When the kids in my neighborhood were really small they liked to dress up in lots of different costumes. Gender was not part of the equation. There were little boys being princesses or wearing fairy wings and little girls with capes showing off their muscles. Then something happened to them when they started going to school. They learned a new set of rules and bullied the younger kids into falling in line. In a few years those younger kids will turn around and do the same thing to their siblings that are just being born.