Are Depression, Alcohol Dependence, and Cannabis Dependence Genetically Linked? A Twin Study

This study used the Vietnam Era Twin Registry, analyzing data from 3,360 male twin pairs (1,868 monozygotic and 1,492 dizygotic). Lifetime DSM-III-R diagnoses were obtained through telephone diagnostic interviews. Structural equation modeling estimated additive genetic, shared environmental, and nonshared environmental effects common and specific to each disorder.

This study provided a mechanistic explanation for why depression and substance dependence so often co-occur. Rather than depression causing substance use or vice versa, the shared genetic risk was largely channeled through antisocial personality traits. This has implications for treatment: addressing antisocial features may be important when treating comorbid depression and substance dependence.

This study was part of a broader effort to understand the genetic architecture of psychiatric comorbidity. The finding that antisocial personality disorder is a genetic hub connecting mood and substance use disorders has been supported by subsequent genomic studies identifying shared genetic loci across these conditions.

The sample was limited to male veterans, who may not represent the general population. The Vietnam Era Twin Registry overrepresents men exposed to military service during a specific historical period. Telephone diagnostic interviews have limitations compared to in-person clinical assessment. The study could not identify specific genes involved.