During ontogeny [development starting with conception], the brain is permanently organized as male- or female-typical in a process called sexual differentiation. Sex-specific brain development supports the emergence of behaviors necessary for reproduction, parenting, and social behaviors such as aggression. Sexual differentiation is driven by sex-specific hormonal signals.
In mammals, the major hormonal signal is the androgen testosterone, which is secreted by the fetal testes. In human males, testosterone secretion occurs prenatally, and this testosterone enters the developing brain and binds to androgen receptors, which act as transcription factors to shape sex differences in gene expression. In humans, the process of sexual differentiation is largely completed by birth. In females (both human and rodent), the process of sexual differentiation proceeds along a “default path” in the absence of an active hormonal signal, but also has a similar critical period.”
“Microglia and Beyond: innate immune Cells As Regulators of Brain Development and Behavioral Function”
Kathryn M. Lenz, and Lars H. Nelson
13 April 2018 doi: 10.3389/fimmu.2018.00698