Many people see smiles as a sign of encouragement, solidarity and friendship. They’re supposed to signal openness and acceptance. But a new study partly funded by the National Institutes of Health showed that not all smiles are equal.

The research team, led by Jared Martin and Dr. Paula Niedenthal at the University of Wisconsin-Madison, examined how men responded to three different kinds of smiles after giving a short speech. In potentially stressful situations, such as giving a speech, the body produces hormones called glucocorticoids that affect many systems throughout the body. The hypothalamic-pituitary-adrenal (HPA) axis — a network involving the hypothalamus and pituitary gland in the brain and the adrenal glands near the kidneys — plays a central role in these effects.

The researchers investigated whether the HPA axis responds to nonverbal feedback, such as facial expressions. They first identified three distinct types of genuine, spontaneous smiles — “reward” smiles, which show happiness and reinforce behavior; “affiliation” smiles, which strengthen social bonds between people, and “dominance” smiles, which are derisive and signal feelings of superior social status. The researchers tested how these different smiles affect HPA axis activity.

The team recruited 90 male college students. The men were asked to give three short speeches about themselves in front of a male evaluator who was watching over a web camera. After each of the speeches, the participants were shown videos of their evaluator’s facial expressions. They were told these represented spontaneous reactions, but the videos were actually prerecorded. The men were shown one reward, affiliation, or dominance smile after each of their responses. Each time, they were also shown a control video with a neutral response, such as face scratching or eye blinks.

The scientists measured levels of the glucocorticoid cortisol in the men’s saliva. They also used an electrocardiograph before, during, and after the speech to measure heart activity.

The team found that dominance smiles increased levels of cortisol, a hormone involved in the stress response, and heart rates. Participants receiving reward or affiliation smiles, however, returned to their base cortisol levels within 30 minutes after their speech, while those who received dominance smiles continued to have significantly higher cortisol levels 30 minutes later.

Heart rate variability — how much the time between heart beats varies — has previously been tied to a sensitivity to social cues such as facial expressions. The scientists found that the men in the study who had higher baseline heart rate variability differed more in their physiological responses to the different types of smiles. These differences between people may fundamentally affect how they respond to social situations.

“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 said in a news release.

Further study will be needed to better understand how people vary in how they understand and respond to nonverbal social cues. Women were excluded from this particular study because of potential measurement complications caused by oral contraceptives.