I came across this quote posted on the A Photo Editor blog, I like it a lot and it's something I've always tried to explain to people why I don't like them to smile in their portraits:

"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." - Rodney Smith

Below is are a couple of portraits of Tony Arcabascio I took for T-world