MADISON, Wis. — Want to make someone’s day? Give them a smile, but make sure it’s the
“right” kind. While they may seem like an expression of warmth and joy, a new study shows that   some smiles can be perceived as offensive or demeaning, which causes a negative physiological response.

Could a smile be harmful to our health? Researchers from the University of Wisconsin-Madison found that when a person displays a smile that expresses dominance, our bodies react in a similar manner to a stressful situation. Conversely, smiles intended in a positive manner have the opposite effect: they create physical buffers to stress in their observers.

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

For the study, researchers subjected 90 participants to media showing what professor Paula Niedenthal defined in previous work as three main types of smiles: dominance, affiliative (communicating a bond and showing you’re not a threat), and reward (showing joy and warmth as a sign of receptiveness and happiness). Martin showed his test subjects short clips of people smiling in all three ways as they delivered short, impromptu speech assignments. Their heart rates were monitored during the assignment and occasional saliva samples were taken to measure for levels of the stress hormone cortisol.

The subjects in the study were all male college students. Martin told them they were being evaluated on their speaking assignments via webcam by a panel of judges. The speakers believed the clips of smiles they were seeing were the judges reacting to their speeches.

“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 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.”

According to Niedenthal, the affiliative smiles were more difficult to quantify because in the judging context, those smiles were likely harder for the participants to understand. Other factors could affect a person’s physiological response, such as heart-rate variability. Participants with higher levels of this had stronger reactions to the smiles, but conditions like obesity, anxiety, depression, or autism could hamper one’s heart-rate variability and thus decrease their ability to recognize and understand social gestures, like 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,” says Martin. “Those things that we carry around with us change the way we perceive the world in very sensitive and personal ways.”

The full study was published in Scientific Reports.