Why would gnawing on a finger make your hair grow?

Nail biting is a non-verbal gesture. In adults it is often a habit that is adopted and carried through from childhood and can become apparent in stressful situations throughout adulthood. To most people, nail biting is associated with nervousness and shyness and is an involuntary response to certain social situations. In many cases, it is an adaptor behaviour that channels nervous energy, inactivity or boredom and does not interfere with normal functioning, the only effect being cosmetic. In severe cases nail biting can become a repetitive behaviour that is intentionally performed. It can result in substantial physical damage

Playing, twirling, fiddling and hair pulling can be indicative with a number of emotions and behaviours. For instance, children that bite their nails and frequently pull or play with their hair may be nervous or have low self-esteem. Hair twirling may be a comforting action for a child who is tense and anxious, and may become habitual later in life in stressful situations. Routine and severe hair pulling (trichotillomania) can be a compulsive disorder that may be linked to genetics, anxiety, depression and obsessive compulsive disorder. It is most commonly thought to be a stress-related condition.