Gender for a lot people is a very sensitive topic. People, especially from the south, don't like when things change away from the traditional values. So now in modern times with people being more open with themselves, others who are stuck in the past are quite against it. I, personally, identify as a girl and am physically a girl therefor giving me the benefit of the doubt in most situations. I have been privileged with the trait of being 'typical' but not everyone is born this way. I have no problem with anyones decision to be who they want to be and I don't think anyone should have a problem with it.
I think a lot of how you identify is in your genes and isn't something you can change. Although, while being raised, little things in movies, around town and other family members will all affect how you grow an understanding of how genders identify. Along with that, being raised in certain areas can definitely influence you on how willing you are to open up to your community which would influence how you identify. If you're in a very closed environment you may be scared to identify as something other than what you're 'supposed to' causing you to force yourself to be the way you don't want to be. While being raised in an open community you may be more willing to experiment with different things.
I think it shows from birth on in to adult hood that girls can identify as a boy a lot easier than boys can identify as girls. In elementary school I can remember lots of girls, including me, who called themselves 'tomboys' and it was normal. They were athletic and liked boyish clothes. But if a boy tried to do it they were, more times than not, ridiculed. As adults, I feel like girls are more open most of time which, once again I think makes it easier for girls to open up about identifying as a boy. While when a boy tries to identifying as a girl, girls may be more okay with it but boys won't be so accepting. Although I say this, it is not always true. You will find a ton of boys who are accepting towards these situations and girls who are not.