Abstract
Understanding critical periods in brain development and how they impact adult functioning is a primary goal of neuroscience. The sexual differentiation of the brain is a unique critical period in that it is initiated by endogenous production of a critical signaling molecule in only one sex, testosterone in fetal males. Females, by contrast, do not produce testosterone but are highly responsive to it and remain sensitive to its masculinizing effects well past the close of the critical period in males. Compared to other well characterized critical periods, such as those for the visual system or barrel cortex, the masculinization of the brain is telescoped into a few short days and initiated prenatally. The slightly longer and postnatal sensitive period in females provides a valuable tool for understanding this challenging but fundamental developmental process.