Q: What can be done to stop a 19-year-old from biting his finger nails? This has been an ongoing habit from early childhood. He’s obviously damaging his fingers and maybe even his teeth. Verbal correction and even punishment has not stopped him. Maybe it is not that big a deal. Please advise.

A: You’ve come to the right guy. I happen to be an expert on nail biting because I bit my nails well into adulthood. Why? Don’t know. When and how did I stop? Don’t know. One day I realized I was no longer biting my nails and had to buy a nail cutter. Like your son, I also chewed the skin around my nails. When I was in my mid-20s, I developed a potentially serious blood infection from doing so. I was given penicillin, to which I developed a serious allergic reaction. That’s probably what cured me, but I don’t recommend blood poisoning as a solution to nail-biting.

My expert answer to the question of how to get your son to stop nail-biting is “I don’t have a clue.” The problem is that nail-biting has no single cause. Yes, many nail-biters are highly anxious, but some are not, and most highly anxious people do not bite their nails. Therefore, every cure for nail-biting has to be customized to the person in question.

Medical doctors and psychologists refer to nail-biting as onychophagia. Most nail-biters don’t develop any serious side effects, but prolonged biting can result in dental problems, viral and bacterial infections, and parasites. But giving this information to nail-biters in an attempt to scare them into stopping rarely works.

I do most definitely know that the more attention people pay to your son’s habit, the more difficult it’s going to be for him to stop. The more things you and others say to him, the more pressure you put on him, the more concern you express, the more you point out to him that he’s biting his nails, the more critical you are or seem to be, the more difficult it’s going to be for him to exercise the requisite self-control. And let me assure you that at his age, the only person who can get your son to stop biting his nails is your son.

Online, one finds various “cures,” mostly involving bitter stuff painted on and around the offender’s nails. Parents who’ve used products of this sort usually tell me their children become accustomed to the taste and keep right on biting. At your son’s age, clinical hypnosis might be helpful, but only if he is highly motivated to stop. Contrary to myth, hypnosis cannot cause someone to do something - give up smoking, for example - that the person does not already want to do. Also, stage hypnosis and clinical hypnosis are horses of two entirely different colors. The purpose of the former is to entertain; the purpose of the latter is to re-train. If there are clinical hypnotists in your area, they can be found under “Hypnosis” in the Yellow Pages.

Family psychologist John Rosemond answers parents’ questions at parentguru.com.