Ask them the question, on a scale of 1 to 10, where 1 is breaking your nail and 10 is learning your mother has cancer, how bad was such and such an incident. The answer is always 10.
He was terrified of the high notes, full of the usual performer's superstitions: a bent nail kept in his pocket, and a quick cry of "Malocchio!" if anyone mentioned bad luck.
Rather, I suggest, nail biting is just the result of a number of factors which - due to random variation - combine in some people to create a bad habit.