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 from the same mother, 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.

The fraternal birth order effect has been described by one of its proponents as "the most consistent biodemographic correlate of sexual orientation in men." 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.

In 1958, it was reported that homosexual men tend to have a greater number of older siblings (i.e., a 'later' or 'higher birth order') than comparable heterosexual men and in 1962, these findings were published in detail. In 1996, Ray Blanchard and Anthony Bogaert demonstrated that the later birth order of homosexual men was solely due to an excess of older brothers and not older sisters. They also showed that each older brother increased the odds of homosexuality in a later-born brother by 33%. Later the same year, Blanchard and Bogaert demonstrated the older brother effect in the Kinsey Interview Data, a "very large and historically significant data base." In a study published in 2004, Blanchard called this phenomenon the fraternal birth order effect as it was determined that male sexual orientation is related only to the number of older brothers, not related to other sibling types (i.e., younger brothers, older sisters, younger sisters), and that this relation between sexual orientation and older brothers is found only in males, not females.

The fraternal birth order effect has been found even in males not raised with their biological brothers. Non-biological siblings, such as adopted or step-brothers, have no effect on male sexual orientation. This demonstrates that the fraternal birth order effect operates during prenatal life, not during childhood or adolescence. To explain how the effect may operate prenatally, a maternal immune response has been hypothesized.

...
Wikipedia