The Structure of Genetic and Environmental Risk Factors for Syndromal and Subsyndromal Common DSM-IV Axis I and All Axis II Disorders

TLDR

The study aimed to clarify the genetic and environmental structure of 22 DSM‑IV Axis I and II disorders in a Norwegian twin cohort. Using 2,111 twin interviews, the authors identified four correlated genetic factors—Axis I internalizing, Axis II internalizing, Axis I externalizing, Axis II externalizing—and three correlated environmental factors—Axis II disorders, Axis I internalizing disorders, and externalizing versus anxiety disorders. The results support a two‑dimension internalizing‑externalizing framework, with most disorders loading on two factors, and show that genetic rather than environmental influences largely drive the coherent grouping of psychiatric disorders. Interpretation should consider the unavoidable limitations of current statistical methods applied to this number of diagnostic categories.

Abstract

The authors sought to clarify the structure of the genetic and environmental risk factors for 22 DSM-IV disorders: 12 common axis I disorders and all 10 axis II disorders.The authors examined syndromal and subsyndromal axis I diagnoses and five categories reflecting number of endorsed criteria for axis II disorders in 2,111 personally interviewed young adult members of the Norwegian Institute of Public Health Twin Panel.Four correlated genetic factors were identified: axis I internalizing, axis II internalizing, axis I externalizing, and axis II externalizing. Factors 1 and 2 and factors 3 and 4 were moderately correlated, supporting the importance of the internalizing-externalizing distinction. Five disorders had substantial loadings on two factors: borderline personality disorder (factors 3 and 4), somatoform disorder (factors 1 and 2), paranoid and dependent personality disorders (factors 2 and 4), and eating disorders (factors 1 and 4). Three correlated environmental factors were identified: axis II disorders, axis I internalizing disorders, and externalizing disorders versus anxiety disorders.Common axis I and II psychiatric disorders have a coherent underlying genetic structure that reflects two major dimensions: internalizing versus externalizing, and axis I versus axis II. The underlying structure of environmental influences is quite different. The organization of common psychiatric disorders into coherent groups results largely from genetic, not environmental, factors. These results should be interpreted in the context of unavoidable limitations of current statistical methods applied to this number of diagnostic categories.