I've been sitting here for the last 20 minutes grinding my left front tooth against one of my lowers. It doesn't make much noise and doesn't really look that "Quasi" (as we used to say in prep school, referring to Quasimodo), so I doubt if anyone around me has even noticed. It's just a little something I do whenever I can't start a story, or my car, or when the other line at the Safeway is moving faster. Once, when I had to give a talk at a Kiwanis club in Michigan, I was grinding so fiercely I thought I had flakes of enamel in my mouth.

I'm not sure how all this started (though I suspect it stems from a childhood ice-skating accident, which ended up giving me false caps for front teeth), but in any case I shouldn't sweat it, according to R. Gregory Nunn. Dr. Nunn, 28, a behavioral psychologist who seems wonder fully free himself of the slightest aberration, is one of America's leading clinical therapists for nervous habits and tics.

He knows all about nail-biters (there are 40 million out there), hairpullers (10 million), stutterers (4 million), compulsive overeaters (maybe as many as 60 million), chain-smokers (50 million), head-jerkers, eye-twitchers, shoulder-shakers, bed-wetters, cheek-suckers, thumb-suckers, and, yes, teeth-grinders like me. In fact, Dr. Dunn has just coauthored a book about all of us.

Not to worry in my instance: The case is apparently unadvanced. "I really don't think it's a problem socially for you," he says, as I lean in to demonstrate my quirk. (We are sitting in a bar on Capitol Hill, drawing mild interest during Happy Hour.) "How much does it bother you that you do it? Are you ashamed? You would have to convince me of real concern before I would take you as a client. You see, I'm not a savior and I don't make perfect specimens, I just try to make life a little less anxious sometimes."

Dr. Nunn, whom you might never guess a clinical therapist for his mod clothes, shag hair and stacked heels (he comes from Southern California), helps make life a little less anxious for people at his Chicago Habits Clinic in Evanston, Ill. - the only clinic of its kind anywhere, he says. There, through motivational techniques, awareness training, situational control and something called "competing reactions," he has successfully treated over 1,000 people in the past couple of years. The patients range from 6-year-olds who wet the bed to 86-year-olds who bite their nails down to the quick. Among a group of 300 persons treated, he says, the average reduction in the habit after six months was 99.5 per cent.

His office, which features desks and chairs made of natural woods ("no chromes or plastics"), plus cacti plants, has been designed to put patients at ease. "Hell, I did it to relax me. Whenever I go to a doctor's office, I always find myself sitting nervously erect. In my office, they're usually slouching."

This sounds awfully "pop," but apparently it works like a dream. (One lady was so grateful for his service she brought him a cow puppet.) The patients have come from all over the world. He charges between $40 and $50 an hour, sometimes a flat $100 for the first session, which might run several hours. Generally, he needs to see a client only two or three times to break his tic or habit. The hairpullers, for instance, he can usually bust in one three-hour session.

It's no joke, some of the anxiety he has seen. Children with stuttering problems so bad they become "blocked." They try to talk and all you get are facial distortions, pumping stomach muscles, a beet-red complexion. Ninety-five per cent of all stuttering begins between the ages of 3 and 8, Nunn says. It's an eminently treatable problem, but should be arrested it early. Some parents let it slide.

(Nunn gives the lie to the old Freudian saw that stuttering derives from being caught at masturbation. "What kid is masturbating at 8? The psychoanalytic theories proliferated because there was no data. Once you get data, the Freudians go away." According to some Freudians, he says, nail-biting and thumb-sucking are also substitutes for masturbation.)

Nunn is not the least psychoanalytically-oriented in his therapies. "These are physical habits, not disorders, I'm treating," he says, "whether it's your eyeball jumping around in its socket or your head that's jerking involuntarily. None of them so far as I'm concerned is organic in origin. I offer short-term therapies with an intensive educational approach. The first step is to realize you're not alone." He's given workshops to psyciatrists - with great success, he adds.

The cornerstone of the Nunn method is the "competing reaction." This is an intentional physical movement that is incompatible with the habit. For nail-biters and hair-pullers, the competing reaction might by grasping an armrest or book with both hands till the urge passes. For a tie sufferer, it is the isometric tensing of muscles opposite ones that have been over-developled and cause the jerking. For stutterers it is a slower breathing technique.

This methodology is explored fully in Nunn's new self-help book. "Habit Control in a Day" (Simon and Schuster, $7.95), which he coauthored with his old psych prof from Southern Illinois University, Dr. Nathan H. Azrin. (Actually, he did most of the work, Nunn says, and used Azrin for Socratic counsel.) He's had a feud with the publisher over the slightly hyped-up title (it often takes more a day), but other than that he's delighted to think of himself as a writer.

Has Dr. Nunn ever had any nervous habits or ties of his own? "None I can remember, though I did have a chronic cough for a time as a kid. I think it was asthma-related." You'd be surprised, though, he says, the number of people who ask. "The first three people who interviewed me were nail-biters. One guy in Detroit used to have the habit so bad he had to have four teeth capped." Then there was the New York TV lady who started biting them down to the quick on the set. "Those kind can be vicious. They know and can't stop."

Maybe I have something I don't know about, I suddenly say. You know, something that's noverbally communicating unrest. "Well," he says, a smile slipping up, "now that you bring it up, I've noticed your head - it seems to move to the left a lot. Maybe your collar's tight. But don't worry about it. It's a normal anxiety reduction. Maybe you can find something to compete with it."

"How about grinding a tooth with a capped upper . . . ?"