Previous research on biomarkers of individual differences in sensitivity to caregiving contexts has largely focused on children's parasympathetic activity, commonly indexed by RSA. Recent work, however, suggests that the parent-child dyadic RSA concordance may also provide important insight into heterogeneity in the links between parenting behaviors and children's adjustment outcomes. This study is among the first to characterize dyadic patterns of RSA concordance between behaviorally inhibited children aged 3.5 to 5 years old (54% female, 47% White) and their caregivers (n = 107 dyads) across tasks designed to mimic children's exposure to novel social interactions while parents observed their children navigating these tasks. Furthermore, we examined dyadic RSA concordance as a potential moderator of the associations between nurturing and intrusive parenting behaviors and children's adjustment problems, as reported by teachers and parents. We found that a more positive concordance (i.e., caregivers and children demonstrated similar patterns of epoch-to-epoch RSA change across tasks) protected against teacher-reported internalizing problems in the context of low parental nurturance. A negative concordance (i.e., caregivers and children demonstrated dissimilar patterns of epoch-to-epoch RSA change across tasks), however, exacerbated the risk for parents' reports of children's externalizing behaviors in the context of high parental intrusion.

The role of the gut microbiome in infant development has gained increasing interest in recent years. Most research on this topic has focused on the first three to four years of life because this is a critical period for developing gut-brain connections. Prior studies have identified associations between the composition and diversity of the gut microbiome in infancy and markers of temperament, including negative affect. However, the specific microbes affected, and the directionality of these associations have differed between studies, likely due to differences in the developmental period of focus and assessment approaches. In the current preregistered study, we examined connections between the gut microbiome, assessed at two time points in infancy (2 weeks and 18 months), and negative affect measured at 30 months of age in a longitudinal study of infants and their caregivers. We found that infants with higher gut microbiome diversity at 2 weeks showed more observed negative affect during a study visit at 30 months. We also found evidence for associations between specific genera of bacteria in infancy and negative affect. These results suggest associations between specific features of the gut microbiome and child behavior may differ based on timing of gut microbiome measurement.

Children's sociability, the motivation and ability to initiate and maintain social interactions, influences how children navigate their social worlds. When sociability supports interactions, it contributes to social competence and positive social-emotional outcomes. Although a substantial body of literature characterizes how specific parenting behaviors have an early and lasting influence on children's social development, far fewer studies have considered how characteristics of, and interactions within the parent-child dyad influence children's sociability. For example, matching affect and behaviors during dyadic interactions may provide insight into children's openness to affiliate, and these interactions provide opportunities to model and practice responses to behavioral and emotional cues common in social interactions. One novel approach to characterizing behavioral matching in dyadic interactions is language style matching (LSM), a linguistic construct characterizing the similarity of subconscious speech patterns (i.e., function words) between members of dyads during social interaction. Although researchers have established links between LSM and other characteristics of the dyad known to influence children's social development (e.g., attachment quality), whether and how LSM predicts children's adaptive sociability among peers is currently unknown. Therefore, we examined the associations between parent-child LSM and classroom observations of children's sociability among 151 preschoolers with elevated behavioral inhibition. Results indicated that LSM is positively associated with observed sociability in the company of classmates over and above other aspects of the parent-child relationship (e.g., parenting). Results supported LSM as an indicator of behavioral matching and contributed to the growing body of research that considers the implications of LSM for children's social development.

The role of the early life gut microbiome in children’s risk for psychopathology has gained increasing interest in recent years. Most research on this topic has focused on the first three to four years of life because this is a critical period for developing connections between the gut microbiome and the brain. Prior research has identified associations between variability in the composition and diversity of the gut microbiome in infancy and markers of temperamental risk for psychopathology, including negative affect. However, the specific microbes affected, and the directionality of these associations have differed between studies, likely due to differences in the developmental period of focus and assessment approaches. In the current preregistered study, we examined connections between the gut microbiome, assessed at two time points in infancy (2 weeks and 18 months of age), and negative affect measured at 30 months of age in a longitudinal study of infants and their caregivers. We found that infants with higher gut microbiome diversity and Veillonella abundance at 2 weeks of age, and higher Ruthenibacterium abundance at 18 months, showed more observed negative affect during a study visit at 30 months. These results suggest timing-specific associations between the gut microbiome and negative affect even within this early critical period.

Behavioral inhibition (BI), a dispositional trait characterized by fear and withdrawal in the face of novelty, is a risk factor for the later diagnosis of anxiety; yet, not all inhibited children go on to develop anxiety disorders. Certain parenting behaviors moderate risk for anxiety among temperamentally inhibited youth, which has motivated the creation of early parenting interventions to mitigate this risk among young children high in BI. These programs vary in complexity, begging the question of who needs more or less intensive intervention. In addition, variability in children's parasympathetic nervous system (PNS) functioning may provide insight into the substantial heterogeneity in anxiety outcomes for children high in BI. The current study investigates the role of vagal flexibility (VF), an index of non-linear change in PNS functioning, in children's response to an early intervention for anxiety (ClinicalTrials.gov registration: NCT02308826). Participants (n = 151, ages of 45-64 months) were randomized to participate in the Turtle Program ("Turtle"), a multi-component intervention comprising an adaptation of parent-child interaction therapy and social skills training, or Cool Little Kids (CLK), an established psychoeducational parenting intervention. Child participants also completed pre-, mid-, and post-treatment laboratory visits, which included a series of social stressor tasks preceded by a non-stressful but emotionally engaging video, during which cardiac physiology was recorded. Longitudinal latent profile analyses were used to identify latent non-linear trajectories of VF across the entire intervention (i.e., pre-, mid-, and post-treatment visits). Two latent classes were distinguished by the presence of high or low child VF across the treatment. Although VF class membership was not associated with intervention group membership, auxiliary within-class prediction models suggest that children who exhibited consistently greater VF demonstrated lower post-treatment clinician severity ratings of anxiety in Turtle, beta = -0.21, p = .01, but not in CLK, beta = 0.13, p = .39. Findings contribute to a broad literature demonstrating the utility of indices of PNS functioning in providing insight into risk for psychopathology and suggest that children's VF moderates treatment response by supporting children's ability to effectively engage in and extract benefits from intervention protocols. Implications for our understanding of the role children's physiological regulation plays in the links between BI, parenting, risk for anxiety, and treatment response are discussed.

Introduction Children with callous-unemotional (CU) traits are at high lifetime risk of antisocial behaviour. Low affiliation (ie, social bonding difficulties) and fearlessness (ie, low threat sensitivity) are proposed risk factors for CU traits. Parenting practices (eg, harshness and low warmth) also predict risk for CU traits. However, few studies in early childhood have identified attentional or physiological markers of low affiliation and fearlessness. Moreover, no studies have tested whether parenting practices are underpinned by low affiliation or fearlessness shared by parents, which could further shape parent–child interactions and exacerbate risk for CU traits. Addressing these questions will inform knowledge of how CU traits develop and isolate novel parent and child targets for future specialised treatments for CU traits.Methods and analysis The Promoting Empathy and Affiliation in Relationships (PEAR) study aims to establish risk factors for CU traits in children aged 3–6 years. The PEAR study will recruit 500 parent–child dyads from two metropolitan areas of the USA. Parents and children will complete questionnaires, computer tasks and observational assessments, alongside collection of eye-tracking and physiological data, when children are aged 3–4 (time 1) and 5–6 (time 2) years. The moderating roles of child sex, race and ethnicity, family and neighbourhood disadvantage, and parental psychopathology will also be assessed. Study aims will be addressed using structural equation modelling, which will allow for flexible characterisation of low affiliation, fearlessness and parenting practices as risk factors for CU traits across multiple domains.Ethics and dissemination Ethical approval was granted by Boston University (#6158E) and the University of Pennsylvania (#850638). Results will be disseminated through conferences and open-access publications. All study and task materials will be made freely available on lab websites and through the Open Science Framework (OSF).

Early behavioral inhibition (BI) is a known risk factor for later anxiety disorder. Variability in children's parasympathetic nervous system (PNS) functioning may provide insight into the substantial heterogeneity in anxiety outcomes for children high in BI. However, gaps persist due to an over-reliance on static measures of functioning, which limits our ability to leverage PNS functioning to identify risk for anxiety. We address these gaps using baseline data from an early intervention study of inhibited preschoolers by characterizing vagal flexibility (VF), an index of non-linear change in PNS functioning, across social stressor tasks and by examining the associations between VF and anxiety. One hundred and fifty-one parents and their 3.5- to 5-year-old children were selected on the basis of BI to participate in an early intervention program (ClinicalTrials.gov registration: NCT02308826). A structural equation modeling framework was used to model children's VF across tasks designed to mimic exposure to novel social interactions and to test the predictive links between VF and anxiety. Children who showed less VF, characterized by less suppression and flatter recovery, were rated by both parents and clinicians as more anxious. Moreover, a multiple group model showed that children meeting diagnostic criteria for social anxiety disorder demonstrated significantly less VF across social stressor tasks. Among inhibited youth, reduced VF is a risk factor for anxiety and may reflect an individual's reduced capacity to actively cope with external demands. Study results contribute to our understanding of the regulatory processes underlying risk for anxiety in early childhood.