We've all seen examples of those haunting sepia-toned Civil War photographs from the mid-nineteenth-century. Many of these photos, including well-known portraits of Abraham Lincoln and generals such as Robert E. Lee and Ulysses S. Grant, are attributed to the famous photographer Mathew Brady. Frequently, however, the actual photographer was someone who worked for Brady.
Many other photographers, now forgotten except for the work they left behind, were in the business of taking pictures which preserved the memory of both the living and the dead. Images of the bloated dead bodies of soldiers killed on vast battlefields remain in stark contrast to portraits of young men going off to war and the loved ones they left behind.
The camera was a relatively new invention by the time that the Civil War broke out. Frenchman Louis Daguerre had developed the first commercially successful method of photography, the daguerreotype, in 1837. Throughout the 1840s and 1850s, the subjects of photographs were mostly wealthy individuals, just as only the rich had had portraits painted of themselves in previous eras.
In the 1860s, however, images of "commoners" become more prevalent as soldiers who had just enlisted wanted to have a "carte de visage" (literally translated as "face card") to give to their loved ones to remember them by, whether or not they returned alive. This practice resulted in a virtual explosion of photographic images in the mid-nineteenth century in which the subjects invariably look very somber and smiling is non-existent. Just do a Google image search for "civil war portraits" and you'll see many examples of the stiff and frowning faces of that era.
Were people just more serious back in those days? Had war made smiling socially unacceptable? Life was certainly more difficult in a number of ways during the Civil War, but despite all the hardships, people of that time did experience happiness as well as the whole range of human emotions. There was indeed a greater level of formality expected when a person was having his or her portrait taken than there is today. Subjects of Civil War photographs were somewhat uncomfortable with the idea of having their likeness preserved mysteriously on film. Although a full-toothed grin is rare, photographic archives are not completely devoid of such images, one example of which can be found at http://www.shorpy.com/node/6058th century.
While dental hygiene was rarely practiced in the nineteenth century, and the subjects of that time might not want to "say CHEESE" as we do today, a lack of teeth is not the fundamental motive for not smiling. The main reason for a lack of smiling in nineteenth-century portraits can be explained by the photographic process itself.
In order to produce an image, a subject would have to remain still for an extended period in order to allow the film to be adequately exposed. It was hard to hold a smile for several minutes (it still is; try it yourself and see). Those who tried to maintain toothy grins during photo shoots ended up with blurry images, rather than the clear portraits they were paying so dearly for.