You may see a lot of old photographs and realize that no one usually smiles when their picture is being taken. There was a time when photographs were only done for the dead. They are propped up to make it seem like they were still alive. Of course, the dead will not have the ability to smile. Another possible reason is the state of people’s teeth back then.
Dental health was not very popular so people were more prone to getting cavities. It seems that for every decade since the birth of the camera, people started smiling more. It was in the 30s when people started smiling a little bit. It has paved the way for people to start smiling whenever their pictures are being taken.
You may have probably looked at old photos and noticed that people back then most do not smile in their photographs. The reasons behind this are first, cameras that were used before have long exposure time. It might take an hour to develop a photo so it is much better not to smile and sit or stand as still as possible.
This is to avoid blurry photos or they will need to retake the picture again. Second is photography back then was heavily influenced by paintings which are also needed to be as still as you can be. Photography in the old times was used to present a person, not to record their lives.