Researchers have identified three distinct, reliably recognised expressions of smile — reward, affiliation and dominance — and the facial muscle combinations that make them.

Each smile hinges on an anatomical feature known as the zygomaticus major, straps of facial muscle below the cheekbones that pull up the corners of the mouth, the researchers said.

"There are so many words people use to describe different smiles, but we see them as describing subtypes of a reward situation or an affiliative situation or a situation of negotiating hierarchy and having disdain for someone else," said Paula Niedenthal, Professor at the University of Wisconsin-Madison.

According to researchers, reward smile is "probably the most intuitive".

This is "the kind of smile you would use with a baby, so he will smile back or do things you like", Niedenthal said. 

It is a like a symmetrical hoist of zygomaticus muscles plus a dash of eyebrow lift and some sharp lip pulling.

On the other hand, affiliative smiles — used to communicate tolerance, acknowledgment or a bond and show that you are not a threat — comes with a similar symmetrical upturn to the mouth, but spread wider and thinner with pressed lips and no exposed teeth.

The third smile in the category is dominance smiles and is used to signify status and manage social hierarchies.

This smile dispenses with the symmetry, pairing a bit of lopsided sneer with the raised brows and lifted cheeks typically associated with expressing enjoyment, the researchers said.

In the study, published in the journal Psychological Science, participants looked at thousands of computer-generated expressions with random combinations of facial muscles activated. But each smile included some action from the zygomaticus — or the 'smile muscle'.

"When distinguishing among smiles, both scientists and laypeople have tended to focus on true and false smiles. The belief is that if you smile when you're not happy, the smile is false," Niedenthal said.

"But people smile in many different circumstances and during many emotional states. So asserting that only smiles that result from states of happiness are 'true' smiles limits our understanding of this important facial expression," Niedenthal added.

The findings may enable people with affiliative and dominance smiles to shift the outcome of games and negotiations as well as help surgeons who repair and reconstruct facial bones and muscles, the researchers noted.