The internal structure of externalizing disorders is often studied using the symmetrical bifactor model, which often shows anomalous results. The bifactor-(S-1) model (Eid et al. 2017) aimed to eliminate anomalous results. In order to evaluate the use of the bifactor-(S-1) model, this study applied the model to externalizing disorders in the Japanese clinical population. We set attention-deficit/hyperactivity disorder-hyperactivity/impulsivity (ADHD-HI) as the general reference factor and ADHD-inattentive (IA), oppositional defiant disorder (ODD), and conduct disorder (CD) as specific factors in this study. The criterion-related validity of questionnaires was assessed using the point biserial correlation coefficient. Anomalous loadings occurred with the symmetrical model, while the bifactor-(S-1) model showed improvement. Model fit for the bifactor-(S-1) model was satisfactory, and point biserial correlation coefficients were significant. One limitation was that a small number of participants endorsing ODD or CD symptoms led to unstable estimates. The results support using the bifactor-(S-1) model for assessing symptoms of externalizing disorders.
