Fraternal birth order and male sexual orientation

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, the greater the probability is that he will have a homosexual orientation. It has sometimes been called the older brother effect. It has been estimated that 15% of the homosexual demographic is associated with fraternal birth order.

Empirical basis
The fraternal birth order effect is the strongest known biodemographic 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
Peter Bearman and Brückner (2008) argue that studies showing a fraternal birth order effect have used non-representative 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 National Longitudinal Study of Adolescent 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).

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 gestating 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.