The cult of the Mona Lisa

She's been stolen, parodied, even used as a political pawn. And, as she nears her 500th birthday, she's still drawing six million visitors a year. So why, asks Philip Hensher, does La Gioconda have such a hold over us all?

If you make a list of 10 or so of the greatest and most famous paintings in Western art, a curious inconsistency immediately emerges. One plausible list might include the Piero Resurrection, Raphael's Sistine Madonna, Titian's Bacchus and Ariadne, the "Mona Lisa", Michelangelo's Last Judgement, The Night Watch, Las Meninas, L'enseigne de Gersaint, Ulysses Deriding Polyphemus and A Bar at the Folies Bergères. We can make our own lists, but there's one unmistakable fact; any list containing the Mona Lisa has a painting whose celebrity is of a completely different order to the others. Everybody, whether they are interested in painting or not, knows exactly what the Mona Lisa looks like.

Given that the overwhelming feeling of visitors to the Louvre, on seeing the Mona Lisa in the flesh, is that it's much smaller than they thought it would be, and subsequently to wonder why they know this painting so well, and not, say, Raphael's magnificent portrait of Castiglione which used to hang alongside it, you have to ask how this celebrity came about. It's not, of course, completely undeserved. La Gioconda, as it is known in Italy, was famous and admired from its creation. It is a major production of Leonardo's second Florentine period, from 1500-1508, and is therefore just about celebrating its 500th birthday. The importance Leonardo placed on it is apparent from the fact that he worked on it over an extended period; the hands are painted much more thinly than the face, in Leonardo's late technique.

It's a portrait of Lisa Gherardini, the wife of Francesco del Giocondo – hence the punning title. Del Giocondo was a political, rather than an aristocratic figure, and Gherardini would have been in her early twenties at the time – something which the monumentality of the portrait rather disguises. In a period when portraiture was frequently a continuation of boasting by other means, this work is remarkable for the simplicity of the presentation; no jewels, no particularly lavish dress, and no richly appointed room. Next to any of Raphael's portraits, it is daringly simple.

There is some kind of meaning here, however. The sinuous and grotesque lines of the landscape, such as the eroded rocks from the Arno valley, and the winding river, are echoed in the sinuous lines of the veil and hair. The painting as a whole is daringly limited in tonality, with the figure's clothes closely echoing the earth tones behind her. The point is to present the human body as an outcrop of the earth itself, as a microcosm of God's creation – a favourite idea of the Renaissance mind.

For Leonardo, it was a chance to demonstrate his virtuosity in sfumato techniques, of suggesting masses through delicately graduated and grained shades of light; the way that the face falls back into darkness at either side was a novel and showy technique. He was certainly very proud of what he had achieved in La Gioconda, and his contemporaries shared his admiration; it was one of the paintings he took with him to the king of France, Francis I, in 1516, and it has remained in France ever since.

For centuries, it remained an object of admiration and imitation, although the 18th century found it difficult to concede the supremacy of a mere portrait, and it fell into some neglect. It was never entirely unknown, but it was not a celebrity painting until the 20th century, and two episodes which had very little to do with art at all.

In 1911, the security at the Louvre was not what it became, and a thief had no difficulty at all in walking out with La Gioconda. In the wake of the theft, the painting acquired massive celebrity. Songs were written – "L'as tu vu La Joconde?" was one of the first, foreshadowing Nat King Cole's famous 1950 hit – along with an enormous amount of newspaper speculation. One shady character, Honore-Joseph Pieret, wrote an article saying he had been purloining statues from the Louvre for some time: the police, having no other leads, hauled in acquaintances of Pieret, including Apollinaire and Picasso, for questioning, before the painting resurfaced in 1913.

It was then the painting took on its status as The Most Famous Painting In The World. The most obvious sign of this is that, in the years after the robbery, the stream of Gioconda parodies began. The most celebrated of these is Duchamp's image, which makes up for the poor woman's lack of eyebrows with a luxuriant moustache, and affixes the mysterious title "L.H.O.O.Q." beneath. It doesn't stand for anything; however, if you say the letters in French, it comes out as "Elle a chaud au cul", a sentiment of unspeakable vulgarity.

Duchamp's joke, and Leger's La Joconde with Keys, had serious artistic intent, but since then, the painting has been through the mill. Stalin, the Kaiser, Fernandel, Miss Piggy, Monica Lewinsky and Dali have been depicted as Mona Lisa. Hundreds of people have independently been struck by the idea of portraying cats, dogs, cows, horses, monkeys and mice in a Mona Lisa wig. It has been executed in almost every foodstuff, for some reason – in pasta, coffee and, with incredible regularity, in toast.

In 1963, La Gioconda became a diplomatic tool. Relations between France and the US had become frayed in the wake of the Bay of Pigs incident, and, just as now, the French government was in search of an emollient gesture. André Malraux, then the minister of culture, persuaded General de Gaulle to lend the painting to America.

It worked. With the full panoply of publicity, including the slightly hilarious gesture of sending it in a first-class cabin on an ocean liner, and some oily speech-making by Malraux, it attracted two million gawping Americans and Jackie Kennedy in a now-famous pink dress. Malraux had no hesitation in blatantly linking art and politics: "There has been talk of the risks this painting took in leaving the Louvre. They are real, though exaggerated. But the risks taken by the boys who landed one day in Normandy – to say nothing of those who had preceded them 25 years before – were much more certain. To the humblest among them... I want to say... the masterpiece to which you are paying historic homage this evening, Mr President, is a painting which he has saved." And, just as in the case of the 1911 robbery, the public use of the painting inspired the creation of more art, in the form of Andy Warhol's Mona Lisa series. More to the point, the tour brought millions of Americans to France to see the painting again. The trick was repeated in 1973, when it was sent to Japan.

By now, it had almost entirely stopped being a painting, and was almost impossible to look at honestly, and not just because of the box it has been encased in since its return from Japan. A panel, a portrait of a woman with no eyebrows in a brown dress, executed in oil on poplar, 600mm x 470mm, 779 in the Louvre inventory with no assigned or estimable value, had long ago become a political tool. The truth is that its fame, and its status as The Greatest Painting In The World, have nothing to do with art, but with its curious 20th-century history.

Laden with meanings, almost none of which are its own, it long ago became, for all purposes, entirely invisible. In the current situation, there is only one thing for it: send it back to America.

'The Secret Life of the Mona Lisa' is on BBC1 on Sunday at 7pm