Two reasons: 1. Smiling for a camera is something you do because you want to be remembered that way (happy) -- it is like positive propaganda of yourself via m

Two reasons: 1. Smiling for a camera is something you do because you want to be remembered that way (happy) -- it is like positive propaganda of yourself via media. Now, figuring out a way to spread a positive image of yourself via the photographic medium is actually a culturally sophisticated trick that we learn by imitating a social habit -- but someone sometime had to invent that habit first. What you are looking at are photos of people taken before that trick was invented (and even if the trick was already invented, the people portrayed wouldn't know about it because they likely had never seen a photo in their life). 2. Even if people knew that they will be remembered more positively if they smile, it took about three minutes to take the kind of photos Prokudin took, and they had to hold absolutely still. It is hard to hold a grin on your face for three minutes without changing it and thus ruining your portrait.