Mother–infant dyadic reparation and individual differences in vagal tone affect 4-month-old infants’ social stress regulation

•Infants social stress regulation encompasses reactivity and recovery processes.•Dyadic functioning and vagal tone have been separately linked to social stress.•4-Month-old infants were exposed to social stress (i.e., Still-Face paradigm).•Dyadic functioning and vagal suppression jointly affected social stress regulation.

Infants’ social stress regulation (i.e., reactivity and recovery) might be affected by mother–infant dyadic functioning and infants’ vagal tone (i.e., respiratory sinus arrhythmia, RSA). This study investigated the role of a specific dyadic functioning feature (i.e., dyadic reparation) and individual differences in vagal tone regulation (i.e., RSA suppression vs. non-suppression) in relation to social stress regulation in 4-month-old infants. A total of 65 mother–infant dyads participated in the face-to-face still-face paradigm. Social stress reactivity and recovery were measured as negative emotionality during Still-Face and Reunion episodes, respectively. RSA was measured during Play, Still-Face, and Reunion episodes. Suppressors had higher dyadic reparation during Play and higher recovery from social stress compared with non-suppressors. Higher reparation during Play was associated with lower reactivity and higher recovery only for suppressors. Findings suggest a joint role of infants’ RSA individual differences and dyadic reparation in affecting infants’ social stress regulation at 4 months of age.