The common classification of human smile patterns was established by the plastic surgeon Leonard Rubin in 1974. He identified three different smile patterns.
Firstly Rubin described the commissural smile, which is evident in two thirds of the world population and also referred to as the Mona Lisa smile. Initially the corners of the mouth are pulled up- and outwards. The following contraction of the M. levator labii superioris shows the upper theet. The lowest point is the incisial edges of the incisors, from where the smile goes convexly up to the molars. The main direction of the movement is directed towards the helix-scalp junction. Jennifer Aniston and Audrey Hepburn can serve as examples for this smile pattern.
Secondly the cuspid smile, that is also known as canine smile, can be found in 31% of world population. The M. levator labii superioris initiates the movement, which is then completed by the contraction of the Mm. zygomatici minor et major. In consequence the lips are visualized like the silhouette of a diamond. The incisial edges of the incisors are at the height of, or even above the molars. Moreover the "gull wing" effect can be detected and classified as the contrary movement pattern to the commissural smile. Inter alii Tiger Woods and Sharon Stone are people, who smile this way.
Thirdly the complexe smile charactertizes two percent of world population, including Will Smith and Julia Roberts. It’s also termed as Hollywood smile or full denture smile. The Mm. zygomatici major et minor, the M. depressor labii inferioris and the M. levator labii superioris contract simultaneously. This causes the upper and lower dental ridge to become visible. In particular the lower lip pulled downward is the feature of this smile pattern.
It should also be stressed that asymmetry can even be beneficial to the human smile. For example, the quality of a smile that does not develop side-by-side within the first 25 to 100ms seems to be superior to a smile that is symmetrical from the very beginning. It should be mentioned that in principle a so-called dynamic symmetry is desirable, i.e. a smile that is very similar but not identical in side comparison. Furthermore, the visibility of the teeth can have both positive and negative effects on the quality of the smile, depending on the angle of the mouth and the extent of the smile. In particular, if the parameters "angle of the mouth" and "extent of the smile" are not very developed, the following applies: The less visible the teeth, the higher the quality of the smile. Exactly the opposite is true if both the corners of the mouth and the smile are strongly developed: The more teeth become visible, the higher the quality of the smile.
Furthermore, it can be stated that the corner of the mouth is generally pulled in the direction of the helix in the course of the smile, forming an average angle of 40 degrees with the horizontal. The direction of the smile shows a symmetry of the left and right halves of the face and is thus in contrast to the extent of the smile. The corner of the mouth is shifted between 7 and 22 mm during the smile process. This shift is asymmetrical with regard to the halves of the face.
With regard to muscle function, smiling can be differentiated into two stages. The first stage is characterized by the contraction of the nasolabial fold. This stage also involves the levator labii superioris muscle, the zygomaticus major muscle and the upper parts of the buccinator muscle. The second stage is defined by the contraction of the periocular (around the eye) muscles. This results in an increased elevation of the upper lip and half-shut eyes.