Scientists have cracked the science behind one of our most common facial expressions – the smile. They say the distinction between sincere and fake smiles that is often made is not entirely accurate.

"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," study author Paula Niedenthal, a psychology professor at the University of Wisconsin-Madison, said in a statement.

"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."

Smiling is perhaps our most flexible facial expression, which we use to convey or hide our emotions, and to manage social interactions. But telling one kind of smile from another is no easy feat.

In a study now published in the Journal of Psychological Science, Niedenthal and colleagues identified three distinct types of smiles - smiles of reward, affiliation and dominance - and described the facial muscle combinations that characterise them.

Classifying the smiles

The scientists recruited participants to look at thousands of computer-generated pictures. In these photos, facial expression of people were depicted with random combinations of facial muscles activated.

"We varied everything that could be varied in an expression, but our stimuli included some action from the smile muscle, the zygomaticus," explained co-author Magdalena Rychlowska, from Cardiff University. "We asked participants to tell us when they see a reward or affiliative or a dominance smile, and when the expression is not a smile."

This allowed them to classify the smiles and to identify their unique features. All smiles were powered by one particular facial muscle below the cheekbones known as the zygomaticus major, but the study suggests that other muscles were also at work depending on the type of smile that was done.

The reward smile, the most common and intuitive type of smile, appears to be a symmetrical hoist of zygomaticus muscles plus a dash of eyebrow lift and some sharp lip pulling.

reward smile
This is the reward smile - a symmetrical hoist of zygomaticus muscles plus a dash of eyebrow lift and some sharp lip pulling. University of Wisconsin-Madison

The 'affiliative smile' on the other hand, which we use to communicate tolerance and acknowledgement, or to show that we are not a threat, appears as a similar symmetrical upturn to the mouth, but spread wider and thinner with pressed lips and no exposed teeth.

This is the affiliative smile - with similar symmetrical upturn to the mouth, but no exposed teeth. University of Wisconsin-Madison

Finally, the dominance smile is the smile we use to manage social hierarchies. This type of smile dispenses with the symmetry, pairing a bit of lopsided sneer with the raised brows and lifted cheeks typically associated with expressing enjoyment.

This is the dominance smile - a lopsided sneer with the raised brows and lifted cheeks. University of Wisconsin-Madison

With this classification in mind, scientists think they will be better equipped to study the use and effects of smiles in pivotal human interactions.

"We now know which movements we should look for when we describe smiles from real life," Rychlowska explained. "We can treat smiles as a set of mathematical parameters, create models of people using different types of smiles, and use them in new studies."

Learning more about smile types could also be helpful to navigate intercultural communication. There is indeed plenty of variation in how often different types of smiles are used from one country to another.