Smiles are not created equally. Some are expressions of warmth and joy, while others can be mean and domineering.

A new study shows that our bodies react differently depending on the message a smile is meant to send.

Research led by Jared Martin, a psychology graduate student at the University of Wisconsin-Madison, shows that smiles meant to convey dominance are associated with a physical reaction — a spike in stress hormones — in their targets. On the other hand, smiles intended as a reward or to reinforce behavior appear to physically buffer recipients against stress.

“Facial expressions really do regulate the world. We have that intuition, but there hasn’t been a lot of science behind it,” said Martin. “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.”

Martin works in the lab of University of Wisconsin-Madison psychology professor and co-author of the study Dr. Paula Niedenthal, whose research on emotions has 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 smile you’d give someone to let them know they are making you happy).

For the study, the researchers stressed out 90 male college students by giving them a series of short, impromptu speaking assignments judged over a webcam by a fellow student who was actually in on the study.

Throughout their speeches, the participants saw brief video clips they believed were their judge’s reactions. In fact, each video was a prerecorded version of a single type of smile — reward, affiliation, or dominance.

Meanwhile, 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,” Niedenthal reported. “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.”

The effect of affiliation smiles was closer to that of reward smiles — interesting, but hard to interpret, according to Niedenthal. That’s because the affiliation message in the judging context was probably hard for the speakers to understand, she explained.

Other research has shown that people with greater variation in the rate at which their hearts beat are better able to understand social cues, such as facial expressions.

“People vary in how tolerant or capable they are at sitting with and understanding or engaging with social information,” said Niedenthal. “The thing about your body that permits you to take in the information and process it fully, or make sense of it, is the functioning of your parasympathetic nervous system, which manages your breathing and heart rate and allows you to be calm in the face of social information.”

The new study found that participants with high heart-rate variability did show stronger physiological reactions to the different smiles.

But Martin noted that heart-rate variability is not innate and unalterable. In fact, a long list of disorders, including obesity, cardiovascular disease, autism, and 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, he said.

The study was published by the journal Scientific Reports.

Source: University of Wisconsin-Madison

Different Smiles Can Elicit Distinct Bodily Reactions