A new study published in the International Journal of Behavioral Development suggests that preschool children with greater physiological flexibility may be more likely to benefit from intensive anxiety interventions. The researchers found that young children with high vagal flexibility — a marker of how well the body adapts to changing emotional and social demands — showed greater reductions in anxiety symptoms following a comprehensive early intervention program known as the Turtle Program. In contrast, children with lower physiological flexibility showed less improvement, regardless of the type of intervention received.
Children who show a pattern of withdrawal, fear, or inhibition in the face of new people or situations — known as behavioral inhibition — are at higher risk of developing anxiety later in life. However, not all behaviorally inhibited children go on to experience anxiety disorders. Researchers have long suspected that parenting practices, social environments, and biological differences play a role in determining which children are more vulnerable.
Parenting-focused interventions can be effective, especially when they aim to reduce overprotective behaviors that may reinforce children’s avoidance. But these programs differ in complexity and intensity. Some include only parent education, while others, like the Turtle Program, provide direct child intervention and parent coaching. Knowing which children benefit most from more intensive treatment could make interventions more efficient and personalized.
The researchers focused on vagal flexibility — a measure of how the parasympathetic nervous system (which calms the body after stress) responds and recovers during emotionally engaging situations. Greater vagal flexibility has been linked to better emotional regulation, attention, and social functioning. The team wanted to know whether vagal flexibility could predict which children would respond best to different types of early intervention.
The study included 151 children between 3.75 and 5.3 years old, all identified as high in behavioral inhibition. Children were randomly assigned to one of two interventions: the Turtle Program or Cool Little Kids (CLK). The Turtle Program combines in-person coaching for parents with group-based social skills training for children, while CLK is a parent-only psychoeducational program.
Children completed lab assessments before, during, and after treatment, where they watched calming videos and took part in mildly stressful social tasks — such as meeting a clown or introducing themselves to imaginary peers. Throughout these sessions, researchers recorded their heart rate patterns using a standard measure called respiratory sinus arrhythmia, which provides insight into parasympathetic nervous system activity. From this data, the team calculated vagal flexibility based on how much the children’s physiological responses varied and recovered across the different tasks.
They then used statistical modeling to group children based on their patterns of vagal flexibility across the treatment timeline and examined how these patterns related to anxiety severity after the intervention.
The children fell into two main groups: those with high vagal flexibility and those with low flexibility. High-flexibility children showed noticeable changes in their physiological responses — including suppression during challenging tasks and recovery during calming periods — while low-flexibility children had flatter, less dynamic patterns.
Importantly, the researchers found that children with high vagal flexibility who participated in the Turtle Program showed the greatest reductions in anxiety symptoms, as measured by clinical interviews with parents. For these children, the more intensive and interactive nature of the Turtle Program appeared to support greater therapeutic gains. In contrast, high-flexibility children in the less intensive CLK program did not show the same level of improvement.
Children in the low-flexibility group showed relatively little change in anxiety severity after treatment, regardless of which program they received. This suggests that their reduced ability to physiologically adapt to changing social and emotional demands may have limited their engagement or responsiveness to the interventions.
The findings suggest that children with high vagal flexibility may be better equipped to engage in and benefit from interventions that involve active participation, both from themselves and their caregivers. These children might respond particularly well to programs like the Turtle Program, which provide real-time coaching, peer interaction, and multiple layers of support.
The study also has limitations. The participants came from relatively educated families, which may limit the generalizability of the results to more diverse populations. The researchers also noted that vagal flexibility was measured during separate lab sessions, not during the actual therapy sessions, which means the exact mechanisms through which vagal flexibility supports treatment engagement are still unclear.
In addition, the study followed children only through the immediate post-treatment phase. Longer-term follow-up would be needed to determine whether the benefits for high-flexibility children in the Turtle Program persist over time and whether interventions can be modified to better support children with low physiological flexibility.
Another open question is whether vagal flexibility itself can be improved. Some evidence suggests that parenting behaviors and specific types of biofeedback or nerve stimulation may help enhance autonomic regulation in children. Future studies could explore whether boosting vagal flexibility directly might improve outcomes for children who are less physiologically adaptable.
The study, “Children’s vagal flexibility is a predictor and moderator of treatment response to early parenting interventions for inhibited youth,” was authored by Nicholas J. Wagner, Nila Shakiba, Hong N.T. Bui, Danielle Novick, Kathy Sem, Christina M. Danko, Kenneth H. Rubin, and Andrea Chronis-Tuscano.
