Oh, what a wondrous modern age are we living in! The things that technology can teach us! Researchers at the University of Amsterdam, who had to work on this in collaboration with the University of Illinois, came up with an algorithim to analyze Mona Lisa's smile.
It seems she is "83% happy, 9% arrogant, 6% scared and 2% furious".
Isn't that just great. We can all sleep better knowing that Mona Lisa was indeed happy.