“Differentiation always occurs in the female direction, regardless of genetic programming, unless an adequate level of testosterone is present In other words, even if the genetic code is masculine, an inadequate amount of testosterone will result in the development of female sexual characteristics. The principle of feminization takes priority over masculinization. During normal female differentiation. the primitive Müllerian duct system develops Into the uterus, the fallopian tubes. and the inner third of the vagina. In males, the Müllerian duct system regresses, and the Wolffian duct system develops. becoming the vasa deferentia, seminal vesicles, and ejaculatory ducts.

Whereas the internal precursors of both male and female sexual organs are thus present for potential development, the precursors for the external genitals are unitypic, that is, the same precursors may develop into either masculine or feminine external sexual organs. Without the presence of adequate levels of androgens (testosterone and dehydrotestosterone) during the critical period of differentiation, beginning with the eight-week fetus. a clitoris, vulva, and vagina will develop. But with the presence of adequate levels of androgen stimulation, the penis, including its glands, and the scrotal sac will form, and the testes will develop as organs within the abdomen. They normally migrate into their scrotal position during the eighth or ninth month of gestation.

Under the influences of circulating fetal hormones, a dimorphic development of certain areas tithe brain takes place following the differentiation of internal and external genitals. The brain is ambitypic, and in it the development of female characteristics also prevails unless there is an adequate level of circulating androgens. Specific hypothalamic and pituitary functions that will be differentiated into the cyclic in women and noncyclic in men are determined by this differentiation. Male/female differentiation of the brain occurs only in the third trimester, after differentiation of external sex organs has taken place, and possibly continues during the first postnatal trimester. In nonprimate mammals, the prenatal hormonal differentiation of the brain preorders subsequent mating behavior. In primates, however, early social communication and learning are overridingly important in determining sexual behavior; the control of actual mating behavior is therefore determined largely by the earliest social interactions.”

— Love Relations: Normality and Pathology, Otto F. Kernberg, M.D