Brain–Sex Differentiation: Biology, Time, and the Aging Brain

Neuroscience has long been fascinated by the question of how sex-related differences emerge in the human brain, not because the answers are simple, but because they consistently reveal complexity rather than clarity (Hall & Hall, 2020). Brain–sex differentiation refers to a set of biological processes through which neural structure and function are shaped by genetic signals, hormonal environments, and developmental timing (Barrett et al., 2019). This process must be carefully distinguished from gonadal sex and from gender identity, both of which follow different biological and psychosocial trajectories (Carlson & Birkett, 2021).