The biology of gender is still in debate even today, the case of David Reimer shows gender isn't fully learned and there is some biological component but nature of this biological component is not known. There seem to be two things kids innately know how to play in gendered ways and their own gender but how to two relate is not known. The possibility are:
- Kids figure out which gender group they fit in to then identify as that gender.
- Gender is hardcoded in how their brain developed. And then learn how to play in gendered ways.
- Both are inborn and really don't have much to do with each other.