The female phenotype: Nature's default?

Abstract

Traditional mammalian developmental models focus on the presence or absence of testosterone as the critical factor differentiating males from females. In this view, a female phenotype occurs by default in the absence of masculinizing hormones. Accumulating evidence, however, suggests that ovarian hormones also play an important role in development of the female brain. The existence of an active ovarian influence on female development (which supplements passive feminization via the absence of testosterone) changes our assumptions and ideas about sexual differentiation and has important theoretical and scientific implications for the study of behavioral similarities and differences between the sexes, and their neural substrates.