Re: nail biting
i think actually this problem meets somewhere with psychological immaturedness or lack of confidence,,i think when a person is doing so then he or she must b thinking some stuff abut dat they feel problem to describe or to solve it or their mind is being partly divided abut specific topic.this is scientific reason n i think by handling it one can get rid of it.
and according `to Freud that in early years of child 's development there must be a fixation, and if its not solved this habit become a part of a person.