Beyond Core ADHD Symptoms: Clinical Evaluation of Antisocial Behaviours Within a Unified Externalizing Framework in Youth With ADHD

Attention-deficit/hyperactivity disorder (ADHD) frequently co-occurs with oppositional and conduct-related behavioural difficulties, raising questions about how these symptoms are structurally related within clinically diagnosed youth. Dimensional models of psychopathology, such as the Hierarchical Taxonomy of Psychopathology (HiTOP), conceptualize these behaviours as part of a broader externalizing spectrum characterized by shared patterns of behavioural dysregulation. However, empirical evidence examining the coherence of antisocial externalizing dimensions within paediatric ADHD clinical samples remains limited. The present study investigated patterns of externalizing symptom covariation in a clinical sample of children and adolescents aged 6-18 years with ADHD diagnoses established through structured clinical interviews conducted by trained clinicians. Parent-reported DSM-oriented scales from the Child Behaviour Checklist (CBCL) were analyzed using confirmatory factor analytic models to compare alternative structural representations of internalizing and externalizing symptoms, including unified and differentiated externalizing configurations. Results indicated substantial covariation among ADHD, oppositional defiant and conduct-related symptom dimensions. Models specifying a unified externalizing structure demonstrated greater parsimony and comparable or superior fit relative to more differentiated models. These findings suggest that, in this clinically diagnosed paediatric ADHD sample, ADHD, oppositional defiant and conduct-related symptom dimensions were more economically represented by a unified externalizing configuration than by a more differentiated alternative. Importantly, these results should be interpreted as reflecting covariance among six parent-reported CBCL DSM-oriented scales within a restricted clinical and measurement context, rather than as broader validation of the hierarchical placement of ADHD within HiTOP or other dimensional taxonomies. From a clinical perspective, the findings highlight the value of integrated assessment of disinhibited and antisocial behavioural features when evaluating youth with ADHD. Future research incorporating multi-informant assessments and broader neurodevelopmental indicators may further clarify how ADHD symptom dimensions relate to overlapping developmental and behavioural domains.