I was a first born who had two sisters follow in quick succession plus my father, who was a reserved intellectual, also did not necessarily provide me with a looming masculine presence. As a result, I remember thinking of myself as almost genderless as a child and did not have a strongly developed sense of being a male.
I eventually did learn to do all the things I was expected to but my core identity remained essentially female. I just kept quiet about it.
How much nature plays into the initial equation is hard to know, but there are many cases of the only boy born into a family of mostly girls who does not become transsexual hence it must be a series of very unique factors in combination that make us who we are.
I believe that these factors are so intricate that you would not be able to precisely reproduce them in another child. It is the variability and unpredictability in a series of perfectly timed events that we do not control which must do it.
The fact that I am okay with all this now makes me no less fascinated with the mechanics.