‘When someone smiles with both their eyebrows raised it shows that they don’t want to dominate the situation and they want come across as unthreatening.’

A full smile is called a Duchenne smile - after 19th century anatomist Duchenne de Boulogne, who produced a ground-breaking study of the muscles involved in facial expressions of emotion. The openness of the expression gives the impression that someone is genuine.

Dr Collett added that the wide open smile is a ‘clever way of getting other people to share one’s positive feelings.'

‘When someone tilts their head back slightly and opens their mouth wide they look like they are laughing rather than smiling.

'Because laughter is so contagious, those of us who see someone doing this are much more likely to be affected by the display and to feel that we share their positive emotions.’

Meanwhile, the ‘look-up’ smile can be used to appear seductive and flirtatious. ‘Some people have a habit of lowering their head slightly when they smile,’ said Dr Collett.

‘Unconsciously, this gives the impression that they’re shorter than they really are and that they’re looking up towards the person they’re addressing.

'This, coupled with the suggestion of embarrassment, makes the look-up smile especially flirtatious, which explains why it’s favoured by women when they’re dealing with impressionable men.’

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