Mutual positive engagement with mother is uniquely associated with increases in human infant heart rate variability.

The ability of human infants to dynamically adapt their physiological states to changing social demands is critical for the development of emotion regulation. We investigated infant parasympathetic dynamics related to emotion regulation-changes in respiratory sinus arrhythmia (RSA)-using a large sample of 974 at-risk infant-mother dyads (infant age: 4 months). Infant RSA was elevated during bouts of positive engagement matching with their mothers, characterized by mutual positive engagement. Specifically, infant RSA began to increase more than 5 seconds before the onset of bouts of positive engagement matching and remained elevated for at least 4 seconds after bout onsets. These periods were substantially extended when the bouts began with positive expressions from the mother. Strikingly, there were no increases in infant RSA before or during bouts of either infant or mother solo positive engagement. Findings from these exploratory analyses suggest the possibility of bi-directional associations between ongoing social interactions and infant parasympathetic reactivity in which positive mother expressions support increases in infant RSA that in turn support prolonged infant engagement in mutual positivity. Thus, early social interaction is not simply an elicitor of infant positive expressions. Instead, mutual positive expressivity may facilitate infant neurophysiological changes that underlie emotion regulation. This article is part of the theme issue 'Mechanisms, development, phylogeny and functions of emotional expressions'.