ABSTRACT
For the last 40 years the dominant theory of sexual differentiation of the vertebrate brain has been a theory of hormonal action. The hormonal hypothesis developed in studies of mammals led to the idea that gonadal steroid hormonal secretions might also be responsible for sexual differentiation of the zebra finch song system. The fundamental paradox of sexual differentiation of the zebra finch song system is that estrogen causes masculinization of genetic females, but that most manipulations of estrogen levels in genetic males fail to prevent masculinization. What are the candidates for alternative causal agents? There are two broad classes. One class are hormonal signals other than gonadal steroids, which might act on the brain to induce or block brain masculinization or feminization. The second class are nonhormonal signals, which called "direct genetic" signals. These represent genes that are expressed in the brain and act within neural cells to promote masculine or feminine neural development.
