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
Sexual differentiation differentiation of cells derived from the genital ridges into testes. In transgenic XX mice (and some human XX males), "SRY" alone is sufficient to induce male differentiation. Various processes are involved in the development of sex differences in humans. Sexual differentiation in humans includes development of different genitalia and the internal genital tracts, breasts, body hair, and plays a role in gender identification. The development of sexual differences begins with the XY sex-determination system that is present in humans, and complex mechanisms are responsible for the development of the phenotypic differences between male and female humans from an undifferentiated zygote.