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PLoS One. 2015 Jun 11;10(6):e0129672. doi: 10.1371/journal.pone.0129672. eCollection 2015.

The Power of an Infant's Smile: Maternal Physiological Responses to Infant Emotional Expressions.

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Graduate School of Education, Kyoto University, Kyoto, Japan.
Faculty of Education, Nagasaki University, Nagasaki, Japan.
Department of Cognitive and Behavioural Sciences, The University of Tokyo, Tokyo, Japan.


Infant emotional expressions, such as distress cries, evoke maternal physiological reactions. Most of which involve accelerated sympathetic nervous activity. Comparatively little is known about effects of positive infant expressions, such as happy smiles, on maternal physiological responses. This study investigated how physiological and psychological maternal states change in response to infants' emotional expressions. Thirty first-time mothers viewed films of their own 6- to 7-month-old infants' affective behavior. Each observed a video of a distress cry followed by a video showing one of two expressions (randomly assigned): a happy smiling face (smile condition) or a calm neutral face (neutral condition). Both before and after the session, participants completed a self-report inventory assessing their emotional states. The results of the self-report inventory revealed no effects of exposure to the infant videos. However, the mothers in the smile condition, but not in the neutral condition, showed deceleration of skin conductance. These findings demonstrate that the mothers who observed their infants smiling showed decreased sympathetic activity. We propose that an infant's positive emotional expression may affect the branch of the maternal stress-response system that modulates the homeostatic balance of the sympathetic and parasympathetic nervous systems.

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