Smiles may reduce or increase physical stress depending upon how they are perceived, a study of nonverbal feedback, just published in Scientific Reports by researchers from Bar-Ilan University in Ramat Gan and the University of Wisconsin-Madison, finds.People who find it unpleasant to speak in public often develop sweaty palms, a racing heart or a faltering voice. The mere anticipation of social evaluation increases the activity of almost all body systems related to stress, with particularly strong activation in the hypothalamic-pituitary-adrenal (HPA) axis, the human body’s central stress-response system.Positive or negative verbal feedback in response to a speech, such as “that was/wasn’t good,” is known to activate the HPA axis. But until now little scientific inquiry has been conducted into how our bodies respond to purely nonverbal feedback, such as facial expressions.University of Wisconsin-Madison researchers Jared D. Martin, Heather Abercrombie and Paula Niedenthal, and psychologist Eva Gilboa-Schechtman of BIU showed that smiles with different social functions have different effects on HPA-axis activity when they are perceived as feedback in stressful social situations.The researchers measured levels of the hormone cortisol in the saliva of 90 male undergraduate students as an indicator of HPA-axis activity. They discovered that “dominance” smiles, which challenge social standing and signal disapproval, were associated with higher HPA-axis activity, such as increases in heart rate and salivary cortisol. Those who perceive dominance smiles also took longer to return to their baseline cortisol levels after the stressful event. These physical responses mirror the influences of negative verbal feedback.By contrast, “reward” and “affiliation” smiles, which variously reinforce behavior, signal lack of threat and facilitate or maintain social bonds, exerted influences similar to the effects of displays of friendliness, positive social evaluation and buffered physiological activity.The authors also found that individuals with higher heart-rate variability – the variation in the time between heart beats – showed more nuanced responses to different smiles. Higher heart-rate variability – an index of parasympathetic nervous system activity – is positively associated with facial expression recognition accuracy.“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,” wrote the researchers.“In addition, cortisol appears to support the detection of social threat and coordinate biological activity needed to adequately respond to the threat.” They also noted that the findings contribute to growing evidence of individual differences in sensitivity to the meaning of facial expression.The authors caution that the small sample of exclusively male participants limits the ability to generalize their findings. Further research is needed to explore whether men and women respond differently to the same kind of smile, and to test the physiological effects of more overtly negative facial expressions.