Funny, someone here just started asking me about my smiles in pictures on FB. (I don't even have many pictures there. Most were uploaded by others). I wonder if it was in connection with this thread.
The person actually said they look "effusive"! I would say my picture smiles are rather forced. (From being shy, and then "having" to do it). Perhaps it's also from being Supine the "introverted Sanguine". (Basically, a variation of the "Behind the Scenes" Interaction Style).
I don't usually smile (Unless I'm making some silly joke or watching or thinking of something funny, as this site even suggests
http://www.intp.org/intprofile.html). I had to run it by my wife to make sure. In person, I'm often all dry information, as she says. (Though with some amount of enthusiasm if it is something I'm really into like type theory).