In non-human vertebrate species, sexual differentiation of the brain is primarily driven by androgens such as testosterone organizing the brains of males in a masculine fashion early in life, while the lower levels of androgen in developing females organizes their brains in a feminine fashion. These principles may be relevant to development of sexual orientation in humans, because retrospective markers of prenatal androgen exposure, namely digit ratios and otoacoustic emissions, indicate that lesbians, on average, were exposed to greater prenatal androgen than were straight women. Thus the even greater levels of prenatal androgen exposure experienced by fetal males may explain why the vast majority of them grow up to be attracted to women. However, the same markers indicate no differences between gay and straight men in terms of average prenatal androgen exposure, so the variance in orientation in men cannot be accounted for by variance in prenatal androgen exposure, but may be due to variance in response to prenatal androgens. These data contradict several popular notions about human sexual orientation.