In most old photos — those taken in the 19th century and early 20th century — people aren't smiling. That's led to the popular belief that people simply didn't smile in old photos. Like in this depressing wedding photo from 1900:

So why did people in old photos look like they'd just heard the worst news of their life? We can't know for sure, but a few theories help us guess what was behind all that black-and-white frowning.

1) Very early technology made it harder to capture smiles

One common explanation for the lack of smiles in old photos is that long exposure times — the time a camera needs to take a picture — made it important for the subject of a picture to stay as still as possible. That way, the picture wouldn't look blurry.

The picture above illustrates why early cameras made it harder to capture a smile. One figure in the center is blurry, most likely because he moved slightly during the long exposure time. In theory, you'd want to maintain as still a position as possible, and it's harder to maintain a smile than a relatively flat facial expression.

But that's only part of the story — and was really only a huge factor in the very early days of photography. As George Eastman House curator Todd Gustavson told me when I was researching the history of the selfie, exposure times had gotten a lot shorter by 1900 with the introduction of the Brownie and other cameras. These cameras were still slow by today's standards, but not so slow that it was impossible to smile.

Yet smiles were still uncommon in the early part of the century. That suggests there were also cultural reasons people didn't smile in old pictures. Any general cultural theories involve a few leaps of faith — but these try to explain why old photos look so sad.

2) Early photography was heavily influenced by painting — which meant no smiling

Today, photography is a means of recording our lives as they're lived. But in the early days of the art, it was indebted to a tradition of portraiture in painting. A photograph was a frozen presentation of a person, not a moment in time. Even the models thought so.

In 1894, the Photographic Journal of America interviewed a model named Elmer Ellsworth Masterman. He had an unusual gig — he professionally modeled as Jesus Christ for paintings and photographs. He also didn't see the distinction between the two art forms. "What is the difference between posing for a photograph and posing for a painting?" he asked.

The photographic tradition of portraiture began in part because of the technological limitations of cameras that had to take pictures slowly. But even once cameras improved, it was difficult to imagine photography as a unique art with its own aesthetics. Even when it was easier to take pictures quickly, cameras still represented an ideal of life, not a slice of it. That meant no smiling.

3) Early photographs were seen as a passage to immortality

When we snap a profile picture today, part of the goal is to look cool or to document fleeting moments. But people didn't think about their Facebook page in the early days of photography. For them, photographs were a passage to immortality.

That's especially evident in the tradition of postmortem photography. In that genre, a recently deceased person, child, or pet would be photographed as if they were still alive. Begun in the early days of photography, it had largely — though not completely — petered out by 1900. But it reveals the mentality of the time: portraiture was used as a way to preserve the living for future generations.

That meant the medium was predisposed to seriousness over the ephemeral. There's no better reflection of that idea than the words of Mark Twain — a man who made a living as a humorist and wrote stories about jumping frogs. Even he said, "I think 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."

4) Victorian and Edwardian culture looked down on smiling

The fourth argument for why people in old photos frowned is one of the most compelling — though also the hardest to prove. It's possible that many people in the early 1900s simply thought smiling was for idiots.

Nicholas Jeeves surveyed smiling in portraits for the Public Domain Review and came to the conclusion that there was a centuries-long history of viewing smiling as something only buffoons did. (Jeeves dismisses the alternative theory that bad teeth kept people from smiling — after all, if everybody had bad teeth, it probably wasn't a problem.)

Like any sweeping cultural thesis, it's a tough statement to prove, and the exceptions are abundant. For example, the Flickr group "Smiling Victorians" has 2,100 photos, and at least some of them show genuine grins. That alone is a significant counterargument. But the prevailing concept of old pictures as humorless relics seems on the mark (and is confirmed, in some ways, by the need to make a special Flickr group for pictures that aren't dour).

In the late 19th and early 20th century, the culture at large may have frowned on the smile, and it took a while to learn to love it.

But then why was this man smiling?

All that is what makes the photograph below, taken around 1904, so striking. It's from a collection of Berthold Laufer's images from his expedition to China (and featured by the American Museum of Natural History Library).

This man is definitely smiling:

We don't know much about the photo itself. But it offers a perfect opportunity to examine why it seems like people in old photos never smiled.

The clues might lie in photographer and subject. Photographer Berthold Laufer was an anthropologist, which meant he had a different mission than other photographers of his time — he sought to record life instead of pose it. That goal meant capturing a wider range of emotions. His rice-loving subject may have been willing to grin because he was from a different culture with its own sensibility concerning photography and public behavior. Both of them were outsiders to the mainstream photographic culture.

Together, they create a picture that's memorable even now. We don't know for sure why one man eating rice looked so happy — but we do know it led to a picture that can still make us smile today.