One of the best-known paintings in the world is the Mona Lisa by Leonardo da Vinci, and undoubtedly the best-known feature of that painting is the enigmatic smile on the woman's face. The smile is a vitally important instrument in the human tool-kit of signals that convey information on the emotional state and intentions of the signaller.
The smile can instantly convey important and subtle information. In certain instances a smile can defuse a situation that otherwise could flare into violence. Smiling is so important that evolution has selected it as an innate characteristic of the human condition. We are born to smile.
Many forms of social greeting and interaction are determined by local culture. The handshake is a generally-used greeting in Ireland but is not used at all in many parts of the world. A kiss on both cheeks is a common form of greeting in mainland Europe, but is rarely used in Ireland. The curtsey was once a popular form of greeting but is no longer used.
But smiling is universal. Wherever you find humans, you find smiling.
There seems little doubt that humans are genetically endowed with the predisposition to smile. The smile displayed by new-borns during the first weeks is called reflex smiling and usually occurs without reference to any external stimulus.
The human infant begins social smiling by about three to four months of age. The smiling habit also kicks in at this stage just as reliably in blind and in blind-and deaf children as it does in hearing and sighted children.
Social smiling is elicited by the sound of human voices and at the sight of a human face, especially one talking to or smiling at the infant.
If we look at our close evolutionary primate cousins we can see apparent precursors of the human smile. Monkeys display a bared-teeth grin and the chimpanzee smiles in an even more recognisable fashion.
The facial configuration that we call the smile is controlled by several facial muscles, principally the risorius and the zygomaticus major muscles. To produce a genuine smile these various muscles must contract in an organised manner, which in turn is controlled by a carefully-orchestrated pattern of nervous impulses genetically programmed into our nervous system.
If we consciously override this to produce an artificial smile, we usually betray ourselves, even to the casual observer, by producing a caricature of the real thing. The ambivalent, and especially the false, smile is hard to disguise.
There are two kinds of smile. First of all there is the spontaneous smile elicited by a joke or a greeting from a friend. Then there is smiling on cue, such as the smile given by the steward to individual passengers as they enter the plane.
There are several medical conditions involving impairments of the nervous system that result in a partial or complete loss of the capacity to smile. Different types of nervous damage can cause the loss of one or the other types of smiling.
For example, nervous damage in Parkinson's disease often produces an emotional deadness in the face - "masked-face syndrome".
It is easy for an adult to induce a smile in a three-month-old infant. Smiling is automatic at this stage and can be elicited even by showing the infant an oval shape with two dark marks placed roughly where the eyes would be if the oval were a face.
As infants grow older they become more discriminating. An oval shape presented to an older infant must closely resemble a human face in order to elicit a smile. The social stimulation, or reinforcement, that the child experiences as it develops also determines the frequency of smiling.
If an infant receives no social stimulation after smiling, such as an adult's approving response, the infant begins to smile less often. Infants raised in an atmosphere of poor social stimulation, for example in badly run orphanages, smile much less often when one year old than infants raised in a loving home.
The first and most fundamental relationship the infant develops is with the mother. This relationship is greatly enhanced by the onset of smiling.
Mothers report that the smiling response from the infant transforms their relationship with the baby from that of a loving attendant to a broader and more fulfilling relationship with a responding person - the baby can now reward the mother for her loving attentions with a "loving" response.
The smile is a key ingredient in the emerging bond between mother and infant. Of course, the smile is not solely responsible for this development.
For example, the infant also develops the capability of prolonged gaze contact at this stage and generally develops a growing attentiveness to the mother and father, which has a marked effect on the parents.
It is not difficult to propose a theory as to why natural selection has hard-wired smiling into the human genetic constitution. The evolutionary function of smiling can be proposed as follows.
It is a device that helps the infant to reinforce the love of the mother, thereby promoting the attention and nourishment needed to grow up and eventually produce babies of its own.
And finally, a little story to raise a smile. The sheriff noticed a dog approaching the town. The dog was limping badly and carrying a shotgun. "Where are you going?" asked the sheriff.
"I'm lookin' for the man who shot my paw," replied the dog.
This article was inspired by a piece written by Melvin Konner, who also wrote The Tangled Wing: Biological Constraints on the Human Spirit (Henry Holt and Co, 1990). (William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.)