In this era, subjects of photographs typically were asked not to smile, since the speed of film then was slow, and a wavering smile would likely result in a blurred photograph.
References:
- Christina Kotchemidova. Why We Say “Cheese”: Producing the Smile in Snapshot Photography, in Critical Studies in Media Communications, 1 March 2005
- Merrell Fabry, Now You Know: Why Do People Always Look So Serious in Old Photos?, in Time magazine, 28 November 2016