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

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.