Nail Biting Is Most Common

Nail-biting is a common stress-relieving habit. You may bite your nails in times of stress or excitement, or in times of boredom or inactivity. It can also be a learned behavior from family members. Nail-biting is the most common of the typical "nervous habits," which include thumb-sucking, nose-picking, hair-twisting or -pulling, tooth-grinding, and picking at skin.
You may bite your nails without realizing you are doing it. You might be involved in another activity, such as reading, watching television, or talking on the phone, and bite your nails without thinking about it.