He divided smiles into two groups: those that involved the crinkling of the eyes -- sincere smiles -- and those that used only the muscles around the mouth -- insincere smiles.
Today, the full smile that uses the muscles around the mouth and eyes is known as a "Duchenne smile" -- and the fake one is the "Pan Am smile", after the kind of smile you might greet someone with if it's part of your job to be friendly.
In 1974, Leonard Rubin described three basic types of smile, based on his study of 100 people:
Cosmetic surgeons, who have to be meticulous in identifying the smiles that their patients are paying good money to try to achieve, call these three types commissure, cuspid and complex, respectively.
Phuong Nguyen, a Philadelphia plastic and reconstructive surgeon, attempts to clarify the matter using celebrities. The Mona Lisa, he says, is the Angelina Jolie. The Tom Cruise smile is a canine smile, and a Julia Roberts is the full dentured smile. This is a subjective matter. Other doctors place Jolie in the second or third categories.
Smiling is not a realm given over to easy description. Contemplate the ideal smile as recounted by one clinician in the Journal of Clinical Orthodontics:
An optimal smile is characterized by an upper lip that reaches the gingival margins, with an upward or straight curvature between the philtrum and commissures; an upper incisal line coincident with the border of the lower lip; minimal or no lateral negative space; a commissural line and occlusal frontal plane parallel to the pupillary line; and harmoniously integrated dental and gingival components.
The total number of different types of smile is frequently debated. A recent BBC article gave the total as 19.
Arguably the most important researcher into smiles over the past 30 years has been Paul Ekman at the University of California. His 1978 Facial Action Coding System, written with Wallace V Friesen, seeks to create an atlas of nearly all possible human expressions. Ekman says, in his book Telling Lies, that their technique for measuring the face can distinguish over 50 different smiles.
Confused? You're not the only one. Perhaps the best approach is to just grin and bear it...