Take my daughter. When she was three, she decided she was a boy. There was nothing anatomical or sexual about this. She had just started daycare and finally noticed the gender differences that are obvious at that age. Boys ran around, climbed on forts, played rough--all her favorites. Girls sat at tables and held tea parties. Can you say boring? She certainly did. Add that most of the girls were in dresses and she was a 100% pants lady at that age and she came home to announce that she was a boy.
My child did, and I think sometimes still does, believe wishing hard enough for something can make it happen. That belief caused her trouble as a child and still makes her life more difficult than it should be now that she is in her twenties...but that's another story. The important thing is I let her be a boy, which lasted for several months. I don't know who set her straight. Not the boys, she always kept up with them, never asked for help, did what they did, and at that age, boys are pretty tolerant. Probably a teacher finally made her see the light, or she just aged enough to realize you can be rough and tumble and still be a girl. That scale thing, remember?
Personality seems to trump dress and even society's dictates. And, again on average, a guy has a different personality. He wants to do, and is impatient with anything that calls for him to be still for long periods of time. And that's not ADD, that's perfectly normal. Unfortunately, reading is one of the things that calls for his muscles to stop flexing while the organ between his ears takes over. That's harder for an active personality than for someone who has no trouble sitting at a table and feeding tea to toys and invisible friends.
As a writer who wants to have both boys and girls read her books, I have to understand that when I consider pacing and the staging of events in my books. If I let things slow down, if I bore him for even a minute, I will lose him. I may have raised a daughter, but she had enough of that energy in her to let me understand that some folks just can't wait around for the good part. It better all be "good parts."