But, I know for damn sure that biology is a big part of it, and it is not simply what sex gets put on your birth certificate — an assignment that is predominantly based on whether or not one has a penis. There are numerous biological conditions, such as androgen insensitivity syndrome, 5-alpha reductase deficiency or Rokitansky syndrome, in which individuals with either XY and XX chromosomes are born without a normal vagina or uterus but also without a penis, and are nearly always assigned to be “a girl.”
It has to do with the biology of your brain.
What most people don’t know is that our brain is both literally and figuratively our biggest sex organ. The parts of our brain that control behaviors that have to do with sex and things that differ by sex are exquisitely sensitive to hormones and chemicals that can mimic hormones. They also express genes in patterns that differ between males and females.