title: Structural models of the comorbidity of internalizing disorders and substance use disorders in a longitudinal birth cohort.
abstract: OBJECTIVE
  The present study examined the associations between internalizing disorders and substance use disorders using structural equation models to examine the relative contributions of common fixed confounding factors and direct causal pathways, and to determine the direction of causality.
  
  
  METHODS
  Data were gathered during the course of the Christchurch Health and Development Study, a 25-year longitudinal study of a birth cohort of New Zealand children (n = 953). Measures included DSM-IV symptom criteria for major depression, anxiety disorders, alcohol abuse/dependence, nicotine dependence, and illicit drug abuse/dependence, ages 18, 21, and 25.
  
  
  RESULTS
  Structural equation modeling showed that for depression and substance use disorder, the best-fitting model was the one in which the associations were explained by a combination of common fixed factors and direct causal pathways from substance use to depression. A similar pattern emerged for anxiety disorders and (1) nicotine dependence, and (2) illicit drug abuse/dependence. The comorbidity of anxiety disorder and alcohol abuse/dependence was best explained by a model that included common fixed factors and a reciprocal pathway between these disorders. Decomposition of the correlations between internalizing disorders and substance use disorders showed that most of the correlation arose from direct causal pathways between disorders.
  
  
  CONCLUSIONS
  The findings suggest that the comorbidity between internalizing disorders and substance use disorders can be attributed to both common fixed factors and direct causal pathways between substance use disorder and internalizing disorder. The evidence suggests that, in most cases, the most plausible explanation of causality is the one in which substance use disorder increases the risk of internalizing disorder.
authors.affiliation: Christchurch School of Medicine and Health Sciences, University of Otago, Christchurch, New Zealand. dm.fergusson@otago.ac.nz
authors.forename: David M
authors.initials: DM
authors.lastname: Fergusson
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Boden
authors.forename: L John
authors.initials: LJ
authors.lastname: Horwood
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D001008:Anxiety Disorders / Q000453:epidemiology*; D015331:Cohort Studies; D015897:Comorbidity; D003865:Major Depressive Disorder / Q000453:epidemiology*; D005260:Female; D006801:Humans; D008297:Male; D008961:Models, Structural; D009520:New Zealand / Q000453:epidemiology; D019966:Substance-Related Disorders / Q000453:epidemiology*; D055815:Young Adult