Sex and age as moderators in the expression of internalizing and externalizing behaviors: insights from the Child Behavior Checklist (CBCL)

BACKGROUND: Children referred for anxiety disorders frequently show both internalizing and externalizing problems, but the role of sex and age in shaping these behavioral expressions remains unclear. Prior research suggests sex differences may shift across development, yet findings have been inconsistent. METHODS: We analyzed data from 600 children (ages 6-17; 53.2% male) referred to a pediatric anxiety specialty clinic in the Northeastern United States between 2013 and 2021. Parents completed the Child Behavior Checklist (CBCL/6-18). Ordinary least squares regression models with an interaction term (sex × age) were estimated using Hayes' PROCESS macro (Model 1). The Johnson-Neyman technique was applied to identify developmental periods where sex differences were significant. RESULTS: Sex significantly moderated the association between age and CBCL scores. Boys displayed higher externalizing scores than girls between ages 6 and 9.7 years, whereas girls showed higher internalizing scores from age 10.4 years onward. Effect sizes for the interactions were modest (ΔR2 range = .03-.05). CONCLUSIONS: Findings indicate that the expression of behavioral problems among anxiety-referred youth differs by both sex and developmental stage. Early externalizing difficulties in boys and later-emerging internalizing difficulties in girls suggest that findings may inform age- and sex-sensitive approaches to assessment and intervention to age- and sex-specific trajectories in pediatric anxiety populations.