
We may assign photographic smiles to four different classes, only one of which is pictorially tolerable.
First, there is what is known as the zizzy smile. I include no illustration of the pure type of the zizzy smile, but thousands of examples may be found in screen magazines and tooth-paste advertisements. It is joyless and violent—a veritable explosion of incisors and bicuspids.
Then there is the grudging smile. This is also too familiar to require demonstrating. It results when the photographer insists on a smile and the bedeviled subject finally yields to the extend of lifting an upper lip in a perfunctory grimace that does not conceal the resentment smoldering in the eyes.
Third, there is the solar-plexus smile.. Or, by analogy with that fine phrase “belly laugh,” it might be called the “belly smile”. This, unlike those described above, is a natural and spontaneous expression, a sudden overflowing of animal joy. (Figure 19.) With an ebullient model a smile of this sort is readily obtained. Unfortunately, although it is actually spontaneous and sincere, the excess and suddenness of this smile cause it to have a violent “zizzy” quality. The sense of restraint and control, so necessary to pictorial representation, is missing.

Finally there is the controlled smile. The physical manifestation is here kept within bounds. (Figure 20.) Whether or not an actual joyous emotion is present is of no account. What is important is that the expression appears to be spontaneous, appears to be joyous, and does not rend the picture asunder with the violence of its presentation.
The smile is a pictorial element that should be used very sparingly. Attractive at first, it palls rapidly. The monotonous fixity of expression in time arouses at first passive annoyance, then active resentment. It is significant that smiles appear but rarely on the faces pictured by the great masters of the past. Franz Hals represented smiles more frequently than most artists, but he was always careful to hold them within pictorial bounds. The famous smile of Leonardo’s Mona Lisa is painted with such great restraint that some persons have disputed its very existence.
William Mortensen, The Model: A Book on the Problems of Posing (1937), p. 39-40
I think “zizzy” is my new favorite word. I think I’m going to name my daughter Zizzy. Her brother will be Cleanth, of course.