Biobehavioral synchrony between three to four months old infants and psychologically healthy mothers during the face-to-face still-face paradigm.

Human social interaction emerges through coordinated behavior, emotion, and physiology. Biobehavioral synchrony supports stress regulation, attachment, and socio-emotional development, yet its manifestation in psychologically healthy mother-infant dyads remains poorly understood. In particular, little is known about how behavioral and physiological modalities interact and how synchrony varies across contexts. To address this, 94 mother-infant dyads with 3-4-months-old infants participated in the Face-to-Face Still-Face paradigm while cardiac activity was recorded simultaneously in mothers and infants. Physiological synchrony was quantified using cross-correlations of heart rhythm and heart rate variability (HRV) time series, averaged in 10-second windows. We coded behavioral synchrony and assessed bonding via maternal self-report. Behavioral synchrony was higher during play but declined after the stressor, while physiological synchrony was primarily evident during play and lagged 10 s, with maternal responses following infant signals. Low behavioral synchrony corresponded to stronger negative physiological synchrony, indicating that physiological co-regulation persisted despite behavioral misalignment. Supporting theories of early co-regulation, higher behavioral synchrony was linked to fewer bonding difficulties. These findings suggest that low-risk mothers show coordinated physiological fluctuations with their infants, highlighting maternal responsiveness as a key co-regulatory mechanism, providing a framework to identify atypical parent-infant dynamics and guiding parenting interventions that support child development.