Different kinds of smiles provoke different kinds of biological responses in the people who see them, according the University of Wisconsin-Madison. For example, friendly smiles intended as a reward to reinforce behavior appear to physically buffer recipients against stress. However, smiles meant to convey dominance lead to a spike in stress hormones.

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

Previous research by the study’s coauthor, psychology professor Paula Niedenthal, PhD, found three major types of smiles: dominance (meant to convey status), affiliation (communicating a bond showing you’re not a threat), and reward (a toothy smile showing people that they are making your happy).

In the recent 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 actually was in on the study. Throughout their speeches, the subjects 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,” said 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.”

The effect of affiliative smiles was closer to that of reward smiles: interesting, but hard to interpret, Niedenthal said, because the affiliative message in the judging context was probably hard for the speakers to understand. Also, 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.”

Subjects with high heart-rate variability showed stronger physiological reactions to the different smiles. But, Martin said, heart-rate variable is not innate and unalterable. Many disorders such as obesity, cardiovascular disease, autism, anxiety, and depression can drag down heart-rate variability. That in turn make people worse at recognizing and reacting to social signals such as dominance and reward smiles.

“We are all individuals walking around in the world with different bodies. You may be really anxious. You may be in really good shape,” Martin said. “Those things that we carry around with us change the way we perceive the world in very sensitive and personal ways.”

The study, “Functionally distinct smiles elicit different physiological responses in an evaluative context,” was published by Scientific Reports.

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