Research press release


Scientific Reports

Psychology: The ‘wrong’ kind of smile could stress you out



今回、Jared Martinたちの研究グループは、ストレスのかかる社会的状況に置かれた人が、笑顔を評価的なフィードバックと認識した時に、さまざまな社会的機能を有する笑顔が、その社会的機能に応じてHPA軸の活性に異なった影響を及ぼすことを実証した。今回の研究では、笑顔を、(1)行動を強化する「報酬」の笑顔、(2)脅威がなく社会的な絆を促進または維持する「親和」の笑顔、(3)社会的地位に反し不承認を示唆する「優越」の笑顔のいずれかとして受け取った被験者の唾液に含まれるコルチゾールの濃度を、HPA軸の活性の指標として用いて比較した。その結果、笑顔を「優越」の笑顔として受け取った者は、他の2種の笑顔として受け取った者に比べて、唾液中コルチゾール濃度がより高く、心拍数が多いことが明らかになった。また、ストレスのかかる状況が消失しコルチゾール濃度が基準値に戻るまでの時間は、「優越」の笑顔を認識する者の方が長かった。これらの身体的応答は、負の言語的フィードバックに対する反応と類似している。



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.

doi: 10.1038/s41598-018-21536-1


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