During the first century A.D., the Roman teacher of oratory Marcus Fabius Quintilianus argued that gestures could enhance the impact of the spoken word. In his 1970 best-seller, Body Language, Julius Fast went one step further. He contended that the way people stand, sit, walk, cross their legs, use their eyes or move their hands offers clues to their unconscious motives and meanings. Fast, who is 59 and the author of 15 books (including What You Should Know about Human Sexual Response and The Pleasure Book,), offers additional proof of his theory in The Body Language of Sex, Power and Aggression, currently out in paperback. Manhattan-born and a graduate of New York University, Fast served as a bacteriologist with the Army during World War II. He met his wife of 31 years, Barbara, on a blind date in 1945, and they are the parents of two daughters, 25 and 26, and a son, 22. While Barbara, 54, awaits the publication of her book on intimacy, the Fasts are working together on a new project, Metacommunication, concerning all that goes into a message in addition to the words. Fast, who each year spends six months in an East Side apartment and six months in Connecticut, discussed his concept of body language with Christopher P. Andersen of PEOPLE.

How soon do infants start using body language?

Amazingly, many start to use gestures that are inherently masculine or feminine when they are only a few weeks old. After several years the average child is definitely using the body language associated with his or her own sex. When my son was 5, he was taking much larger steps than when my daughters were at that age. He would always look me straight in the eye when I scolded him, while my daughters would lower their eyes.

Does body language develop in adolescence?

Yes, the male throws his shoulders back to emphasize their breadth, and when he walks he swings his arms from the shoulders. The female learns either to carry her breasts provocatively or to hunch forward shyly to hide them. And when she walks, her arms are held close to her body, swinging from the elbow down.

Why is this?

The behavior is clearly learned. We reward boys for acting bolder, more aggressive. Girls are expected to be quiet, gentle, passive—and to use their bodies accordingly. As our perceptions of male-female roles change, our body language will almost certainly change.

What are some other "gender signals"?

For starters, women tend to close their eyes more slowly than men. And while men seldom show the palms of their hands, women use this as a courting gesture.

Are there many courting gestures?

Yes. For both sexes, prolonged eye contact—I call it the "eye lock"—is the most important. If you hold another person's eye longer than, say, two seconds, it's a clear sign that you're interested.

How can a man usually tell if a woman is interested in him?

Courting signals are generally unconscious. Touching the lips with the tongue a la Marilyn Monroe is an obvious one, as is brushing back one's hair away from the face with the hand. Crossing the legs to expose some thigh is common. Equally common to men and women is fondling something—a key chain, a glass, a cigarette.

What are courting signals for gays?

Eye contact is as standard a signal among gay men as it is among heterosexuals. Once a red necktie was a sign to announce that a man was gay. Now the most common signal is a single earring or a bunch of keys clipped to the belt—on the left to indicate that a man is aggressive and on the right to indicate that a man is passive. Handkerchiefs, half-tucked in the back pocket, are also widely used in the gay community. Black handkerchiefs signify a predilection for S&M, green is worn by fanciers of bondage and discipline. Blue handkerchiefs are worn by practitioners of conventional gay sex.

How significant is the way a person crosses his or her legs?

Men usually cross their legs with knees open. Strangely, we spend a lot more time analyzing the way women cross their legs. Psychologist John Blazer has even come up with several categories. The conformist, says Blazer, sits on one leg. The schemer crosses above the knees and dangles one shoe, but she seldom delivers. The perfectionist crosses above the knees and twists her legs. The organizer keeps her crossed legs parallel. But unless you know the person involved, I don't think these types always apply. After all, a woman may cross her legs a certain way because she's wearing a skirt instead of pants.

Are men or women better at reading body language?

Women are usually more sensitive to body language. Five Harvard psychologists ran a series of tests and discovered that in 81 out of 98 groups, women were better at interpreting gestures and facial expressions than men.

How does the size of a person's eye pupils influence the way others see him?

University of Chicago psychology professor Eckhard Hess showed two identical photographs of the same woman to a group of men. The only difference between the pictures was that in one the pupils were retouched to look bigger. None of the men noticed the difference but all thought the woman with the small pupils was cold and selfish, the same woman with larger pupils soft and much more feminine.

Why do larger pupils apparently make a person more attractive?

Your pupils dilate when you are interested in the subject you are viewing. Hence large pupils create the impression that the individual is more interested, more involved. So does an increased rate of blinking.

What can the way a person handles his eyeglasses tell you?

Eyeglasses can be curiously Freudian in their symbolism. For instance, if you don't see "eye to eye" with someone, you may unconsciously remove your glasses. Some body-language watchers think pushing glasses up on the forehead is a sign of honesty and openness, while touching both ends together is a sign of stress. Glasses on the tip of the nose may denote skepticism.

How does body language work in an office setting?

You can tell a lot about your fellow workers just by watching the way they use their bodies in the office. A boss, for example, will often emphasize his position of superiority by looming over the desk of his employees. In meetings, he will acknowledge his executives with a quick glance and an instantaneous flash of a smile—anything more would make him look as if he were playing favorites. Fellow executives, on the other hand, will avoid catching and holding somebody else's eye while the boss is talking. That could give an impression they were plotting against him. These are subconscious feelings, but real nonetheless.

You have said that we can learn a great deal about body language from talk show hosts. What do you mean?

Merv Griffin is perhaps the most obvious case. He is overly intense and wide-eyed, and his guests and viewers alike are convinced of his interest in the subject at hand. Mike Douglas nods at every statement and frequently clutches at his guests' arms or shoulders to project the same image. David Frost leans into his guests, though he seldom touches them.

What about Johnny Carson?

He's at the other end of the spectrum, with that desk between himself and everybody else. He seldom leans toward his guests—that would be a sign of intense interest—and remains somewhat aloof. People come to pay homage to him as much as they come to be interviewed—or at least that is the message his body conveys.

Is it possible to lie with body language?

Yes, but it is much more difficult than it is to lie verbally. The most graphic example I can think of was Richard Nixon's 1970 press conference in which he tried to explain our incursion into Cambodia. His voice was smooth and all the body movements projected sincerity. But one TV cameraman zeroed in on the President's fist. It was clasped so tightly that his knuckles were white—a tense gesture that to me contradicted everything he was saying.

What do you think of Jimmy Carter's body language?

Actually, Carter is not very good at concealing his emotions. He has a quick, nervous smile that he flashes as soon as he is confronted with a question he would prefer not to answer. It's a dead giveaway, and one the President should work on controlling.