All common dimensions of psychopathology in children and adolescents are positively correlated to varying degrees, often quite substantially.1-3 Like others,4, 5 we posit that the correlated nature of psychopathology reflects the underlying etiologic structure of psychopathology. In particular, Kendler hypothesized that many genes pleiotropically influence risk for multiple mental disorders.6 This hypothesis is supported by studies of adult twins showing that major depressive disorder (MDD) and generalized anxiety disorder (GAD) are substantially influenced by common genes, as are antisocial behavior and multiple forms of substance abuse.7, 8 Perhaps most importantly, a twin study of categorical mental disorders in adults identified two broad genetic factors accounting for most of the genetic variance in the mental disorders that loaded on them: An internalizing (anxiety disorders and depression) and an externalizing (conduct problems, antisocial personality disorder, and substance use disorders) factor.8
