This is Elizabeth Santorum's sincere smile after she's held it bit too long (for which a politician or his daughter has many an occasion) and thus starts to display her bottom teeth. Ms. Santorum's eyes display the required (for a sincere smile) partial closure as well as the concave-up furrow on each lower lid (see also: Secret # 353: Brad Pitt's Eyes - Sincere and Insincere Smiles). Her forehead is relaxed and smooth without any muscle contraction - also a must for a sincere smile. Elizabeth's upper teeth are perfectly displayed which is also characteristic of the joy-happiness emotion - however the lower teeth are beginning to show. A few seconds before this photo, her lower teeth were covered. Her sincere smile has been held too long (externally displayed as well as emotionally-internally felt) and so it has "degraded" to an 80% sincerity level (still an outstanding "Social Smile" - and indeed much better than most). It now has to be partially forced - so the corners of the mouth pull a bit less upwards, and a bit more laterally.
If Elizabeth's mouth were the same as in this photo, but her eyes were open in a relaxed-resting state, her smile would be insincere. The partially closed eyelids with the concave-up furrow is a body language absolute when measuring the authenticity of a smile.
Rick Santorum sent Elizabeth, his eldest child, to Hawaii a couple days ago to campaign for him. The photogenic and articulate woman had her job cut out for her. Her father needs all the primary & caucus victories he can get. At the time of this post, the polls in the 50th State are not yet closed.