Smiles may reduce or increase physical stress responses in situations where people are being evaluated, depending on what they perceive a smile to mean, according to a study involving 90 male undergraduate students in Scientific Reports.
Verbal feedback cues - such as “that was/wasn’t good” - in evaluative situations, for example when people are giving a speech, are known to activate the hypothalamic-pituitary adrenal (HPA) axis, the human body’s central stress response system. However, only a few studies have investigated whether or not the HPA axis responds to non-verbal cues.
Jared Martin and colleagues demonstrate that smiles with different social functions have different effects on HPA axis activity when they are perceived as evaluative feedback in stressful social situations. Measuring cortisol levels in the saliva of participants as an indicator of HPA axis activity, the authors found that compared to ‘reward’ and ‘affiliation’ smiles - which reinforce behaviour, signal lack of threat and facilitate or maintain social bonds, respectively - ‘dominance’ smiles - which challenge social standing and signal disapproval - were associated with higher cortisol levels, and higher heart rates in people who were perceiving the smiles. Individuals perceiving ‘dominance’ smiles took longer to return to their baseline cortisol levels after the stressful situation was over. These physical responses are similar to the reaction to negative verbal feedback.
The authors also found that individuals with higher heart-rate variability - the variation in the time between each heart beat - showed more nuanced responses to different smiles. Heart-rate variability has been associated with facial recognition accuracy.
The findings provide further evidence that smiles do not necessarily constitute positive non-verbal feedback, and that they may impact social interactions by affecting the physiological reaction of people who perceive them. The authors caution that the small sample of exclusively male participants limits the generalizability of the findings. Further research is needed to explore if men and women respond differently to the same kind of smile, and to test the physiological effects of more overtly negative facial expressions.