When and why did smiling at strangers and in photos become customary in the United States?

Question

In the US, in my experience, it's customary to smile when meeting a stranger. Even in an interaction as casual as passing someone in the street without a word, a smile is normal and expected.

In other countries this is not the case. For example in Russia, smiling at a stranger is seen as odd and possibly suspicious.

In the US people also typically smile in photographs, often irrespective of what their expression was before they saw the camera. In Russia, they don't.

In very old US photographs people seldom smiled, although this could be explained by the difficulty of maintaining a smile through the lengthy exposure process.

When did smiling become the norm in the United States, and why did it happen? Or, when and why did smiling fall out of fashion elsewhere?

Answer 1

Nicholas Jeeves of the Cambridge School of Art addresses this question well in a lengthy essay:
The Serious and the Smirk: The Smile in Portraiture.

In this sense, a portrait was never so much a record of a person, but a formalised ideal. The ambition was not to capture a moment, but a moral certainty. Politicians were particularly sensitive to this. For a more modern, photographic example of the principle, we may consider Abraham Lincoln. Here was a man better known than most, in his day, for his sense of humour, there being a number of well-known stories about him regularly drawing hoots of laughter from those in his company. While there are some informal images of him looking distinctly avuncular, a wit doesn’t abolish slavery without tough critical opposition, and in his best-known image, the ‘Gettysburg portrait’, he takes on the gravest expression imaginable.

For centuries many echoed the sentiments of Mark Twain:

A photograph is a most important document, and there is nothing more damning to go down to posterity than a silly, foolish smile caught and fixed forever.

However not all schools of formal portraiture were in agreement, as witness the Dutch painters of the 17th century:

To see the smile at its biggest and best, we have to leave the upper classes and instead visit our attentions on those lower in the social order. 17th century Dutch painters were fascinated with recording the fullness of life, and deliberately sought out the smiles found within it. Here there are almost no end of artists to choose from, and in consequence ‘Dutchness’ in painting, and in life, was often a society shorthand for licentiousness. Jan Steen, Franz Hals, and Judith Leyster were all followers of this style, all painted broad smiles, and all were said to be good company, there being no attempt at separation between the artist, the viewer, and the subject. With the artists as complicit as they were explicit, it was a mutual love affair that put them firmly at the centre of contemporary life.

But the true turning point was probably in 1877:

By 1877 the photographic pioneer Eadweard Muybridge had solved the problem of fleeting movement with his series of photographs entitled The Horse In Motion. As we know from artists’ previous attempts to paint running horses, the horse’s movement was impossible to capture accurately in paint. Thanks to Muybridge’s pictures, almost overnight all the painted horses became transformed from awkward caricature into great galloping beasts. And before you could say ‘cheese’ photographers found themselves able to capture another fleeting thing: the true smile.

Other links:

1. Say 'Prunes' not 'Cheese': The History of Smiling in Photographs by Michael Zhang
2. "Smile!": A Polemic on Fine Art Portraiture by Stephanie Dean
3. Archive body starts debate on smiling for photographs by Steven McKenzie