While going to the Great Wall, we got into a conversation with our driver about this. He said that he remembered when he was in school, he had to fill out some forms, and one of the questions dealt with his ethnicity. He didn't know the answer, so he asked the teacher. The teacher said to put Han which he did. So, now he is Han for the rest of his life. A piece of unverified trivia. The Chinese flag's five stars were supposed to represent the five major ethnic groups - Han (the big star), Manchu, Mongolians, Tibetans and Uyghurs. I think revisionist history now gives the stars political significance, with the big star representing the CP.