Fraternal birth order
Encyclopedia
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. The observation is that the more older brothers a man has, the greater the probability is that he will have a homosexual
orientation
. It has sometimes been called the older brother effect.
The fraternal birth order effect has also 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, the United Kingdom, the Netherlands, and Polynesia.
The effect has been found even in males not raised with their biological brothers, suggesting an in-utero environmental causation. To explain this finding, a maternal immune response has been hypothesized. Male fetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates. Other studies have suggested the influence of birth order was not due to a biological, but a social process.
and Brückner (2008) argue 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.)
a boy that affects subsequent sons. An in-utero maternal immune response has been hypothesized for this effect. 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 step brothers.
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.
Bogaert (2006) replicated the fraternal birth order effect on male sexual orientation, in a sample including both biological siblings and adopted siblings. 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.
McConaghy (2006) published in a sociological journal that he found no relationship between the strength of the effect and degree of homosexual feelings, rather than homosexual identity or homosexual behavior, 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.
Ray Blanchard
Ray Milton Blanchard is an American-Canadian sexologist, best known for his research studies on pedophilia, gender dysphoria, and sexual orientation. He has also published research studies on phallometry and several paraphilias, including transvestism and autoerotic asphyxia.-Education and...
identified the association and referred to it as the fraternal birth order effect. The observation is that the more older brothers a man has, the greater the probability is that he will have a homosexual
Homosexuality
Homosexuality is romantic or sexual attraction or behavior between members of the same sex or gender. As a sexual orientation, homosexuality refers to "an enduring pattern of or disposition to experience sexual, affectional, or romantic attractions" primarily or exclusively to people of the same...
orientation
Sexual orientation
Sexual orientation describes a pattern of emotional, romantic, or sexual attractions to the opposite sex, the same sex, both, or neither, and the genders that accompany them. By the convention of organized researchers, these attractions are subsumed under heterosexuality, homosexuality,...
. It has sometimes been called the older brother effect.
Empirical basis
The fraternal birth order effect is the strongest known predictor of sexual orientation. According to several studies, each older brother increases a man's odds of having a homosexual orientation by 28–48%. The fraternal birth order effect accounts for approximately one seventh of the prevalence of homosexuality in men. There seems to be no effect on sexual orientation in women, and no effect related to the number of older sisters.The fraternal birth order effect has also 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, the United Kingdom, the Netherlands, and Polynesia.
The effect has been found even in males not raised with their biological brothers, suggesting an in-utero environmental causation. To explain this finding, a maternal immune response has been hypothesized. Male fetuses produce H-Y antigens which may be involved in the sexual differentiation of vertebrates. Other studies have suggested the influence of birth order was not due to a biological, but a social process.
Contrary evidence
BearmanPeter Bearman
Peter Shawn Bearman is an American sociologist. He is Jonathan Cole Professor of the Social Sciences in the Department of Sociology at Columbia University....
and Brückner (2008) argue 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
National Longitudinal Study of Adolescent Health
The National Longitudinal Study of Adolescent Health is the only nationally representative study of adolescent sexuality, which has spawned over one thousand peer-reviewed publications on many issues related to adolescent health and sexuality, and other adolescent health risk behaviors.The Add...
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.)
Theories on the cause
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 gestatingGestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
a boy that affects subsequent sons. An in-utero maternal immune response has been hypothesized for this effect. 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 step brothers.
The fraternal birth order effect appears to interact with handedness
Handedness
Handedness is a human attribute defined by unequal distribution of fine motor skills between the left and right hands. An individual who is more dexterous with the right hand is called right-handed and one who is more skilled with the left is said to be left-handed...
, as the incidence of homosexuality correlated with an increase in older brothers is seen only in right-handed males.
Bogaert (2006) replicated the fraternal birth order effect on male sexual orientation, in a sample including both biological siblings and adopted siblings. 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.
McConaghy (2006) published in a sociological journal that he found no relationship between the strength of the effect and degree of homosexual feelings, rather than homosexual identity or homosexual behavior, 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.
See also
- Biology and sexual orientationBiology and sexual orientationBiology and sexual orientation is the subject of research into the role of biology in the development of human sexual orientation. No simple, single cause for sexual orientation has been conclusively demonstrated, but research suggests that it is by a combination of genetic, hormonal, and...
- Birth orderBirth orderBirth order is defined as a person's rank by age among his or her siblings. Birth order is often believed to have a profound and lasting effect on psychological development...
- Handedness and sexual orientationHandedness and sexual orientationA relationship between handedness and sexual orientation has been suggested but not verified by a number of researchers, who report that heterosexual individuals are somewhat more likely to be right-handed than homosexual individuals...
- Prenatal hormones and sexual orientationPrenatal hormones and sexual orientationThe hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the adult...