This is not true. When cameras are first introduced to most societies, people don't smile for them. Often they even very intentionally stop smiling and adopt a serious/formal expression. Always smiling for the camera is a learned cultural behavior reinforced through pervasive social pressure, which took many decades to form and which was helped along by the heavy use of photography for advertising/propaganda where showing smiles increased sales, not a natural or predestined result.

That is a fair point. Back maybe 20 or 30 years ago, the average person would bring their camera on vacations and that was probably the extent of it. But it's a bit different now that most people carry a camera in their pocket so these moments of "fake" display are much more common.

Not in all the really old photos I've seen. Lots of stern serious looks there. Smiles came later.

Hard to smile when you had to sit motionless for an hour while the image burned into the film back then.

That stopped being an issue very quickly.

Also, in the beginning it was not unusual that the only portrait of a person was post-mortem - to preserve the memory (as in 100 years of solitude). Difficult to smile in that case.

However, photographs were serious business for quite a while. You don't see people smiling on official portraits for the same reason. Imagine that portrait of Napoleon on a horse, but with a smile - doesn't have the same ring to it, does it?

Same for early photographs; they were expected to be solemn.