well, I also think that you are born with it.
because when I see that one kid in my kindergarten, she is acting like a boy and if you ask her if she is a girl or a boy she always says " I´m a boy". She just likes her hair short, she is just wearing pants etc.
I´m pretty sure she is born in the wrong body.
Or we also had this one boy, he acted like a girl and he liked girly coulours. like pink.