Measured environmental contributions to cannabis abuse/dependence in an offspring of twins design

Genetic and environmental factors are known to contribute to cannabis abuse/dependence (CAD). We sought to determine the magnitude of the contribution from measured environmental variables to offspring cannabis dependence in a design that controls for familial vulnerability. Data come from a study of 725 twin members of the Vietnam Era Twin Registry, 720 of their biological offspring (age 18–32 years) and 427 mothers. Data were obtained on offspring perception of family and peer support and substance use behaviors and offspring CAD. After adjusting for familial risk, and environmental covariates, CAD was significantly more likely among male offspring (OR = 2.73; 95% CI: 1.69–4.41). Offspring CAD was associated with reporting: siblings used illicit drugs (OR = 3.40; 95% CI: 1.81–6.38), a few friends used drugs (OR = 2.72; 95% CI: 1.04–7.09), a quarter or more friends used drugs (OR = 8.30; 95% CI: 3.09–22.33) and one-half or more 12th grade peers used drugs (OR = 3.17; 95% CI: 1.42–7.08). Perceived sibling, friend and school peer substance use are strongly associated with CAD in young adults even after accounting for latent familial risk and for multiple measured intra-family and extra-family environmental influences.