Researchers crack the smile, describing 3 types by muscle movement


Credit: Courtesy of Paula Niedenthal


But how do we tell one kind of smile from another?



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. But it's not the only muscle at work.


"We varied everything that could be varied in an expression, but our stimuli included some action from the smile muscle, the zygomaticus," says Magdalena Rychlowska, a postdoctoral researcher at Cardiff. "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."


For example, a reward smile — "probably the most intuitive," Niedenthal says, "the kind of smile you would use with a baby, so he will smile back or do things you like" — is a symmetrical hoist of zygomaticus muscles plus a dash of eyebrow lift and some sharp lip pulling.

Affiliative smiles — used to communicate tolerance, acknowledgment, or a bond, and show that you're not a threat — come with a similar symmetrical upturn to the mouth, but spread wider and thinner with pressed lips and no exposed teeth.


"This facial expression has evolved to solve basic tasks of human living in social groups: Thanks, I like this. Don't worry, I'm not going to hurt you. Hey, I'm in charge here," Niedenthal says. "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."


"We now know which movements we should look for when we describe smiles from real life," says Rychlowska. "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."


"They may have to make choices that will affect a patient's expression for the rest of their life," Niedenthal says. "It's useful for them to know how different kinds of smiles are used in the world, and which muscles are involved in making them."


"Americans smile so much that people from other countries are taught to smile more when they interact with us," she says. "The problem is, they're almost always taught one kind of smile, and that can cause confusion. "Simply teaching people about the existence of different types of 'true' smiles can help people pay more attention and avoid some of those misunderstandings."


The research was supported by the National Institutes of Health (grant T32-MH018931-26), the National Science Foundation (grant BVS-1251101), the Wellcome Trust, Multidisciplinary University Research Initiative/Engineering and Physical Sciences Research Council (grant EP/N019261/1) and the United States-Israel Binational Science Foundation (grant 2013205). Chris Barncard, 608-890-0465,

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Paula Niedenthal

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