At least one reason that smiling didn't persist for people of a certain time was that dental care wasn't the greatest, and people were reluctant to be captured

At least one reason that smiling didn't persist for people of a certain time was that dental care wasn't the greatest, and people were reluctant to be captured with their poor teeth showing. Having had the opportunity to sort through thousands of studio photographs spanning most of the 20th century, I could almost date the age of the subject independent of the time the photograph was taken by seeing whether they were smiling or not.