The Serious and the Smirk: The Smile in Portraiture

Is Mona Lisa smiling? Just a bit. 

"Today when someone points a camera at us, we smile. This is the cultural and social reflex of our time, and such are our expectations of a picture portrait. But in the long history of portraiture the open smile has been largely, as it were, frowned upon."

http://publicdomainreview.org/2013/09/18/the-serious-and-the-smirk-the-smile-in-portraiture/