As people asking Google this question have accurately observed, smiles are grimly absent from early photographs. Portraiture was at the heart of photography’s appeal from its very invention. In 1852, for instance, a girl posed for her Daguerreotype, her head slightly turned, giving the lens a steady, confident, unsmiling look. She is preserved forever as a very serious girl indeed.

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That severity is everywhere in Victorian photographs. Charles Darwin, by all accounts a warm character and a loving, playful parent, looks frozen in glumness in photographs. In Julia Margaret Cameron’s great 1867 portrait of the astronomer John Frederick William Herschel, his deep melancholy introspection and wild hair kissed by the light give him the air of a tragic King Lear. Why did our ancestors, from unknown sitters for family portraits to the great and famous, become so mirthless in front of the lens?

You don’t have to look very long at these unsmiling old photos to see how incomplete the apparently obvious answer is – that they are freezing their faces in order to keep still for the long exposure times. In Julia Margaret Cameron’s portrait of Tennyson the poet broods and dreams, his face a shadowed mask of genius. This is not simply a technical quirk. It’s an aesthetic and emotional choice.

People in the past were not necessarily more gloomy than we are. They did not go around in a perpetual state of sorrow – though they might be forgiven for doing so, in a world with much higher mortality rates than in the west today, and medicine that was puny indeed by our standards. In fact, the Victorians had a sense of humour even about the darkest aspects of their society. Jerome K Jerome’s book Three Men in a Boat is a revelatory insight into the Victorian sense of humour – it’s rollicking and irreverent. When the narrator drinks some water from the river Thames, his friends chaff him that he will probably catch cholera. It’s a startling joke to make in 1889 just a few decades after cholera had ravaged London. But then Chaucer wrote The Canterbury Tales, which can still raise laughs, in the century of the Black Death. And Jane Austen found plenty to giggle about in the era of the Napoleonic wars.

Facebook Twitter Pinterest Summer Days by Julia Margaret Cameron. Photograph: Stapleton Collection/Corbis

Laughter and gaiety (to use an old word) were not just common in the past but institutionalised far more than they are today, from medieval carnivals in which entire communities indulged in riotous comic antics to Georgian printshops where people gathered to look at the latest funnies. Far from suppressing festivals and fun, the Victorians, who invented photography, also created Christmas as the secular feast it is today.

Today, we take so many smiling snaps the idea of anyone finding true depth and poetry in most of them is absurd.

So the severity of people in 19th-century photographs cannot be evidence of generalised gloom and depression. This was not a society in permanent despair. Instead, the true answer has to do with attitudes to portraiture itself.

People who posed for early photographs, from earnest middle-class families recording their status to celebrities captured by the lens, understood it as a significant moment. Photography was still rare. Having your picture taken was not something that happened every day. For many people it might be a once-in-a-lifetime experience.

Facebook Twitter Pinterest ‘Rembrandt’s portraits would look very different if everyone was smiling in them.’ Rembrandt’s The Jewish Bride. Photograph: Alamy

Posing for the camera, in other words, did not seem that different from having your portrait painted. It was cheaper, quicker (even with those slow exposure times) and meant that people who never had a chance to be painted could now be portrayed; but people seem to have taken it seriously in the same way they would a painted portrait. This was not a “snap”. Like a portrait painting, it was intended as a timeless record of a person.

Oil portraits are not that packed with smiles, either. Rembrandt’s portraits would look very different if everyone was smiling in them. In fact they are full of the consciousness of mortality and the mystery of existence - nothing to smile about there. From the ruddy glare of Velazquez’s Pope Innocent X to Titian’s intimately serious Violante, few of the painted portraits in museums are smiley faces.

Facebook Twitter Pinterest ‘Jerome K Jerome’s book Three Men in a Boat is a revelatory insight into the Victorian sense of humour - it’s rollicking and irreverent.’ Photograph: Popperfoto/Getty Images

The most famous exception is of course the Mona Lisa – and Leonardo da Vinci laboured for years to make that smile “work”. His contemporaries were amazed to see a smiling portrait. In the 18th century, artists painted smiling people – the sculptor Houdon even gave Voltaire a smile in marble – to capture the new, sociable, smiling attitude of the Enlightenment. But on the whole, melancholy and introspection haunt the oil portrait and this sense of the seriousness of life passes on from painting into early photography.

In fact this question might be reframed: Why are old photographs so much more moving than modern ones?

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For the existential grandeur of traditional portraiture, the gravitas of Rembrandt, still survives in Victorian photography. Today, we take so many smiling snaps the idea of anyone finding true depth and poetry in most of them is absurd. Photos are about being social. We want to communicate ourselves as happy social people. So we smile, laugh and cavort in endless and endlessly shared selfies.

A grinning selfie is the opposite of a serious portrait. It’s just a momentary performance of happiness. It has zero profundity and therefore zero artistic value. As a human document it is disturbingly throwaway. (In fact, not even solid enough to throw away – just press delete).

How beautiful and haunting old photographs are in comparison with our silly selfies. Those unsmiling people probably had as much fun as we do, if not more. But they felt no hysterical need to prove it with pictures. Instead, when they posed for a photograph, they thought about time, death and memory. The presence of those grave realities in old photographs makes them worth far more than our inanely happy Instagram snaps. Perhaps we should stop smiling sometimes, too.