Dynamic associations between mothers' and fathers' parenting behaviors and infant physiological emotion regulation.

Emotion regulation (ER) is a key developmental process shaped through parent-infant interactions, during which parents and infants dynamically co-regulate the infant's emotions. Successful co-regulation depends on parents' sensitivity to the infant's emotional cues; unadjusted parental responses, such as intrusiveness or withdrawal, can dysregulate the infant, reflected in physiological indicators such as heart rate (HR). Although research has advanced understanding of infant ER, most studies have focused on mothers and dyadic contexts and primarily compared infant HR during stressful versus non-stressful interactions, without examining the moment-to-moment dynamics of naturalistic interactions. We examined associations between infant HR and parental behaviors in three interaction contexts-mother-infant, father-infant, and triadic (mother-father-infant). Eighty families with three-month-old infants completed three successive two-minute free-play sessions corresponding to these contexts. Each session was divided into 5-s intervals, with parental intrusive and withdrawn behaviors (IWPBs) coded for intensity (0-4) and infant HR averaged per interval. Multilevel vector autoregression models were applied to assess lag-1 and contemporaneous effects at within- and between-family levels. IWPBs were associated with elevated infant HR, with cross-lagged effects showing that increases in IWPBs predicted subsequent increases in infant HR. For intrusive behaviors, associations with infant HR were bidirectional, suggesting dynamic co-regulation processes. Context differences also emerged: fathers showed delayed responses to infant arousal, whereas triadic interactions elicited stronger associations and higher HR levels when considering the whole sample. Overall, findings highlighted that parent-infant co-regulation is moment-to-moment, context-sensitive, and shaped by both parental behavior and infant physiological reactivity.