I have to correct my original comment, it's actually the presence of the Y chromosome, and consequently the reaction of androgen receptors to testosterone in t

I have to correct my original comment, it's actually the presence of the Y chromosome, and consequently the reaction of androgen receptors to testosterone in the womb that causes the physiological change on the male fetus. https://en.wikipedia.org/wiki/Androgen_receptor#Effect_on_development https://en.wikipedia.org/wiki/Androgen_receptor#Effect_on_de... This event early on in the embryo's development is tantamount to dividing males from females.