The following is a collection of some of the earliest known images of people smiling, starting with a pair of soldiers in the Mexican American War in 1847, up to a group of soldiers near the end of the Civil War. If early images of people smiling do not come as a surprise to you, there are a few things to note. Among other things, a portrait of a person with a grin of any kind is quite a rare find in the early decades of photography.
There are many reasons why this was so, mostly [hypothesized] that cameras had simply too long of an exposure time for the subject to hold anything but a mute expression. Early daguerreotype images took about 60-90 seconds to expose, though photographic technology was advancing and exposure times were quickly being reduced.
Other theories include widespread poor dental hygiene. But maybe most simply of all, smiling in a photograph was just not the cultural norm it is a today.
Given the following excerpt was written 150 years before these photographs, it still allows some possible insight to the phenomena or phenomenot rather, of smiling during photography's early years.
"There are some people who raise their upper lip so high… that their teeth are almost entirely visible. This is entirely contradictory to decorum, which forbids you to allow your teeth to be uncovered, since nature gave us lips to conceal them." - Jean-Baptiste De La Salle, The Rules of Christian Decorum and Civility, 1703
In 1900 with Kodak's introduction of the Brownie camera, photography became safe, affordable, and easy to use for the general public in a way that it had never been before. With shutter speeds in the fractions of a second, Kodak coined the concept and usage of the term "Snapshot."
Kodak began using images of smiling people in their marketing campaigns. The pitch of the Brownie camera was "You Press the Button, we do the Rest."
Shortly after, Kodak began using the phrase "Kodak Moment" to sell not just a camera but a lifestyle. The staple ideology of brands today.
Big thanks to YouTube creator Chubachus who did the research on this for his YouTube channel devoted to historical photography videos.
The Serious and the Smirk: The Smile in Portraiture - Nicholas Jeeves