Gender?

"Testosterone from the fetal testis is aromatized to estradiol within the fetal brain.
During the critical period estradiol causes organization of the CNS. The
organizational effects which occur during fetal development in long gestation
species or postnatally in short gestation species are permanent. In contrast the
activational effects of the hormones are transient and occur in adulthood, ie
puberty. Exposure to testosterone / estradiol during the fetal period
masculinizes and defeminizes the brain. Masculinization of the nuclei is essential
for male-typical adult behaviors. Defeminization is the loss of adult femaletypical behaviors and suppression of estradiol to induce an LH surge."


http://www.cvmbs.colostate.edu/bms/PDF/640_TD_sexdiffnotes.pdf

http://www.wiringthebrain.com/2010/04/wired-for-sex.html

http://endo.endojournals.org/content/150/9/3980.full.pdf+html