Duchenne was interested in the mechanics of facial expressions, including how the muscles of the face contract to produce a smile.

The Duchenne‘ smile is long and intense, though it involves the contraction of just two muscles. First the zygomatic major, which resides in the cheek, tugs at the corners of the mouth, then the orbicularis oculi, which surrounds the eye, pulls up the cheeks, leading to the characteristic ‘twinkling eyes’.

The ‘miserable smile’ is a stoical grin-and-bear-it expression – a slight, asymmetric smile with an expression of deep sadness pasted over the top.

Since Landis’ classic study, psychologists have found this tell-tale smirk on the faces those watching gory films – they were filmed by a hidden camera – and among patients suffering from depression. It's a socially acceptable way of showing that you’re sad or in pain.

Most people – around 71% - can voluntarily contract the inner portion of the orbicularis oculi. 

Judged by facial expressions alone, people are judged as most truthful when they are lying. As the American humourist Kin Hubbard once said: “If you haven’t seen your wife smile at a traffic cop, you haven’t seen her smile her prettiest.”

The dampened smile is an attempt to control an automatic, happy one and exists because some muscles, such as the ones controlling the mouth, are easier to suppress than others. “The cheeks will be raised but we pull the corners of the mouth downwards or press the lips together."

Not all cultures invite a broad smile.  In Japan, where etiquette dictates that emotions are stifled in public, there’s a greater emphasis on smiling with the eyes. 

Translating roughly as ‘malicious joy’, schadenfreude is the thrill of discovering another’s misfortune.

“If individuals are alone and feel unobserved, they usually express feelings of schadenfreude by so-called ‘Duchenne smiles’ and ‘Duchenne laughs’,” says Jennifer Hofmann, a psychologist at the University of Zurich.

“When bonobo chimpanzees are afraid they’ll expose their teeth and draw their lips back so that their gums are exposed,” says Zanna Clay, a primatologist at the University of Birmingham.

In babies, a broad grin can either mean they’re happy or distressed and studies have shown that men tend to smile more around those considered to be higher status.

The ‘qualifier smile’ aims to take the edge off bad news. 

It begins abruptly, raising the lower lip slightly, and is occasionally accompanied by a slightly downwards and sideways tilt of the head.

The ‘contempt smile’ indicates a mixture of disgust and resentment and is disconcertingly similar to a smile of true delight, except for the corners of the lips which appear tightened.