When they started to say "Smile for the camera" or "Say cheese" :)

Jo says with the advent of good dentistry :)

When it became Instant to take the picture, so you could smile briefly as opposed to having to Hold a pose for so long.

It never changed for mug shots

Maybe when the purpose and function of photography changed?

Perhaps although it's a painting and not a photo, that's why the Mona Lisa is so special ...at least she was smiling. Seriously, I suppose it was because the purpose was to capture their likeness and not as an indication that they were having fun.

So funny that you mentioned it. I have a family picture of my mother's family and no one was smiling. They were all dressed up (for then) but no one smiled.

Maybe the photographer never told them to.

I think because to have your photo taken was a very expensive once in a lifetime event so you had to look serious.

Experts say that the deeper reason for the lack of smiles early on is that photography took guidance from pre-existing customs in painting—an art form in which many found grins uncouth and inappropriate for portraiture.

My theory is that being serious meant that you were serious. Smiling abd laughing was was children and lower classes

I agree with Mitch. My son doesn’t smile in pictures. He says people look crazy when they smile. He thinks serious pictures are more handsome.

Scott hit it on the head, long exposure times back then meant you had to sit still in one position. Otherwise the picture would come out blurry. George Eastman of Kodak introduced the first flexible roll in the 1880's. It was the first camera that you sent back to the factory to be developed, like disposable camera today. No plates, quicker exposure times as well.

So here goes….from Time magazine, Nov.28, 2016….
One possibility is dental; bad teeth caused tight lipped expressions, except that most people in the early 1800’s had bad, if any, teeth.
Another reason is because it took so long to capture a photograph it was too hard to hold a smile long enough; by the 1850’s it was possible in the right conditions to take a photo in a much shorter time, so more smiles appeared.
Photography followed customs in painting in which many found grins uncouth and inappropriate for portraiture.
A smile, or grimace, was often found in context….where 2 people might be playing poker the winner could be pictured smiling while the loser….not!
Although saints were depicted with faint smiles, wider smiles were “associated with madness, lewdness, loudness, drunkenness and all sorts of states less than decorous! What probably changed all that most was the rise of snapshot photography, which helped to democratize the art form.