The broad grin acquired its current status as a universal symbol of health and happiness comparatively recently. Like so many of our personal habits, it emerged at the end of the 19th century. Dentistry took care of the infrastructure. Around 150 years ago, bad teeth were largely irrelevant to most assessments of facial beauty. Lord Palmerston, a Whig prime minister under Queen Victoria, was considered a model of manhood, possibly the first person to be called "tall, dark and handsome". He had a clear complexion, black hair, a small nose and piercing blue eyes. That he was missing several prominent teeth because of hunting accidents was regarded as an additional demonstration of his dash and vigour, and by no means inconsistent with his looks.
Queen Victoria herself had a short upper lip and, according to various tactful sources, exposed her front teeth more than she might have wished. Observers stated with barely disguised relief that the queen's teeth, though stubbornly visible, were small and white.
Improved dental hygiene changed all that, bringing teeth out into the open and focusing attention on especially large ones. The US president Theodore Roosevelt set the pattern of public smiling for his 20th-century successors. His big, flashing teeth fascinated Europeans, who thought them magnificent. Roosevelt became so well known for his peppy grin that when he died, in 1919, it was widely reported that the cause of death was an infected tooth (which was not true).
If better teeth had some impact on smiling habits, the invention of motion pictures utterly transformed them. The moving image captured more effectively than ever before those evanescent qualities of smiling that may be discerned only in the movement from facial repose to animation and back again. Films captured this again and again, in close-up, for a vast new picture-hungry audience, whose expectations of real smiling soared as a result.
Apart from in a small number of masterpieces, such as Frans Hals's exuberant Lute Player, these fleeting qualities had eluded the static medium of oil painting in earlier centuries. The sitters in a few famous portraits - for example, Leonardo da Vinci's Mona Lisa and Hals's Laughing Cavalier (laughing? I don't think so) - have been miscast in our time as enigmatic smilers, mysterious smilers and urbane smilers, the faintness of whose smiles is held to conceal something deep or, in Mona Lisa's case, possibly even lurid. In Old Master painting, these are exceptions. Most artists tackled open-mouthed smiling rarely, and with mixed success.
It is, without doubt, time and motion that allow us to observe in context the progress of the smile. If we are lucky, this experience will let us understand why a particular smile occurs and what it may mean. Yet there are no guarantees about that. At times, we get it fearfully wrong - such as when we mistake the desperate smile of a dinner guest trapped between two bores for a smile of genuine amusement. Instead of responding with the much-needed rescue attempt, we beam back pleasantly. Only the seasoned host knows the difference.
In the 17th century, broad grins were more readily associated with debauchery, madness or sadistic tax collectors than with what polite people did in public. Conventions of decorum made room for volatile situations in which the smile could be exchanged privately between lovers, deployed strategically in conversation and even used to provoke, enrage or, in the case of Gothic kings, seem royal. The smile brimmed over with meaning. By contrast, we have inherited something rather new: the smile as a default position. It is, without question, a devalued currency.
There are any number of situations in which we exchange smiles purely out of politeness. When, for instance, we meet people for the first time, we notice if they do not smile and may form the impression that they are cold, unfriendly or French. In some cases, we will recognise that a shy person finds this situation painfully difficult and doesn't smile for that reason.
But if the person we have just met does smile, we tend to accept it as an expression of good manners and refrain from reading that smile in any particular way. This may change within minutes, even seconds, when we begin to observe mannerisms, peculiarities or the tendency of specks of saliva to accumulate at the corner of the mouth. This may or may not make a difference to whether we find we like the person, find him or her attractive or irritating (or both), or else conclude that we have little in common. No matter what follows, the gambit tends to be conventional.
One of the most remarkable characters in Lewis Carroll's Alice's Adventures in Wonderland is the Cheshire Cat. Carroll provides no clue as to the origin or meaning of the cat or, indeed, the grin. Alice is immediately impressed by both: "The only two creatures in the kitchen that did not sneeze," writes Carroll, "were the cook, and a large cat, which was lying on the hearth and grinning from ear to ear."
There has never been a breed of cat known as the Cheshire. But neither it nor the phrase "to grin like a Cheshire cat" was invented by Carroll. The saying was well established in Georgian England. Where did it come from?
One intriguing explanation relates to a heraldic lion that was badly painted and therefore easily mistaken for the rounded form of a well-fed cat. The Egertons of Tatton Park and Arley Hall in Cheshire had a lion on their coat of arms, which was apparently painted on the signs outside inns associated with the family and their estate workers. Certainly, there was once no shortage of inns in Cheshire with a strong feline connection: The Red Cat at Brimstage, The Cat and Fiddle at Buxton, The Cat and the Lion at Stretton (not far from Daresbury, where Carroll was born) and two called The Cheshire Cat, one at Nantwich and the other at Brinnington.
But why does the Cheshire Cat grin? Elsewhere in Alice, it is made clear that while it grins all the time, some variation is possible - for example, when Alice asks it for directions and it "grinned a little wider". Perhaps it is as simple as this: Carroll was fond of cats, and Alice Liddell, his subject and muse, owned a tabby cat called Dinah, whom she loved. Be that as it may, Carroll's Cheshire Cat, much assisted by John Tenniel's remarkable illustrations, is a startling prophecy of the default-position smile that has flourished in our time.
What will happen to the smile in future? If trends in dental science continue to develop as they have done over the past century, we can expect more and more effective cosmetic treatments to enhance the appearance of our teeth. They will get whiter, and maybe bigger. Their shape and disposition will become more regular. "Gummy" smiles will gradually disappear, as will all sorts of other irregularities.
As a result, the smile, already somewhat elasticised and tooth-exposing, will probably get wider and fiercer. A faint smile will increasingly be considered to fall short of a "true" smile. Gestures of modesty that rein in the smile in other parts of the world will be further challenged by the habits that children learn from the mass media. The old remark about somebody who smiles an awful lot - namely, that "they are all eyes and teeth" - may never be heard again.
Angus Trumble is curator of paintings and sculpture at the Yale Centre for British Art, New Haven, Connecticut, and author of A Brief History of the Smile (Basic Books)