How strange, this very question occurred to me this morning. Here’s what I found:
Smiling for portraits was frowned upon pardon well into the 1900s for several reasons:
Getting one’s picture taken was a big deal, since photography was only done by professionals. Having a portrait done was not undertaken casually; one was creating a singular record for posterity, and wanted to inspire respect.
The public smile was generally seen as indicating lower social class. The lower classes smiled to ingratiate themselves to their betters. The better bred wanted at all times to project that they were in perfect control of their emotions and expressions, as exemplified by the European aristocracy. So they adopted the dour expressions portrayed in centuries of European portraiture.
Dentistry didn’t come of age until well into the 20th century. Until then, most full-toothed smiles were not a treat to behold.
This began to change with the advent of the Kodak camera. Portraits became far more common, and so weren’t considered such a big deal. Candid, unposed portraits started to appear.
Hollywood broke the social strictures against smiling. Movie stars became America’s “royalty”, and their constantly beaming smiles gave social acceptance to public smiling. People began to want to look like Gary Cooper in their portraits instead of Frederick III.
And finally smiles became more presentable as dental hygiene took hold.
The change wasn’t instantaneous, however. One study of High School yearbook pictures turned up no smiles prior to 1920. By 1970, 60% of men and 80% of women showed at least a partial smile.