The trait impulsivity theory suggests that a single, highly heritable externalizing liability factor, expressed as temperamental trait impulsivity, represents the core vulnerability for externalizing disorders. The present study sought to test the application of latent factor models derived from this theory to a clinical sample of children. Participants were 474 German children (age 6-12 years, 81% male) with symptoms of attention-deficit/hyperactivity disorder and externalizing behavior problems participating in an ongoing multicenter intervention study. Using confirmatory factor analyses (CFA) and exploratory structural equation modeling (ESEM), we evaluated several factor models of externalizing spectrum disorders (unidimensional; first-order correlated factors; higher-order factor; fully symmetrical bifactor; bifactor S-1 model). Furthermore, we assessed our prevailing factor models for measurement invariance across raters (clinicians, parents, teachers) and assessment modes (interview, questionnaires). While both CFA and ESEM approaches provided valuable insights into the multidimensionality, ESEM solutions were generally superior since they showed a substantially better model fit and less biased factor loadings. Among the models tested, the bifactor S-1 CFA/ESEM models, with a general hyperactivity-impulsivity reference factor, displayed a statistically sound factor structure and allowed for straightforward interpretability. Furthermore, these models showed the same organization of factors and loading patterns, but not equivalent item thresholds across raters and assessment modes, highlighting cross-situational variability in child behavior. Our findings are consistent with the assumption of the trait impulsivity theory that a common trait, presented as hyperactivity-impulsivity symptoms, underlies all externalizing disorders.

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To enhance the understanding of how symptoms of attention-deficit/hyperactivity disorder (ADHD) and disruptive behavior disorders such as oppositional defiant disorder (ODD), conduct disorder (CD), including callous-unemotional (CU) traits, differentially relate to functional impairment (FI). Participants were 474 German school-age children (age: M = 8.90, SD = 1.49, 81% male) registered for participation in the ESCAschool trial (ESCAschool: Evidence-based, Stepped Care of ADHD in school-aged children). Clinicians assessed the severity of individual symptoms and five FI domains specifically associated with ADHD symptoms or ODD/CD/CU symptoms using a semi-structured clinical interview. We conducted two multiple linear regression analyses, combined with relative importance analyses, to determine the impact of individual symptoms on global FI associated with ADHD and ODD/CD/CU symptoms. Next, we estimated two networks and identified the strongest associations of ADHD symptoms or ODD/CD/CU symptoms with the five FI domains. Symptoms varied substantially in their associations with global FI. The ADHD symptom Easily Distracted (15%) and ODD symptom Argues with Adults (10%) contributed most strongly to the total explained variance. FI related to academic performance, home life and family members, and psychological strain were most strongly associated with ADHD inattention symptoms, whereas FI related to relationships with adults and relationships with children and recreational activities were most strongly associated with hyperactivity-impulsivity symptoms. By comparison, the ODD/CD/CU symptoms most closely linked to FI domains originated from the ODD and CD dimensions. Our findings contribute to a growing body of literature on the importance of analyzing individual symptoms and highlight that symptom-based approaches can be clinically useful.