I learned the way a monkey learns-by watching its parents (Prince Charles)

Many children and adults bite their nails. Very often, these parents and children live(d) in the same house.

Its original cause probably is stress, but very often nail-biting has grown into a habit. It can do a lot of damage to the front teeth. They can be chipped or simply wear out, because front teeth are not meant for chewing. The gums can also be damaged and infections can be spread. Both the nails and teeth look neglected and people hate to see or hear (....) someone else biting his or her nails.

Some doctors prescribe anti-depressants to stop the habit. I think that is a 'kill or cure remedy' - why on earth should someone who is not depressed take anti-depressants? There are better solutions:

Two golden-agers were discussing their husbands over tea: "I do wish that my Elmer would stop biting his nails. He makes me terribly nervous." "My William used to do the same thing," the other woman replied. "But I broke him of the habit." "How?" "I hid his teeth."