You can find articles that explain various reasons why people don't smile in old photographs, at least when they know they are being photographed. Mimicking wh

You can find articles that explain various reasons why people don't smile in old photographs, at least when they know they are being photographed. Mimicking what they saw in paintings seems the most likely to me. The advertising around cameras and photos tended to make them painting like, and had more serious subject matter. Though other reasons are cited too...bad teeth, inertia from when exposure times had to be long, etc. So the smiles could just be the effect of them not knowing they were being photographed.