Abstract
Smartphones can absorb attention and abruptly interrupt social interactions, a dynamic particularly critical in early parent-infant exchanges where infants rely on emotionally available caregivers for regulation. While previous research highlights the negative effects of parental media use on parenting and infant behavior, little is known about how maternal smartphone use affects both mothers' and infants' physiological and behavioral stress responses during early interactions. Consequently, we observed 67 mothers and their 6-month-old infants during an extended still-face paradigm including five phases in total: (1) an interaction baseline, (2) a still-face interruption, (3) a subsequent reunion, (4) a smartphone interruption, and (5) a subsequent reunion. The order of interruptions was randomized. Maternal and infant heart rates were continuously recorded via electrocardiogram. Infant protest and self-regulatory behaviors, as well as maternal co-regulatory behaviors, were coded from video recordings on a frame-by-frame basis. Infants showed significantly more protest behavior, higher cardiac arousal, and reduced parasympathetic activity during maternal smartphone use compared to baseline. During smartphone use, mothers reduced social engagement, accompanied by increased parasympathetic activity and decreased physiological arousal, which remained lower during reunions. We also found that dyadic physiological coupling emerged during still-face and smartphone disruptions, whereas dyadic behavioral coupling was observed only during the baseline interaction. These findings highlight the disruptive effects of maternal smartphone use and raise important questions about the potential cumulative effects of repeated smartphone interruptions on early socioemotional development.