AS your comment re: environment of the mother's uterus earlier brought to mind an article I read forever ago. My DD1 is 100% tomboy ... the girl has been into boys things since before she was 2. She would walk into a store and pick out trucks and never bat an eye at princess stuff; wouldn't wear a skirt/dress from the time she was old enough to object. At preschool and kindy she has always chosen to be friends with the boys. I always thought it was fascinating because she didn't have an older brother or some male cousin or anything, it's just been part of who she is. She's not at all gender confused in case anyone was wondering that though; she's completely comfortable with the fact that she is a girl, she just says "I'm a girl who likes boy stuff."
Anyhow, I had mostly forgotten about this study I read, but it says that "tomboys are born and not made" ... basically, the level of maternal testosterone during pregnancy might influence the gender-role behavior of preschool girls. It's pretty fascinating to me in particular, because I would say that DD1 (who was a BCP-fail baby) was conceived during one of the times in my life that was undoubtedly a very high T time.
Study Suggests That Tomboys May Be Born, Not Made