And why we're terrible at faking them
The neuronal signals for smiles usually start in the cortex of our brain. From there they travel to the deeper part, the brainstem -- which, in terms of evolution, also happens to be one of the oldest parts of our brain. From there, a nerve that's large enough to be visible to the naked eye, called the seventh cranial nerve, carries the signal in front of the ear to the more central part of the face, where it reaches the smile muscle.
The smile muscle is attached from the mouth to the cheekbone. When this nerve fires, the muscle is activated, the corners of our mouth are pulled up, and we look happy. And if it is a true smile, one that signifies real enjoyment by its wearer, then a branch of the facial nerve also activates little muscles around the eyes, leading to wrinkling around the eyes in addition to a mouth smile.
Baseball players who had the broadest smiles lived, on average, seven years longer than those who smiled the least.
The scientific analysis of the smile really began with the French anatomist Guillaume-Benjamin-Amand Duchenne de Boulogne. In the 1860s, he used electrical currents to make his subjects' "facial muscles contract to speak the language of the emotions and the sentiments." Duchenne believed that one could gain insight into the ways the face expresses emotions by studying the muscles underlying facial movement. He recorded the expressions produced by the electrical stimulation by taking photographs and was the first to use photography to prove a scientific theory. Duchenne showed that in the particular smile he called the "smile of joy," the muscle that is just to the side of the eyes (called orbicularis oculi) is activated. When this muscle contracts, it creates creases, sometimes called crow's feet. He called this the "true smile," the pure smile of enjoyment. Duchenne concluded that the mouth smile obeys the will, but the eye smile does not. He said, "The muscle around the eye ... is only brought into play by a true feeling, an agreeable emotion. Its inertia in smiling unmasks a false friend."
In the image at right, Duchenne applies electrodes to his cooperative subject in order to stimulate the contraction of the smile muscle (zygomaticus major) that pulls the corners of the mouth up when we smile. His subject is clearly enjoying this. Duchenne's jolt of electricity activates the smile muscle around the mouth, but the gentleman's eyes are also smiling, as seen by the creases formed just to the side of his eyes so the happiness comes from inside. When Duchenne first applied the electricity, only the smile muscle around the mouth was activated; he realized that it didn't look like a true, natural smile, so he told this gentleman a joke to make his eyes smile as well.
Paul Ekman, who led the scientific investigation of facial expressions in the twentieth century, has shown that Duchenne was right. Few of us can fake an eye smile. If you're sitting across from a new business associate and he smiles at you but you're still not sure if you should sign off on a new business venture, look at those little lines around the outside of his eyes. If they wrinkle up like crow's feet, then his smile is a real signifier of pleasure or happiness. The absence of smiling eyes should alert you that your new associate's smile is not necessarily as friendly as it looks.
A century after Duchenne recorded his remarkable experiments, Dr. Paul Ekman honored the anatomist by terming the smile involving the eyes the "Duchenne smile." Ekman found that the failure to distinguish between different types of smiles showed up in much scientific research over the years and might explain contradictory findings regarding the universal meaning of the smile. Other scientists confirmed their findings and found that the Duchenne smile appears significantly more often when people are freely enjoying themselves than in situations that would require feigned smiles. The eyes do not lie unless, of course, the person has received Botox around the eyes, in which case the skin around the eyes is unable to wrinkle up, no matter how true a smile.
Certainly what we see greatly influences when we smile, but it is not necessary. We need no visual feedback to smile. Darwin discovered that those who are born blind will still smile appropriately during a conversation. You can smile when all alone, but smiling is definitely enhanced by socializing; it happens six times more frequently in social settings.
Some of the complexities of smiling can be appreciated by examining those who can't do it normally. People who have suffered brain damage may not be able to smile when asked to but will still involuntarily smile at a joke. Conversely, patients suffering from Parkinson's disease, a disease of dopamine-containing neurons in the brain, may be able to turn up the corners of the mouth when asked to smile but after getting a joke may lack the ability to smile as a natural, automatic response. Patients who have had a stroke leading to paralysis of half of their face lack an ability to voluntarily move one side of their face. They show a crooked smile when asked to grin but a normal smile on hearing a joke, indicating intact nerve pathways beyond their conscious control. Clearly, the pathways for smiling are quite elaborate, with both unconscious and conscious connections that receive inputs from different parts of the brain.