Perhaps this question is strange…. But a smile is meant to convey some sort of social information. The information that is conveyed depends on the cultural context (e.g., a smile in one culture may convey information that won't be conveyed in another culture).
What is it about our cultural history (e.g., the West) that made the smile the requisite expression for a posed photograph?
From cross-cultural emotion research, I can surmise that Western culture's focus on self-esteem was influential. But I'm curious about whether smiling for a posed photo can be traced back to a more specific historical point.
first of all, people do smile in old pictures. there are some examples here. even the Mona Lisa appears to be smiling slightly according to some (take a look yourself). even just googling old photos reveals ones where people are smiling, but it certainly wasn't the standard pose.
it was seen as more proper to keep a straight face, especially at formal events (and still is). for example this picture of President Obama is very presidential and serious in nature. however this photo is a bit more light hearted.
here's a quote from Mark Twain on photographs (from the article posted above)
A photograph is a most important document, and there is nothing more damning to go down to posterity than a silly, foolish smile caught and fixed forever.
tl;dr people have been smiling AND not smiling in photos or portraits for hundreds of years. it all depends on context of the painter/photographer/subjects
