LAUSR

Simple Summary Sensitive caregiving implies the mother’s moment-by-moment adaptation to the infant’s states and signals, and such online coordination supports the child’s social and neurobiological development. In contrast, intrusive mothering is characterized by overstimulation and social interactions guided by the maternal agenda rather than the infant’s interactive cues. These two maternal behavioral styles have been extensively studied and repeatedly shown to predict positive and negative social-emotional outcomes, respectively. Here we show that these two styles, sensitivity and intrusiveness, are differentially related to mechanisms of mother–infant brain-to-brain synchrony; while sensitivity is linked with higher mother–infant neural synchrony, intrusiveness is associated with diminished inter-brain coordination. We believe that the enhancement or limitation on coordinated interactive inputs to the infant’s social brain during its maturational period may be one mechanism by which maternal sensitivity and intrusiveness exert their differential long-term effects on children’s brains and behaviors. Abstract Biobehavioral synchrony, the coordination of physiological and behavioral signals between mother and infant during social contact, tunes the child’s brain to the social world. Probing this mechanism from a two-brain perspective, we examine the associations between patterns of mother–infant inter-brain synchrony and the two well-studied maternal behavioral orientations—sensitivity and intrusiveness—which have repeatedly been shown to predict positive and negative socio-emotional outcomes, respectively. Using dual-electroencephalogram (EEG) recordings, we measure inter-brain connectivity between 60 mothers and their 5- to 12-month-old infants during face-to-face interaction. Thirty inter-brain connections show significantly higher correlations during the real mother–infant face-to-face interaction compared to surrogate data. Brain–behavior correlations indicate that higher maternal sensitivity linked with greater mother–infant neural synchrony, whereas higher maternal intrusiveness is associated with lower inter-brain coordination. Post hoc analysis reveals that the mother-right-frontal–infant-left-temporal connection is particularly sensitive to the mother’s sensitive style, while the mother-left-frontal–infant-right-temporal connection indexes the intrusive style. Our results support the perspective that inter-brain synchrony is a mechanism by which mature brains externally regulate immature brains to social living and suggest that one pathway by which sensitivity and intrusiveness exert their long-term effect may relate to the provision of coordinated inputs to the social brain during its sensitive period of maturation.