Getting your Trinity Audio player ready...

When Jude Law wanted to make himself look like a rotten, no-good, slimy scumbag in the movie “The Road to Perdition,” he knew just what to do: He gave himself lousy teeth.

Those teeth, which one critic described as “antique Chiclets,” were a dead giveaway, the incisor equivalent of a neon sign flashing, “Caution! Creep Ahead!”

Law was counting on the audience’s recognition of a primitive visual polarity: White teeth mean good guy. Yellow teeth, bad guy.

And while the film, released last year, is set in 1930s America, the use of teeth color as an ethical tip-off harks back centuries and ignores national borders. The whole world wants white teeth — boldly, brashly, vividly, sometimes obscenely and insanely white teeth.

But why? Why are white teeth — which, after all, are no healthier than not-so-white teeth — so desirable? What cultural siren-song is emitted by a dazzling half-moon display of charismatic choppers?

One thing is clear, say scholars and anthropologists: The wild yen for gleaming teeth didn’t start with Crest Whitestrips.

Yet those sticky little tabs have joined an array of new teeth-whitening products and procedures that can do the job better and more cheaply than ever before. According to a consumer research group, 132 new teeth-whitening devices have been introduced in the past five years, including peroxide-laden strips pressed on teeth, liquids brushed on teeth, teeth-whitening toothpaste and teeth-whitening gum. Over the same period, the American Academy of Cosmetic Dentistry reports, customer requests for dentists to perform teeth whitening procedures — which can include power bleaching or trays filled with a whitening solution that are fitted on patients’ teeth — have risen more than 300 percent.

The phenomenon is not restricted to America, even though we’re sometimes derided as a land of shallow, superficial consumers with the leisure time and disposable income to worry about trivial matters such as white teeth. People in Germany, Finland, Canada, Brazil, Bosnia, Hungary, Malaysia, Austria, Czech Republic (the name as published has been corrected in this text) and Iran also desire alabaster smiles, according to dentists who tend to their teeth.

The simple explanation: White teeth are coveted because, now more than ever, we can attain them. Teeth whitening has become so cheap and easy that the only excuse these days for dingy grins is apathy or deliberate contrariness. It’s a quickie route to aesthetic self-improvement, much easier than, say, grunting through a gym workout or enduring Botox injections.

The same process occurred with orthodontics. Once the exclusive province of the wealthy, straight teeth moved into the middle classes as the price of braces came down. Kimberly Moffitt, a communication professor at DePaul University who studies conceptions of beauty, notes that anthropologists used to be able to discern people’s social class by peering at their teeth — straight meant rich, not-so-straight meant straightened circumstances — yet as orthodontics became more affordable, the classification technique lost its legitimacy.

White teeth, however, reach even deeper into the psyche. We yearn for them because somewhere in our souls, we associate whiteness with hygiene, morality and youth. Herman Melville , that is, was on to something when he added a chapter titled “The Whiteness of the Whale” to his novel “Moby Dick” (1851), rhapsodizing, “Whiteness refiningly enhances beauty, as if imparting some special virtue of its own, as in marbles, japonicas, and pearls; and . . . various nations have in some way recognized a certain royal pre-eminence in the hue.”

Even though teeth naturally darken as people age, helped along by savage stain-inducers such as coffee, cigarettes and red wine, most still want their teeth to look like they did before the bad habits kicked in.

“We’re all programmed to appreciate white teeth,” says Michael Jorgensen, associate professor at the University of Southern California’s School of Dentistry. Hollywood, moreover, has helped with the hard-wiring. Movie and TV actors sport bigger, bolder, more bizarrely white teeth than ever before, adding to the cultural pressure for regular folks to sport perfect smiles as well.

Jorgensen and a colleague recently published a study proving the safety of over-the-counter teeth whiteners. He attributes the increase in the number of such products to the fact that we’ve conquered other dental challenges — the conditions that cause pain and suffering. What’s next to obsess over? Appearance, naturally.

“Over the past 20 years or so, the incidence of tooth decay has decreased quite a bit,” Jorgensen says, citing the fluoridation of water and other factors. “So people have gotten beyond treating decay and gone on to a higher level of improving what you’re born with.”

But why white?

But why white teeth? Who decided that white teeth were the ultimate in oral allure, when did they decide it and why did the rest of the world go along?

It’s one of those questions that sounds too obvious even to ask out loud — why, of course everybody wants white teeth; what are you, stupid? — until you try to answer it, whereupon it stumps even veteran dentists.

“I think you need to talk to the anthropologists and psychologists,” shrugs Dr. Saul Legator, a Chicago dentist who practices on the Northwest Side, when asked about the origin of white teeth as a beauty ideal. Like so many people who muse on the topic, however, he can’t resist taking an anthropological flier: “Wedding gowns are white. And there may be religious overtones.”

White functions as a symbol of virility and cleanliness in areas other than teeth — but teeth are a perfect public stage for white’s come-hither quality.

Moreover, Legator adds, styles of smiling changed dramatically in the 20th Century. Movie stars in the 1920s, ’30s and ’40s generally offered thin, tepid, lips-only smiles — what Legator calls “demure smiles” — to the camera; by the 1990s, big, bodacious smiles from the likes of Julia Roberts and Tom Cruise had become the standard. The reason? Better dental techniques meant better-looking smiles for stars as well as the rest of us, he says. And if you’ve got it, flaunt it.

In a study of Americans’ romantic proclivities conducted by a toothbrush manufacturer last year and published by The Christian Science Monitor, some 75 percent of women surveyed said white teeth were a major factor in male sex appeal. And one of the most hotly discussed novels of recent times is Zadie Smith’s “White Teeth” (2000), a novel that uses teeth as a metaphor for a cultural ideal that survives the mingling of cultures, religions and generations.

Smith was clearly on to something.

“In ancient India and Egypt, in Summaria and Babylon, they had all kinds of materials for whitening teeth,” says David Chernin, a dentist and clinical instructor at Harvard University. He and a colleague, Gerald Sklar, are the authors of “A Sourcebook of Dental Medicine” (2002). “Humankind doesn’t change. This crosses times and religions and cultures. In India in 1500-800 B.C., they used a compound of honey and oil and other elements as a paste on a twig” to whiten teeth, Chernin adds. “In China between 33 and 1400 A.D., it was white ash and honey, using a small cloth.”

But why white when, especially for ancient cultures, maintaining white teeth was a pain in the neck?

White, Chernin speculates, always has been a cultural touchstone indicating freshness, vigor and value. “Virgins in Roman times were clad in white.”

Ellen Steinberg, an anthropologist who has taught at the University of Illinois at Chicago and other universities, notes that for centuries and across cultures, teeth have been canvases for political and social statements. Messages have been conveyed through whitening, filing and even knocking out certain teeth to indicate standing in a group. In contemporary times, “inlaying teeth with diamonds or rimming them with gold” is a widespread practice, she adds. Another teeth-altering trend, some dentists report, is tattooing.

Youth and vigor

But whiteness was and is the benchmark, perhaps because white teeth long have been associated with youth and vigor, says John Burton, a history professor at DePaul University.

“It’s not so much that the standard of beauty has changed” that explains our ravenous desire for white teeth, he adds. “It’s that it’s obtainable now.” People in the 18th Century — the area of Burton’s specialization — would have flocked to dentists to have those sepia-toned stains removed, had such procedures been available. Discolored teeth were associated with old age, bad breath, withered hopes. But what could an 18th Century guy do?

Dr. Matthew Messina, a Cleveland dentist who serves as the American Dental Association’s consumer adviser, concurs. “In years past, there was a fatalistic attitude toward teeth.” People assumed they were going to lose them anyway, so why bother with whiteness? As better care and dental techniques improved, however, “People think, `If I’m going to keep my teeth, let’s make them look good.’

“At first, teeth whitening was seen as a personal indulgence,” Messina continues. “But now, it’s like getting your nails done. It’s OK.”

It’s OK around the world as well. “Whitening therapies constantly grew in the last few years in Germany,” reports Dr. Sebastian Ziller of the German Dental Association.

“People in Iran love to have whiter teeth,” says Dr. Ali Yazdani of the Iranian Dental Association.

Canada on bandwagon

In Canada, the desire for white teeth is “absolutely” on the upswing, declares Dr. Tom Breneman, president of the Canadian Dental Association. “It’s very interesting. As we go through life and use our teeth, they darken.” They’re supposed to darken. But just try telling that to parents, Breneman says.

“If the tooth isn’t white, the parents think it’s not perfect. A child’s first teeth are so white. As soon as the other teeth start coming in, the parents say, `What’s wrong with these other teeth?'”

No discussion of black and white — be it about teeth or any other entity — can overlook race.

Yet if teeth-whitening ever was a race issue, Moffitt believes, it was only because it was a class issue first. The wealthy alone could afford to spruce up their smiles. Now that teeth-whitening has become available to all classes, blacks and whites alike go for the big-wattage welcomes, she notes.

“We do know that when we look in the dictionary, white and black mean good and bad,” Moffitt says. “But white teeth aren’t a race issue. White teeth are valued across the board” by both races.

Thus white teeth will always be the standard for a beautiful smile.

Or maybe not, Messina speculates. “We might be into blue one day. Who knows? And it would probably start in California.”