Time-Varying Impact of Steatosis and Alcohol on Mortality: A Marginal Structural Model of the Canadian Longitudinal Study on Aging.

Human social interaction is inherently reciprocal, involving dynamic coordination of behavior, emotion, and physiological states between individuals. This reciprocity, known as , is particularly crucial in infancy, when caregivers support infants’ stress and emotion regulation before self-regulatory capacities emerge. The Theory of Biobehavioral Synchrony proposes that such early interpersonal coordination shapes infants’ expectations of social exchange and forms the foundation for attachment, socio-emotional development, and later self-regulation. Importantly, synchrony is dynamic and not defined by constant maximum coordination. Natural fluctuations and the ability to re-engage after brief disruptions or mismatches reflect functional interaction and are particularly relevant for developmental outcomes. Characterizing these early synchrony processes is therefore essential for identifying deviations from typical patterns, enabling early detection of developmental risks, and informing parenting-focused interventions. At the behavioral level, early parent-infant interactions unfold through reciprocal exchanges of gaze, affect, and vocalizations, often described as a synchronized “dance”. Through this coordination, dyads develop mutually regulated interaction patterns that shape the quality and stability of their relationship. Parental behaviors such as infant-directed speech activate infants’ physiological systems, creating a bidirectional link between behavior and physiology. Situations that disrupt the interaction challenge infants’ emerging regulatory capacities and further elicit active parental co-regulation efforts. These dynamics are commonly investigated using the Face-to-Face Still-Face (FFSF) paradigm, a validated social-stressor task comprising three episodes: emotionally attuned play, a still-face episode where the caregiver becomes unresponsive, and a reunion phase marked by renewed co-regulatory engagement. The still-face episode induces a marked disruption of dyadic interaction. Although engagement typically resumes during the reunion phase, behavioral synchrony remains attenuated relative to the initial play episode. Accordingly, the dyad’s capacity to re-establish coordinated engagement is considered an index of interaction quality. Physiological coordination, particularly within the autonomic nervous system (ANS), plays a central role to these processes. During the first year postpartum, parasympathetic nervous system (PNS) functioning is pivotal for characterizing infants’ emerging self-regulatory capacities and is typically assessed via heart rate variability (HRV), with elevated HRV levels reflecting enhanced PNS activity. Consistent with models of biobehavioral synchrony positing an interplay between physiology and behavior, research in earliest infancy demonstrates that PNS activity predicts infant social interaction behaviors. Physiological synchrony, defined as correlated fluctuations in HRV between parent and infant, provides a window into dyadic attunement. occurs when HRV changes in the same direction in both partners, (i.e., higher parental HRV is associated with higher infant HRV), whereas reflects opposing physiological patterns (i.e., increase in one partner’s HRV corresponds with a decrease in the other’s). These patterns can emerge simultaneously, referred to as concurrent synchrony, or with a temporal lag (i.e., changes in the infant are followed by subsequent changes in the mother, or vice versa), highlighting the dynamic nature of physiological coordination.