Advances in cosmetic dentistry and plastic surgery have made it possible to correct facial birth defects, repair damaged teeth and tissue, and prevent or greatly delay the onset of tooth decay and gum disease. As a result, more people smile more often and more openly today than ever in the past, and we can expect more smiles in the future.

Evidence of the smile's ascent may be seen in famous paintings in museums and galleries throughout the world. The vast majority of prosperous bigwigs, voluptuous nudes, or middle-class family members in formal portraits and domestic scenes appear to have their mouths firmly closed. Soldiers in battle, children at play, beggars, old people, and especially villains may have their mouths open; but their smiles are seldom attractive, and more often suggest strain or violence than joy.

Smiles convey a wide range of meanings in different eras and cultures, says art historian Angus Trumble, currently curator of Yale University's Center for British Art, in his book A Brief history of the Smile. Compare, for instance, the varying impressions made by the shy dimples of Leonardo's Mona Lisa; the rosy-cheeked, mustachioed Laughing Cavalier of Frans Hals; and the "Smiley Face" logo perfected (though not invented) in 1963 by American graphic artist Harvey R. Ball.

In some non-Western cultures, Trumble notes, even a warm, open smile does not necessarily indicate pleasure or agreement. It can simply be a polite mask to cover emotions considered too rude or shocking to be openly displayed.

Subtle differences in muscle movement can convey enormous differences in emotion, from the tranquility of bronze Buddhas, to the erotic bliss of couples entwined in stone on Hindu temples, to the fierce smirk of a guardian demon at the entrance to a Chinese tomb.

Trumble expects the impact of Western medicine and mass media to further increase the pressure on people to grin broadly and laugh openly in public. "Faint smiles are increasingly thought of in scientific and psychological circles as something that falls short of the true smile," and therefore suggest insincerity or lack of enthusiasm, he says.

With tattooing, body piercing, and permanent cosmetics already well established as fashion trends, one can imagine tomorrow's beauty shops adding plastic surgeons and dentists to their staffs. These corner-store cosmeticians would offer style makeovers to reshape our lips, teeth, and jaw lines to mimic the signature smile of one's favorite celebrity.

What can you say to that except "Have a nice day?"