Categorical and dimensional approaches to the developmental relationship between ADHD and irritability

Background: Attention deficit hyperactivity disorder (ADHD) and irritability commonly co-occur, and follow similar developmental trajectories from childhood to adolescence. Understanding of the developmental relationship between these co-occurrences is limited. This study provides a longitudinal assessment of how ADHD diagnostic status and symptom patterns predict change in irritability. Methods: A community sample of 337 participants (45.2% ADHD), recruited for the Childhood Attention Project, completed the Affective Reactivity Index (ARI) to measure irritability at baseline (mean age 10.5 years) and follow-up after 18-months. Latent change score models were used to assess how (a) baseline ADHD vs. control group status, (b) baseline symptom domain (inattention, hyperactivity–impulsivity) and (c) longitudinal change in ADHD symptom severity predicted change in irritability. Results: Irritability was significantly higher among the ADHD group than controls; however, change in irritability over time did not differ between groups. When assessed across the entire cohort, change in irritability was predicted by higher symptom count in the hyperactive–impulsive domain, but not the inattentive domain. Greater declines in ADHD symptoms over time significantly predicted greater declines in irritability. Baseline ADHD symptom severity was found to significantly predict change in irritability; however, baseline irritability did not significantly predict change in ADHD symptoms. Conclusions: ADHD symptoms—particularly hyperactive–impulsive symptoms—predict the degree and trajectory of irritability during childhood and adolescence, even when symptoms are below diagnostic thresholds. The use of longitudinal, dimensional and symptom domain-specific measures provides additional insight into this relationship.