’TIS THE SEASON to be merry. You can hardly fail to have noticed — not when every shop manager has pumped out the jingle with such nerve-jangling insistence. No wonder I spent the past week staring neurotically into the happy faces around me. I had to resort to Angus Trumble, the art historian, for help. His book, A Brief History of the Smile, may have started out as a lecture for maxillofacial surgeons, but it developed into an overflowing cornucopia of riveting snippets, strung together to explore the many permutations of this most complex of non-verbal expressions.

The smile — from the benign curve of the contemplative Buddha to the cosmetic rictus of the on-screen presenter, from the slowly-carved simper of the ancient Greek kouros to the chemically-captured grin of the camera snap — is a fantastically versatile contortion that cultures the world over have consistently relished and constantly reassessed.

But one of the more indicative smiles you may encounter this week could be that of the Queen as Rolf Harris represents it in his recently unveiled 80th-birthday portrait, the subject of a BBC One programme on New Year’s Day.

“I’ve painted hundreds of portraits in my time,” pants an over-thrilled Harris as his chauffer-driven Mercedes sweeps up to the Palace. “But this — this one is very special: the most important portrait of my life.”

Why? How does the Queen keep such a hold on the imagination? We live in an era that distrusts the trappings of traditional symbolism, in an age of rising republicanism and rampant celebrity. Maybe the mouth that Harris fumbled so hard to capture — “I had to muck about with those teeth over and over again” — offers an answer. If the eyes are the window, then the mouth is the door to the soul.

The smile turns the key. It is, Trumble suggests, the most immediately expressive muscular contraction of which the body is capable. And yet the portrait-lined corridors of our palaces are not grinning parades. This is not only because bared teeth, as Harris discovered, are notoriously awkward to paint. The open mouth in art, explains Trumble, tends to belong to the likes of dirty old men, misers, whores and drunks, tax collectors, lunatics and ghosts.

Small wonder that our monarchs remained traditionally unamused. Power rested upon an air of decorum. The ruler’s smile was associated less with spontaneity than with measured restraint. That most immediate of human expressions remained a complex mask.

But now we live in the age of the camera. The spontaneous smile can be frozen. The private can be dished up to the public at any convenient date. Perhaps the Queen feels, as her Christmas message suggested, a little sombre this year, but a celebrity painter presents her smiling on cue for his picture. The same is true for politicians. Even the dour Gordon Brown is cajoled into showing his teeth for the camera. The smile has become a currency that purchases the allegiance of an electorate in the same way as it buys custom in the marketplace.

Just as McDonald’s staff don a grin with their uniform, politicians must smirk for the camera. They want to convince us that they are trustworthy, fresh and upbeat. And nothing, says Trumble, can achieve this as effectively as an open smile. But he adds the proviso that we have to find it plausible. And that is our problem. A smile chucked into the equation, like a free gift in a cereal packet, starts to look as cheap as some plastic toy. We are slowly debasing our most precious currency. I look forward to a season when we may be a little less merry again.

• THE PERILS of too much smiling were made plain in an article that appeared in The Times some two centuries ago. “A grinning match lately took place at Bridlington,” the paper reported on June 11, 1804. “There were three competitors for the prize, all of whom were speedily seized with the most painful symptoms in consequence of their violent contortions, and two of them died in a few days; the third lies dangerously ill.” On first reading, such “gurning” competitions sound, if not faintly old-fashioned, then anthropologically baffling. Then you recall election time. Suddenly the article seems to sound a solemn warning.

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