It seems rather tame today. Elisabeth Louise Vigee Le Brun's 1786 self-portrait shows the French painter cradling her daughter. As the artist smiles, teeth gleam between her lips.

Yet this picture caused outrage. A critic wrote: "artists, people of good taste and collectors all disapprove". Vigee Le Brun's toothy smile was a huge breach of etiquetteThe set of rules that inform behaviour among a social group.. Showing one's teeth was bad enough. Painting them was even worse.

In a new essay, historian Colin Jones tells the story of the smile. Smiling is one of our most common expressions. A smile is produced when we flex a single muscle.

They express, Jones writes: "sensory pleasure and delight, gaietyAn old-fashioned word for being light-hearted and cheerful. and amusement, satisfaction, contentment, affection, seduction, relief, stress, nervousness, annoyance, anger, shame, aggression, fear and contempt".

Humans have probably always smiled. The muscles required are present in the womb. Our close relations, the great apes, can also smile, suggesting that the expression comes from a common ancestor.

Yet there is a debate on whether we should smile at all.

The key point of tension was the flashing of the teeth. For the Renaissance humanistThe Renaissance was a period from the 14th to 16th Centuries in which Europeans discovered ancient knowledge and began to apply it to art, politics and philosophy. The humanists were the intellectuals who inspired this change.  Baldassarre Castiglione, opening one's mouth for anything but speaking, drinking and eating was at best uncouthLacking good manners.. At worst, it revealed the horrible inside of the human body, as well as rotten teeth and diseased gums.

A century later, the painter Charles Le Brun developed a theory of facial expressions. Inspired by the ideas of philosopher Rene DescartesA French philosopher, scientist and mathematician (1596-1650). Ideas related to his work are described as "Cartesian"., he held that a calm soul was reflected by a resting face.

There was more acceptance by Vigee Le Brun's time, which coincided with the advent of dentistry in France. People could have their teeth cleaned, filled and replaced. The sentimental novels of the time saw the open-mouthed smile as a symbol of virtue.

After the violence of the French RevolutionThe overthrow (and subsequent execution) of France's king and aristocratic government in 1789. The revolution became infamous and influential worldwide., smiling again became taboo. It was associated with the excess of the past. It took until the 20th Century for smiling to become en vogueFrench term meaning in fashion. again.

Debate over smiling continues. Some claim that smiling creates a positive chemical reaction that releases hormonesChemicals created inside living creatures. They are used to send messages from one part of the body to another., including dopamine and serotonin, into our brain. As neurologist Dr Isha Gupta explains: "Dopamine increases our feelings of happiness. Serotonin release is associated with reduced stress."

Yes: The huge quantity of smiles we see in celebrity photographs and on social media has ruined the special power of smiling. We should save them for those occasions when we feel truly overcome by emotion.

No: We should let our faces show how we feel. Hiding our emotions is never a good thing. And as the great essayist William Hazlitt wrote: "a good-natured smile can work wonders and accomplish miracles".

Or... There is no such thing as too much or too little. Smiling is a feature of our biology, outside our control. We can try to stifle our smiles, but in the end our bodies call the shots.