Examining processes that characterize the ebb and flow of emotions offers insight into how infants modulate their own emotional experience as well as how both mothers and infants jointly regulate their emotional states. Drawing from polyvagal theory, which posits that vagal tone supports the capacity to quickly, flexibly, and adaptively respond to contextual demands (Porges, 2003, 2007), I hypothesized that infants with greater vagal tone (indexed by respiratory sinus arrhythmia; RSA) would show stronger evidence of emotion regulation and coregulation processes during free play and a frustrating task at 24 weeks child age. To evaluate these hypotheses, I used dynamic structural equation modeling (DSEM; Asparouhov, Hamaker, & Muthén, 2018) to examine biologically-based differences in second-by-second infant emotion regulation (equilibria, volatility, carryover, and feedback loops in positive and negative affect engagement) and mother- and infant-driven coregulation processes, among a sample of 210 low-income, Mexican-origin mother-infant dyads. Results offered evidence of both mother-driven and infant-driven emotion coregulatory processes during free play, which did not differ based on infant RSA. Results offered limited support for RSA-based differences in infant self-regulation processes during the teaching task, such that infants with below average RSA tended to respond to increased negative affect with subsequent increases in positive affect engagement. Prenatal maternal depressive symptoms also accounted for greater infant emotional volatility and weaker mother-driven emotion coregulation. Results highlight the unique roles mothers and infants play in achieving emotion regulation, as well as between-dyad differences in these processes, suggesting multiple pathways towards resilience among low-income, Mexican-origin families.

The theory of biological sensitivity to context (BSC; Boyce & Ellis, 2005) posits that specific biological characteristics, such as vagal tone, may confer risk for physical and mental health outcomes for some children but promote health for others. High levels of resting respiratory sinus arrhythmia (RSA), an index of vagal tone, may confer susceptibility to the effects of the caregiving environment on child development. Consistent with BSC, I expected that, relative to infants with lower RSA, infants with higher RSA would demonstrate fewer behavior problems if their mothers reported fewer postpartum depressive symptoms, but more behavior problems if their mothers reported more postpartum depressive symptoms. I also evaluated whether observed child social engagement with their mothers mediated children's biological sensitivity to the effects of postpartum depressive symptoms on behavior problems in early childhood. I evaluated a mediated moderation model among a sample of 322 low-income Mexican American mother-infant dyads. As expected, the RSA x maternal depressive symptoms interaction, controlling for covariates, was a significant predictor of internalizing, externalizing and total behavior problems, and high vagal tone conferred susceptibility for externalizing behavior problems. Contrary to my hypothesis, children with low RSA may be more susceptible to the effects of maternal postpartum depressive symptoms on children's internalizing and total behavior problems, and child social engagement did not account for these effects. Among infants in economically disadvantaged families, lower RSA and fewer maternal depressive symptoms may promote resilience, and more research is needed to understand behavioral mediators of biological sensitivity.