Over at Mind Hacks, Tom Stafford looks at the scientific literature around onychophagia, aka nail-biting:
Psychiatrists classify it as an impulse control problem, alongside things like obsessive compulsive disorder. But this is for extreme cases, where psychiatric help is beneficial, as with other excessive grooming habits like skin picking or hair pulling. I'm not at that stage, falling instead among the majority of nail biters who carry on the habit without serious side effects. Up to 45% of teenagers bite their nails, for example; teenagers may be a handful but you wouldn't argue that nearly half of them need medical intervention. I want to understand the 'subclinical' side of the phenomenon – nail biting that isn't a major problem, but still enough of an issue for me to want to be rid of it.