BOSTON -- Speaking at a banquet for White House photographers a few years ago, President Ronald Reagan couldn't resist taking advantage of a moment when he thought cameras were forbidden.

The president raised his hands, stuck his thumbs in his ears and wiggled his fingers at the audience of shutterbugs. Without saying a word, he got his message across.

The meanings of individual gestures -- from pounding a fist for emphasis to outlining a woman's silhouette in the air -- often seem clear. Yet the purpose that hand gestures serve for speakers and listeners is a puzzle, says Robert Krauss, professor of psychology at Columbia University.

"Emblem" motions probably are best understood. Waving goodbye, making the thumbs-up sign and drawing the index finger across the throat like a knife are so unambiguous to those who know their meanings that they can serve in place of words.

But even such clear-cut signs have different meanings in different cultures.

For example, twirling the index finger around the ear -- a sign Americans use to mean "crazy" or "loony" -- is used in the Netherlands to indicate that someone is wanted on the phone, Krauss said. He spoke (gesturing occasionally) at a symposium on nonverbal communication during the annual meeting of the American Association for the Advancement of Science.

More perplexing are "illustrators" or "speech focus gestures," ones that almost never occur without speech, Krauss said. Although they can't substitute for words, illustrators seem to be related to the speech they accompany.

In this category are rhythmic hand beats that lend emphasis, much as voice intonations do, and gestural pictures, such as a golfer would use while describing a ball's arching path through the air.

By studying videotapes of people describing pictures, researchers learned that women tend to move their hands more rapidly, but no more often, than men do. The duration of gestures and the distance they covered was about the same for both sexes, Krauss said.

Not too surprisingly, people describing scenes that suggested action used more rapid motions.

Perhaps the most intriguing findings, however, come from studies that try to tease out the contribution that gestures make to conversation.

In one set of experiments, researchers asked subjects to describe an abstract pattern of lines to a person sitting across the table. Another group of subjects then tried to pick the right pattern from a collection of similar designs after either viewing a sound videotape of the speaker's description or listening only to the soundtrack.

Seeing the speaker seemed to give listeners an edge: Those who saw the audio-video tape picked the correct pattern more often than did those who only heard the sound track, said Krauss.

But when the experiment was repeated with a slight twist, the results were different.

This time, researchers videotaped people talking over an intercom to describe the figures to someone in another room. When sound videos or soundtracks alone were played back to observers, being able to see the speaker did not improve their ability to pick the right pattern.

However, subjects in the intercom experiment were more accurate than those in the face-to-face experiment. Perhaps this was because people talking over the intercom couldn't see their partners, so they put more effort into verbal descriptions.

Experimenters also have found links between the duration of a gesture and the speaker's familiarity with whatever he or she is talking about. The less familiar the word or subject, the longer the gesture. This suggests that gesturing somehow helps retrieve words or concepts from memory, Krauss said.

Taken together, the evidence on hand gestures points to this: You may be understood just as well without them, but you may have to try harder to say what you mean.