Variation in Everyday Maternal Sensitivity Predicts Infants' Real-Time Physiological Regulation.

While maternal sensitivity is generally considered a stable trait, recent work has found that caregiver sensitivity varies substantially across the day as caregivers respond to their infants amid everyday demands and distractions. It remains unknown how variation in everyday caregiver sensitivity influences infants' real-time physiological regulation during naturally occurring distress. In a sample of 23 infants (1-10 months old, 48% female), we synchronized infant-worn full-day home audio recordings and heart rate monitors to examine changes in infants' respiratory sinus arrhythmia (RSA) following the onset of spontaneous distress, that is, episodes of fussing or crying, occurring across the day (N = 332 episodes, M = 14.34 per dyad). Caregiver sensitivity was coded individually for each episode of infant distress on a 1-7 scale, and RSA was time-locked to distress onset. Between-person analyses found that infants of caregivers with above-average sensitivity (i.e., sensitivity greater than the sample mean) showed adaptive decreases in RSA following distress onset. Additionally, within-person analyses found that infants also displayed significant decreases in RSA only on occasions when caregivers responded more sensitively relative to their own average. These findings demonstrate that both stable differences in caregiving quality and moment-to-moment variation in sensitivity shape infant's physiological regulation during everyday distress interactions.