I always wondered this. I think the first black and white photos I ever saw were from the Depression era, and I just assumed people were…depressed. But I noticed later that a frown is typical of most old photos. I’ve also noticed that this is not just an American thing, but true of other cultures. When people would show me pictures of their grandparents, they would have a stern expression, and pictures of their grandchildren at the same age would be all smiles. You even see pictures of families together where the grandparents look pretty fierce and everyone else is smiling.

This video says that early on people treated a photo as though it were a portrait being painted. It was a rare thing that might only happen once and was expensive. There would just be this one picture for people to remember you by. There was also the practical matter that early on, you had to sit still for 10-15 minutes and it was hard to hold a smile that long. The video also talks about the practice of taking pictures of the dead, including children, which I found sad but it makes a certain sense. In some cases that might have been the only photo taken of the person.