Here's two of the happiest faces I know. Oldest granddaughter, Kaylyn, 14, with youngest grandchild, Jena, 4 months. The photo reminds me of the old saying; "Smile and the whole world will smile with you". The joy reflected in these faces sure make me smile.
Wilkepedia identifies the following two kinds of smiles.
The "Duchenne smile", after the researcher Guillaume Duchenne, is the most studied, and involves the movement of both the zygomaticus major muscle near the mouth and the orbicularis oculi muscle near the eyes. It is believed that the Duchenne smile is only produced as an involuntary response to genuine emotion, and is therefore what one could call the "genuine" smile. Due to the involvement of the muscle near the eyes, it is sometimes said that one can tell whether or not a smile is "real" by whether or not it "reaches the eyes". (Notice Kaylyn and Jenna's eyes.)
The "Pan American smile", on the other hand, is the voluntary smile involving only the zygomaticus major muscle to show politeness; for example, by a flight attendant on the former airline of the same name. Considered "insincere", this type of smile has also been called the "Professional Smile" by David Foster Wallace in his comedic short story A Supposedly Fun Thing I'll Never Do Again".