From the abstract: "Consistent with the gustatory–vagal hypothesis, vagal stimulation during breastfeeding may contribute to infants' physiological regulatory development independent of caregiving effects.This study examined whether breastfeeding predicted 6-month-old infants' (N = 151) and their mothers' vagal regulation during the face-to-face still-face (FFSF). Although breastfed and nonbreastfed infants showed expected vagal withdrawal during the Still-Face episode, only breastfed infants showed continued withdrawal during the reunion episode, suggesting greater physiological mobilization to repair the interaction. Breastfeeding mothers showed higher vagal tone than nonbreastfeeding mothers at baseline, suggesting greater capacity for regulation, and throughout the FFSF, suggesting calmer states. Breastfeeding effects were independent of maternal sensitivity. Findings suggest that infants' and mothers' physiological regulation may be shaped by breastfeeding independently of associated social factors."