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From the very first stage of the postpartum period, infants show various emotional expressions such as crying and smiling, and their caregivers typically respond sensitively and automatically to these expressions. Many studies have investigated maternal physical and mental responses toward infants’ emotional expressions. For example, distress cries of infants evoke physiological responses in mothers, most of which involve accelerated cardiac activity, increased skin conductance, and a higher rate of respiration. Several researchers have suggested that this physiological arousal caused by infant cries functions as ‘preparation for action’. When a mother finds that her infant is crying, her stress response will prompt her to approach, pick up, and attempt to console her infant.

Given that these stressful responses are a fairly typical feature of parenting, how are such responses modulated, and can they be decreased? Levenson suggests that positive emotions facilitate the process of recovery from physiological arousal provoked by negative emotions. This is called the undoing effect. Indeed, Fredrickson and Levenson showed that cardiovascular activity induced by watching a negative film returns to baseline more quickly when followed by watching a cheerful film than after a sad or neutral film. The stimulation of positive emotions associated with the undoing effect may result in the restoration of homeostatic balance. Homeostasis is dependent on the dual operation of both sympathetic and parasympathetic autonomic nervous systems. When a person faces a stressful situation, sympathetic activity becomes dominant, causing an increase in skin conductance and heart rate, which helps prepare the person for an emergency. After the person is released from the stressful situation, parasympathetic activity becomes dominant and sympathetic activity decreases, with an associated reduction in skin conductance and heart rate, which is commonly associated with a person experiencing a (relatively) quiet, relaxed state.

In the course of daily childrearing experiences, mothers experience stress reactions to their infants’ expressions of negative emotion, and subsequent positive emotional expressions of the infants may moderate or ameliorate these stress reactions. Although a number of researchers have suggested that positive infant emotional expressions are important for effective mother-infant interaction, exactly how infant smiling affects maternal physiological states remains unknown. Some functional magnetic resonance imaging (fMRI) studies have reported that infants’ positive emotional expressions activate various maternal brain regions, including those underlying the reward system, motor planning, and inhibition of negative emotion; the activation of these regions is thought to be necessary to initiate positive parenting behaviors. However, these studies emphasized the effects of infant smiling on changes from a calm maternal state. It is possible infant smiling might have stronger recovery effects on physiologically negative stressful states, by means of the undoing effect. These studies could therefore have underestimated the positive effects of infant smiling on maternal physiological states. Thus, it is necessary to examine whether and how infant smiling brings about positive recovery effects on mothers' physiologically stressful states, including those states caused by exposure to infants’ distress cries. In the present study, we investigated whether the happy smiling of infants attenuate their mothers' physiological responses to their preceding cries.