Nail biting usually begins during childhood, can increase significantly during adolescence, and declines gradually, although the habit may continue into adulthood. The considerable increase in onychophagia (nail biting) in teens may be explained by anxiousness relating to the transitional phases in an individual's life, and the feeling of uncertainty associated with those phases.
According to the Diagnostic and Statistical Manual of Mental Disorders, fifth edition, nail biting is classified with "obsessive-compulsive and related disorders." In the 10th edition of the International Classification of Diseases, nail biting is classified with "other specified behavioral and emotional disorders with onset usually occurring in childhood and adolescence," such as thumb sucking, excessive masturbation, and nose picking.
Nail biting is an underrecognised habit, and its precise cause is debatable. The causes of nail-biting may include psychological, acquired, or familial factors, although it is unknown whether familial factors relate to imitating the nail-biting behavior of other family members, or to a genetic predisposition to nail biting.
Of interest, nail biting is more prevalent among children whose parents were nail biters, even when parents reported ceasing the habit before their children were born. Onychophagia (nail biting) can sometimes affect individuals who are psychologically stable, but it is usually a sign of loss of control over difficult tasks.
The principal psychological factors found to be associated with this behavior are stress, nervousness, anxiety, and low mood. In fact, nail biting was shown to be a coping mechanism in stressful conditions, and patients whose nail biting may be described as an addiction may experience more distress when trying to refrain from the nail biting habit.
On the other hand, a lack of stimulation (low activity, boredom) may also trigger or exacerbate nail biting. Hunger and low self-esteem are also known to be possible causes for this oral habit.
Onychophagia is thought to be an automatic, unintentional behavior. In adulthood, researchers suspect that alternatives to nail biting are smoking and chewing gum. One theory is that onychophagia is a continuation of the habit of thumb sucking.
In most cases, onychophagia is isolated and considered to be only a cosmetic problem. However, when co-occurring are conditions present, they are usually psychiatric in origin. At least one comorbid psychiatric disorder was present in more than two thirds of children referred to a psychiatry clinic who were nail biters, and more than one half of the parents of those children had at least one psychiatric illness, especially major depressive disorder. However, in one study, no link between the frequency or age of onset of onychophagia and psychiatric illness was found.
The mental illnesses most commonly reported in association with nail biting in children and adolescents presenting to a mental healthcare clinic are attention-deficit/hyperactivity disorder (74%), oppositional defiant disorder (36%), and separation anxiety disorder (20%), followed by enuresis (15%), tic disorder (12%), obsessive-compulsive disorder (11%), mental retardation (9%), and major depressive disorder (6%). Generalized anxiety disorder and panic disorder have also been reported.