its called dermatillomania. its a form of SI, but not because you WANT to bleed or feel pain etc, but its a nervous habit. I have been doing it since i was a young child. ive had several hair stylists tell me to get anti-dandruff shampoo cause when i scratch, a lot of skin flakes fill my hair, like dandruff. its really embarrassing. when i was in middle school i was picking at my scalp in the girls locker room before gym and a bunch of people saw me and thought i had lice and one kid grabbed my head and started looking through my scalp to see if she could see lice, i couldnt get away cause she was too strong. i was so embarrassed. from then on there was a rumor everywhere at school that i had lice. everyone hated me from then on. it sucked. but thats middle school for ya.