Courtney's question made me think of a video i watched in sociology while i was in high school called the Boy Who Was Turned into A Girl. It was about twin boys who were born and while being circumcised one boy's penis was burnt off accidently and could not be repaired. The parents decided to try changing him to a girl and raised him and dressed him as a girl. The child never felt right though. She didn't like playing with girls and knew something was different about her. Her parents were finally forced to tell her what had happened, and after suicide attempts she was eventually turned back into a male and he married and led a normal life for awhile.
I don't kow if anyone else has seen this video, but I think it does a good job of showing that nurture, the way someone is raised and what they are exposed to as a child, isn't the determining factor of what gender they feel like they most identify with. It shows that biological factors have a lot to do with the gender of a person, and it isn't a choice or a mental disorder.
Here's a link to an article related to the same video I was talking about
http://www.cbc.ca/news/background/reimer/