Structure and Etiology of Co-occurring Internalizing and Externalizing Disorders in Adolescents

Abstract

Several studies suggest that a two-factor model positing internalizing and externalizing factors explains the interrelationships among psychiatric disorders. However, it is unclear whether the covariation between internalizing and externalizing disorders is due to common genetic or environmental influences. We examined whether a model positing two latent factors, internalizing and externalizing, explained the interrelationships among six psychiatric disorders (major depressive disorder, generalized anxiety disorder, separation anxiety disorder, attention-deficit/ hyperactivity disorder, oppositional defiant disorder, and conduct disorder) in adolescents, and whether there are common genetic and environmental influences on internalizing and externalizing latent factors. Multivariate behavior genetic analyses of data from 1162 twin pairs and 426 siblings ascertained from the general population via the Colorado Center for Antisocial Drug Dependence (CADD) were conducted. We found support for a model positing two latent factors (internalizing and externalizing). These factors were moderately heritable and influenced by significant common genetic and nonshared environmental influences. These findings suggest that co-occurrence of internalizing and externalizing psychopathology in adolescents results from both genetic and environmental influences. Keywordsadolescence; psychopathology; comorbidity; genetics; environment Psychiatric comorbidity, or co-occurrence of disorders, is commonly observed in general population samples, and alternative theoretical models explaining the causes of comorbidity have been proposed (Angold, Costello, & Erkanli, 1999). Many behavioral genetic studies have examined the etiology of psychiatric comorbidity in adolescents and adults, as increased knowledge regarding the etiology of comorbidity informs the understanding of the etiology, course, and treatment of psychiatric disorders. An Internalizing-Externalizing Model for Co-Occurring DisordersAn internalizing-externalizing model has received considerable attention as a potential theoretical framework for understanding co-occurring psychiatric disorders (Krueger, 1999). Internalization is the propensity to express distress inwards; common internalizing disorders include mood disorders (e.g., major depressive disorder, dysthymia) and anxiety disorders (e.g., generalized anxiety disorder, separation anxiety disorder, phobias, obsessivecompulsive disorder). In contrast, externalization describes the propensity to express distress outwards; commonly recognized externalizing disorders include attention-deficit/ hyperactivity disorder, oppositional defiant disorder, conduct disorder, antisocial personality disorder, and substance use disorders. NIH Public AccessAuthor Manuscript J Abnorm Child Psychol. Author manuscript; available in PMC 2011 February 10. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptFactor analytic studies of psychiatric disorders have found support for two factors, internalizing and external...

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