The Headcase has a long piece in this month's APS Observer on the psychological study of smiling. The article is a broad overview of several decades of smiling research. But one of the most interesting things I learned is that most psychologists believe a "genuine" smile of enjoyment is distinguishable from all other smiles — those that occur when we're embarrassed, frightening, or lying, for instance — by the movement of two facial muscles: the zygomatic major, which pulls the cheek upward, and the orbicularis oculi, which scrunches the corners of our eyes.
As I describe in the introduction, the discovery of this genuine smile was made by a 19th-century French anatomist named Guillaume Duchenne:
Other muscles can simulate a smile, but only the peculiar tango of the zygomatic major and the orbicularis oculi produces a genuine expression of positive emotion. Psychologists call this the “Duchenne smile,” and most consider it the sole indicator of true enjoyment. The name is a nod to French anatomist Guillaume Duchenne, who studied emotional expression by stimulating various facial muscles with electrical currents. (The technique hurt so much, it’s been said, that Duchenne performed some of his tests on the severed heads of executed criminals.) In his 1862 book Mecanisme de la Physionomie Humaine, Duchenne wrote that the zygomatic major can be willed into action, but that only the “sweet emotions of the soul” force the orbicularis oculi to contract. “Its inertia, in smiling,” Duchenne wrote, “unmasks a false friend.”
Psychological scientists no longer study beheaded rogues — just graduate students, mainly — but they have advanced our understanding of smiles since Duchenne’s discoveries. We now know that genuine smiles may indeed reflect a “sweet soul.” The intensity of a true grin can predict marital happiness, personal well-being, and even longevity. We know that some smiles — Duchenne’s false friends — do not reflect enjoyment at all, but rather a wide range of emotions, including embarrassment, deceit, and grief. We know that variables (age, gender, culture, and social setting, among them) influence the frequency and character of a grin, and what purpose smiles play in the broader scheme of existence. In short, scientists have learned that one of humanity’s simplest expressions is beautifully complex. [end excerpt]
Another fascinating nugget came via a study published in a 2007 issue of the Journal of Experimental Social Psychology (pdf). Researchers used emoticons to observe the different ways Americans and Japanese interpret smiles. They found that Americans, who tend to be emotionally expressive, focus on the mouth, seeing :) as happy and :( as sad. The Japanese, who tend to suppress emotions, focus on the eyes when looking at a smile, seeing ^_^ as joyful and ;_; as tearful. (As Duchenne pointed out, the mouth can be manipulated into a smile more easily than the eyes — hence its attraction to an expressive culture.)