OBJECTIVE: Callous-unemotional traits (CU), characterized as a lack of guilt and empathy, and irritability, a tendency to show anger and frustration, are 2 risk factors for externalizing behavioral problems. Externalizing problems, CU, and irritability are all heritable. However, there is a dearth of studies examining the genetic and environmental associations between the 3 domains. The present study partitioned joint and independent etiological pathways from CU and irritability to externalizing problems. METHOD: The sample consisted of 614 pairs of 3-year-old twins from the Boston University Twin Project. Primary caregivers reported twins' externalizing problems, CU, and irritability using the Child Behavior Checklist. Biometric Cholesky models were used to estimate common and unique genetic and environmental variances among the 3 domains. RESULTS: There were common genetic, shared environmental and nonshared environmental factors operating across all 3 domains. In addition, there were unique genetic and nonshared environmental factors, independent of the common effects, linking externalizing problems and CU, and externalizing problems and irritability, respectively. There were also genetic and nonshared environmental influences unique to externalizing problems, independent of CU and irritability. CONCLUSION: Common genetic as well as shared and nonshared environmental associations among externalizing problems, CU, and irritability suggest, to some extent, that etiological influences are common to all 3 constructs. However, distinct genetic and child-specific nonshared environmental links separately from CU and irritability to externalizing problems, reveals the heterogeneity of externalizing problems, and suggests that they should not be considered a unitary outcome. STUDY PREREGISTRATION INFORMATION: Study Preregistration: Understanding the Etiology of Externalizing Problems in Young Children: The Roles of Callous-Unemotional Traits and Irritability
https://doi.org/10.1016/j.jaac.2023.09.549.