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. These results thus support a prenatal origin to sexual orientation development in men and indicate that the fraternal birth order effect is probably the result of the mother's body's "memory" for both male births as well as male gestations (i.e., every instance of being pregnant with a male fetus). So even if a male fetus was not carried to term, he still increases the likelihood of homosexuality in any subsequent male fetuses gestated by the same mother.
