A group of American psychologists are trying to find how people will respond to smiles with different implicit meanings, either warm or mean.
A study published Thursday in the journal Scientific Reports has shown that smiles meant to convey dominance are associated with a physical reaction in their targets: a spike in stress hormones. On the contrary, smiles intended as a reward appear to physically buffer recipients against stress.
It showed that our bodies react differently depending on the message a smile is meant to send.
"Our results show that subtle differences in the way you make facial expressions while someone is talking to you can fundamentally change their experience, their body, and the way they feel like you're evaluating them," said the paper's co-author Jared Martin, a psychology graduate student at the University of Wisconsin-Madison.
Researchers have established three major types of smiles: dominance (meant to convey status), affiliation (which communicates a bond and shows you're not a threat), and reward (the sort of beaming, toothy smile you'd give someone to let them know they're making you happy).
They stressed out 90 male college students by giving them a series of short, impromptu speaking assignments. They were judged over a webcam by a fellow student and throughout their speeches, the participants saw brief video clips they believed were their judge's reactions, but in fact, each video was a prerecorded version of a single type of smile: reward, affiliation or dominance.
The researchers were monitoring the speakers' heart rates and periodically taking saliva samples to measure cortisol, a hormone associated with stress.
"If they received dominance smiles, which they would interpret as negative and critical, they felt more stress, and their cortisol went up and stayed up longer after their speech," says co-author Paula Niedenthal.
"If they received reward smiles, they reacted to that as approval, and it kept them from feeling as much stress and producing as much cortisol," Paula said.
Smile study participants with high heart-rate variability have shown stronger physiological reactions to the different smiles.
But the heart-rate variability is not innate and unalterable. Disorders like obesity, cardiovascular disease, anxiety and depression can drag down heart-rate variability.
That may, in turn, make people worse at recognizing and reacting to social signals such as dominance and reward smiles, Martin said.