Believe it or not, estrogen plays the opposite of its expected role during fetal development in the brain and masculinizes the brain. Under normal circumstances, the female fetus is exposed to no hormones and takes the default development phenotype (female). Meanwhile, the male fetus is full of aromatase enzymes in the brain, and when the surge of testosterone occurs, the sexually dimorphic circuits grab those androgens and turn them into estradiol.
The part of the brain that’s sexually dimorphic and has lots of aromatase enzyme becomes more “male” when the estrogen receptors are activated, derived from testosterone converted to estrogen. A female fetus has an absence of hormones, and takes the default “female” pathway. This process takes place independently of androgens, which is why a male that has low estrogenic activity pre birth could look physically masculine but have an attraction to men or be more “feminine” mentally.
