The truth is no portrait of substance has people smiling. Look at the history of painting, Rembrandt, Titian, Goya, Velasquez, Sargent, Vermeer, DaVinci, etc., the subjects gaze to the viewer is neutral at best, neither inviting nor forbidding. It is there for the viewer to see and feel.
Smiling is like much of American popular culture, superficial and misleading. It is part of our vernacular, but it should be expunged in photographs.
You can find some famous portrait paintings made throughout history here. Virtually all of them support this argument.