Informant-discrepancy in the Affective Reactivity Index Reflects the Multifaceted Nature of Childhood and Adolescent Irritability

Objective: The Affective Reactivity Index (ARI) is widely used to assess young people’s irritability symptoms, but youth and caregivers often diverge in their assessments. Such informant discrepancy might stem from poor reliability. However, evidence from genetic, imaging and treatment studies suggests that irritability may not be a unitary construct. Hence informants might be sensitive to different aspects of irritability. We use an out-of-sample replication approach and a longitudinal design to test these hypotheses. Method: Across two independent samples (N1=765, 8-21 years; N2=1910, 6-21 years), we investigate the reliability and measurement invariance of the ARI, examine sociodemographic and clinical predictors of discrepant reporting, and probe the utility of a bifactor model for cross-informant integration. Results: Despite good internal consistency and 6-week-retest-reliability of parent and youth forms, we confirm substantial informant discrepancy in ARI ratings, which is stable over six weeks (n1=177). Measurement invariance across informants was weak, indicating that parents and youth may interpret ARI items differently. Informant-discrepancy increased with irritability severity, but which informant reported higher child-irritability depended on the child’s diagnostic status. In both datasets, a bifactor model parsing informant-specific from shared irritability-related variance fit the data well. Conclusion: Parent and youth ARI reports and their discrepancy are reliable. However, parent and youth ratings may reflect different interpretations of the scale items; hence they should not be averaged. Our findings suggest that irritability is not a unitary construct. Future work should investigate and model different aspects of irritability, which might be more accessible to specific informants.