for habitual nail biting
This is a fairly common human behavior. It is most common among those between 10 and 18 years old, and is more common in women than in men (about 1.5:1). The severity of the habit ranges from mild, in which one just uses their teeth to shorten their fingernails instead of the clippers, to severe, in which one has a powerful compulsion to gnaw at their nails and cuticles, inflicting serious tissue damage. It can be associated with certain psychological disorders, such as Tourette's Syndrome and Obsessive Compulsive Disorder.
What causes nail biting? Some research indicates it's reflective of inner tensions, and engaging in the habit helps to relieve those tensions. It's also possible that, at least in severe cases, it's associated with serotonin deficiency. There is evidence that SSRIs (selective serotonin reuptake inhibitors) can help those with a bad case resist the urge to chew their nails, pull out their hair, bite their cheeks, etc.