Externalizing behavioral problems in young children are associated with later delinquency and crime,¹ which can cause burdens at both personal and socialeconomic levels. The heterogeneity of externalizing problems emphasizes the importance of examining the etiological mechanisms that underlie externalizing problems and related behaviors. The present study focuses on 2 risk factors for externalizing behavioral problems in early childhood: callous-unemotional traits (CU), characterized as a lack of guilt and empathy,² and irritability, a tendency to show anger and frustration.³ Behavioral genetic studies find that externalizing problems, CU, and irritability are heritable,^4,5 raising the possibility of common genetic effects linking the 3 behaviors, but this has not been previously explored. Neurological evidence suggests distinct pathways from CU and irritability to externalizing problems,⁶ implying that the genetic and environmental factors linking externalizing problems and CU may differ from those linking externalizing problems and irritability. We predict that there will be common genetic influences operating across externalizing problems, CU, and irritability; but we also predict unique genetic and environmental influences representing distinctive risks shared between externalizing problems and CU, and between externalizing problems and irritability, respectively.