New research finds that smiles can be used to both soothe or attack — at least, as far as the brain’s stress pathways are concerned.

Image via Pixabay.

A faltering voice. A racing heart. Sweaty palms. Those are some of the symptoms of stress that we’ve all experienced at one time or another — during an exam, before an exciting date, before speaking in public. We know that such events can instill stress even in the most level-headed out there, but new research shows that a single smile can also have the same effect — if done well.

The study, published by researchers from the University of Wisconsin-Madison and Bar-Ilan University, looked at the interaction between nonverbal feedback and the activity of the hypothalamic-pituitary-adrenal (HPA) axis, our body’s central stress response system.

We know that verbal feedback, such as telling someone “that was/wasn’t good” following a speech can influence the activity of the HPA axis, either determining a rise or a lowering of stress-hormone cortisol. However, there was very little scientific research looking into how our HPA axis responds to purely nonverbal feedback, such as facial expressions.

Turns out your brain is actually paying a lot of attention to all of these cues. The team reports that smiles can reduce or increase physical stress, depending on how they are perceived. They also showed that smiles with different social functions have different effects on the HPA axis, when perceived as feedback in the context stressful social situations.

For the study, the team worked with 90 male undergrad students, using the cortisol levels in their saliva as a measure of their HPA activity. They report that ‘dominance’ smiles, those smirky things we use to convey disapproval or to challenge social standings, were associated with higher HPA axis activity — this type of smile correlated to an increase in heart rates and levels of cortisol in the participants’ saliva. Those who perceived ‘dominance’ smiles also needed a longer period to return to their baseline cortisol levels after experiencing a stressful event. All in all, the team notes, the physical responses to dominance smiles mirror the effect of negative verbal feedback on the HPA axis.

On the other hand, ‘reward’ and ‘affiliation’ smiles — which reinforce behavior, grease social wheels, or are meant to signal the lack of threats — have an effect similar to displays of friendliness or positive verbal feedback on the HPA axis, lowering the participants’ stress and improving their psychological resilience to stress.

Reward smiles (left) reinforce desired behavior by signaling positive feelings; affiliation smiles (center) promote approachability by signaling non-threat; dominance smiles (right) influence social hierarchies by signaling superiority.
Image credits Jared D. Martin, Magdalena Rychlowska.

The authors further report that individuals with higher heart-rate variability (the variation in time between heartbeats) showed the widest range of responses to different smiles. Higher heart rate variability has previously been correlated with a higher ability to recognize facial expressions.

“The findings provide further evidence for the view that smiles do not necessarily constitute positive nonverbal feedback, and that they may impact social interactions by affecting the physiological reaction of people who perceive them,” the authors write.

“In addition, cortisol appears to support the detection of social threat and coordinate biological activity needed to adequately respond to the threat.”

The findings help patch in our understanding of the depth of nonverbal communication in human language — as a tool to unnerve, a hand to soothe, and as an outside effect on our psychological state. However, the team cautions that because of the small sample of exclusively male participants, the findings shouldn’t be generalized until replicated. Thus, further research will need to explore whether or not men and women react differently to the same kind of smile, and to test more overt (both negative and positive) facial expressions.

The paper ” Functionally distinct smiles elicit different physiological responses in an evaluative context” has been published in the journal Scientific Reports.