One thing is certain – smiles never go up in price or down in value. Life is like a mirror. If you frown at it, it frowns back. If you smile, it returns the greeting.

One thing particularly pleases me while visiting USA. As I walked the pavements of the small suburban town where our son lives, passing women almost invariably reward with a smile when I smile at them – most smiles that way are started by another smile. Maybe, I am a smallish man in my seventies, with an unthreatening demeanour. Passing men muttering "Good Day" are welcome but the really satisfying part comes when women, young and old, pushing prams or carrying groceries, complete strangers, smile back at you in greeting.

Note that people don’t smile in my own country or elsewhere. But Americans smiling so often and so easily may well have something to do with their history and tradition.

That country has no rigid class structure or caste system. The American smile is a democratic version of a courtesy or doffed hat. Every stranger is deemed a potential equal. People getting better teeth could be a plus factor. Bad teeth do discourage smiling.

Smiles are commonly associated with joy, relief, and amusement. But smiles are by no means limited to the expression of positive emotions. People in different cultures smile when frightened, embarrassed, angry or miserable. With Japanese, for instance, a smile is often used to hide pain or sorrow.

Smile – one of the most ambiguous human expression involving 26 facial muscles – conveys a thousand different meanings, says the anatomist Charles Bell.

Psychologist Paul Ekman, the head of the University of California’s Human Interaction Lab, has actually identified 18 distinct types of smiles. There are joyous smiles, dampened smiles, compliance smiles, flirtatious smiles, and so on.

The smile of true merriment is characterized by heightened circulation, and the employment of two major facial muscles; of the lower face and the skin crinkles around the eyes. In social smiles, not backed by an actual state of happiness, often only the lower facial muscles come into play.

True smiles last between two-thirds of a second and four seconds, depending upon intensity. But false smiles often linger longer, like awkward guests after a party. Since these smiles lack an authentic inner prompt, we don’t know when to retire them – felt smiles fade away naturally. In fact any expression over ten seconds long, and most over five, are probably false. And the smile that arrives too early or late is probably an imposter.

And ‘smiles’ happens to be ‘longest’ word in English language; there’s a ‘mile’ between the first and the last letter. Desmond Morris in Babywatching has a chapter on baby smiles. The general, unselective smile, he says, arrives at about four weeks of age. It is apt to be offered to any adult human face brought close to the baby’s face.

Specific smiling – directed at faces of the loved ones – comes between five to six months. It is this uncontrollable honesty that makes the specific smile to the loved parents so immensely rewarding.

Morris makes out that we are genetically engineered to smile. Even born-blind babies smile, though not as much as sighted ones. While the monkey baby can cling to its mother’s body for comfort, the human infant has no such ability. It makes up by winsome smiles to keep its mother close.

Smiling for women is a far more complex affair than with men. There is more social pressure on today’s women to smile.

This has not been always so. Actually social laws governing women’s smiles almost stand reversed. For centuries women were discouraged from displaying animation or indulging in immoderate laughter. No wonder, most medieval portrait paintings of women show them devoid of any responsive expression: Mona Lisa perhaps the sole exception with her enigmatic smile.