Parent-child synchrony of respiratory sinus arrhythmia (RSA) varies by risk, but novel approaches are needed to capture individual contributions to synchrony. Multilevel state-trait modeling was applied to examine how parental psychological distress and parent and child average RSA during challenge (reflecting individual regulatory capacities) shaped RSA synchrony in mother-child (n=71) and father-child (n=47) interactions. RSA synchrony was curvilinear such that greater in-the-moment RSA reactivity in one partner prompted greater reactivity in the other. Higher risk (lower average RSA; higher distress) predicted in-the-moment RSA withdrawal to partner RSA changes, whereas lower risk (higher average RSA; lower distress) predicted in-the-moment RSA augmentation. In some models, one's higher average RSA prompted the partner's greater reactivity and thus synchrony when parental distress was higher. However, the presence and direction of synchrony was not consistently adaptive nor maladaptive across models, suggesting its meaning relies on theory and the parent and risk factors in question. [This paper was published in "Biological Psychology" v161 2021.]