Prenatal hormones and sexual orientation Only 23% of childhood gender problems will result in transsexuality in adulthood. Drawing on some transsexualism cases, Garcia-Falgueras and Swaab state that "[f]rom these examples it appears that the direct action of testosterone on the developing brain in boys and the lack of such action on the developing brain in girls are crucial factors in the development of male and female gender identity and sexual orientation ... ." Countless studies have been run on peripheral levels of sex steroids in male and female homosexuals, a considerable number of which claimed to find "less 'male hormone' and/or more 'female hormone' in
Fraternal birth order and male sexual orientation 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. The existence of the fraternal birth order effect on male sexual orientation has been confirmed many times. One approach for establishing the ubiquity of this effect