Functionally Different Smiles Cause Distinct Physiological Responses in Humans

By Maryna Mullerman ‘20

smile.jpeg

Figure 1. A smile could activate a hypothalamic-pituitary-adrenal response that is like the responses caused by verbal cues.

A smile is often associated with positive feedback and friendly gestures. Jared D. Martin and researchers from the University of Wisconsin-Madison aimed to reevaluate this nonverbal social tool. They suggested that there are three distinct smiles that could be encountered  reward, affiliation, and dominance with each smile playing a different role in human society. Verbal communication activates the hypothalamic-pituitary-adrenal (HPA) axis, which controls stress, mood, and emotions. The researchers hypothesized that nonverbal smile communication could activate HPA axis responses and cause high-frequency heart rate variabilities (HF-HRV).

The data was collected from 90 male university students, who were randomly assigned to one of the three smile conditions. The participants responded to three personal questions and were evaluated by the confederate – a research associate – via Skype. After the speech, the subject saw pre-recorded clips of the evaluators’ facial expressions. The participant was exposed to three smile videos and three neutral videos. In the pre-recorded videos, the evaluator expressed one of the three smiles. The participants were unaware of the nature of the experiment and believed that the expressions occurred spontaneously. Saliva samples for cortisol level measurements were taken from subjects before the experiment, after the anticipation period, after the speech, and during 10-minute periods after the tasks. Electrocardiography (EKG) was used throughout the entire procedure to measure heart rate variability.

Dominance smiles were associated with higher HPA axis activity. Men that received dominance smiles had higher cortisol levels lasting for longer periods of time. HF-HRV was positively associated with variability in HPA axis activity, indicating physiological responses to social evaluation. Females were excluded from the procedure due to significant differences in the accuracy of recognition of positive facial expressions.

In this study, the researchers suggested that smiles with different social functions could influence physiological responses. Dominance smiles increased HR and cortisol levels, while reward and affiliation smiles displayed similar effects but lowered physiological activity. The findings suggested the existence of heterogeneity in the “positive” nonverbal feedback category, as well as the importance of facial expressions in social environments. The interpretation of facial expressions can further be studied in the context of social anxiety.

 

References:

  1. J. D. Martin, et al., Functionally distinct smiles elicit different physiological responses in an evaluative context. Scientific Reports 8, (2018). doi: 10.1038/s41598-018-21536-1 .
  2. Image retrieved from: https://www.pexels.com/photo/men-s-gray-crew-neck-shirt-160914/
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