternal birth order with birth weight an obviously prenatally determined trait suggests that the mechanism of the fraternal birth order effect operates before the individual's birth i. They found, in a sample of men, that the fraternal birth order effect was contingent on handedness: The effect of older brothers on the likelihood of homosexuality only occurred in right-handed males; the effect of older brothers did not alter the likelihood of being gay in left-handed and ambidextrous men.
Inresearchers discovered a biological mechanism of gay people who tend to have older brothers. They think Neuroligin 4 Y-linked protein is responsible for a later son being gay. The result also indicates that number of pregnancies, 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. Bogaert provided a direct test pitting prenatal against postnatal e.
He examined the association between male sexual orientation and biological siblings i. Whether and how long participants were reared with these siblings was also examined.
If a prenatal factor underlies the fraternal birth order effect, then only biological older brothers predict sexual orientation because only biological older brothers share prenatal characteristics e. The study found that only biological older brothers predicted sexual orientation. Even when the number of non-biological older brothers significantly exceeded the number of biological older brothers, and hence the opportunity for an effect via being reared with non-biological older brothers was high, only the number of biological older brothers and not non-biological older brothers predicted sexual orientation in men.
Further, if rearing or social factors underlie the fraternal birth order effect, then the amount of time reared with older brothers, either biological or non-biological, should predict sexual orientation because rearing time determines the relative opportunity that older brothers have to affect their younger sibling's postnatal sociosexual development.
If a prenatal factor underlies the fraternal birth order effect, then a postnatal factor such as rearing time with older siblings be they biological or non-biological should have no impact on the sexual orientation of younger male siblings. The study found that the amount of time reared with older
Fraternal birth order effect homosexuality in japan, 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.
Finally, if rearing or social factors underlie the fraternal birth order effect, then the number of biological older brothers with whom the participants were not reared should not predict sexual orientation because they should have no impact on the postnatal sociosexual environment of their younger brothers. If a prenatal factor underlies the fraternal birth order effect, then biological older brothers with whom the participants were not reared should still predict sexual orientation because all biological older brothers, even those not reared with the participants, share prenatal characteristics e.
The study found that the number of biological brothers does predict men's sexual orientation even if the participants were not reared with the biological older brothers.
In summation, it was found that biological older brothers significantly predicted male sexual orientation regardless of whether or how long participants were reared with these brothers whereas the remaining sibling categories, including non-biological older brothers, did not.
The existence of the fraternal birth order effect on male sexual orientation has been confirmed many times. The fraternal birth order effect has been found in homosexual males from different races,  including White, Black, Hispanic, East Indian, Asian, Middle Eastern and Polynesian.
The effect has also been demonstrated in homosexual males from different cultures: Despite how variable cultures can be, cross-cultural universals in the development of homosexual males appear to exist. For example, in Western cultures, homosexual males exhibit comparatively more gender-nonconforming behavior during childhood than heterosexual males.
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 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 "Fraternal birth order effect homosexuality in japan" 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 fraternal birth order effect has also been demonstrated in widely separated geographic regions and in countries such as Brazil,  Canada,  Finland,  Iran,  Italy,  The Netherlands,  Independent Samoa,  Spain,  Turkey,  the United Kingdom,  and the United States.
The fraternal birth order effect has been observed in androphilic male-to-female MtF transsexuals: MtF transsexuals who are sexually interested in men also called "homosexual transsexuals" 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,  the United Kingdom,  the Netherlands,  and Polynesia. Three studies have investigated whether sexual orientation also correlates with fraternal birth order in men attracted to physically immature males.
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 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.
Each type of pedophilic group was compared with a control group that consisted of gynephilic men i. 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. Previous studies had established that the fraternal birth order effect affects what gender a man is sexually attracted to; Blanchard et al.
Results from the study indicate that fraternal birth order does not affect the age of preferred erotic targets and that fraternal birth order does not correlate with pedophilia. So the fraternal birth order mechanism only causes males to be attracted to other males; whatever mechanism causes males to be pedophilic as opposed to teleiophilic i. Other than fraternal birth order, no common feature has been identified in the developmental histories or family demographics of androphiles and pedophiles, while key features distinguish the two groups, for example, most androphilic males display gender non-conformity during childhood whereas same-sex attracted pedophiles do not.
The finding of the fraternal birth order effect in cisgender and transgender male androphiles, men attracted to boys and attracted to adult men—same-sex attracted groups who differ as widely as possible in their own characteristics and in the characteristics of their desired partners—would suggest that fraternal birth order or the underlying variable it reflects may be the first universal factor to be identified in development of same-sex attraction in males.
Their analysis, focusing on opposite-sex twins, did not Fraternal birth order effect homosexuality in japan an association "between same-sex attraction and number of older siblings, older brothers, or older sisters".
The failure of these studies to demonstrate the fraternal birth order effect has been attributed to their methodological flaws. Participants in the study were split into two groups: To determine what label participants used to identify their sexual orientation, participants were asked, "How would you classify your sexual orientation? To assess sexual attraction, participants were asked, "How sexually attracted are you to men?
Participants were also asked "How sexually attracted are you to women? People who selected 1 not at all when answering the same sex sexual attraction question were
Fraternal birth order effect homosexuality in japan 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? Individuals who identified having any same-sex sexual partner i.
Each participant was also asked "What percent of your sexual fantasies during masturbation involve women?