Abstract

The etiological connection between internalizing and externalizing disorders is poorly understood. This manuscript aims to investigate the roles of genes and then environment in the development of Major Depressive Disorder (MDD) and Antisocial Personality Disorder (ASPD) and the nature of their comorbidity in young adults. Data from a sample of 2,291 young adult male and female twins from the Young Adult Follow Up Study (YAFU) of the Virginia Twin Study of Adolescent Behavioral Development (VTSABD) were used to estimate the proportion of variation in these traits that can be accounted for by additive genetic, shared environmental, and unique environmental factors. In addition to traditional methods of measurement of psychiatric disorders (symptom sum scores), a more novel approach to measurement through item response theory (IRT) was employed. Through both measurement methods, variation in depression was found to be influenced almost entirely by environmental factors, however IRT analysis revealed genetic influences related to specific symptoms that cannot be gleaned from a traditional univariate analysis. While the symptoms of MDD appear to work well together to define the latent construct, symptoms of ASPD are less cohesive. ASPD item loadings are more disparate and the item response models are somewhat unstable compared with the MDD item analyses. Consistent with the literature, results using traditional sum score data indicated that ASPD was influenced by additive genetic and unique environmental effects. IRT analysis, however, did indicate a role for the shared environment in ASPD variation.ASPD/MDD comorbidity analyses suggest that more than one mechanism may reasonably explain the relationship between the disorders. It appears to be more likely that common genetic factors account for some of the observed comorbidity in females than in males. Conversely, the shared environment is that the most likely link between the two disorders in males. The etiological nature of these disorders is complex and analysis may be further complicated in a population spanning a critical developmental period, such as young adulthood. The item response approach has the potential to provide new insight into how these disorders develop and differ between the sexes and different age groups.