Norwegian Twin Study Confirms Genetic Factors Account for 58-81% of Variation in Drug Use and Dependence

Twin study of 1,386 complete young adult twin pairs from the Norwegian Institute of Public Health Twin Panel. Personal interviews assessed lifetime use, DSM-IV abuse/dependence symptoms and diagnoses for five drug categories. Twin model fitting using Mx statistical package compared genetic, shared environmental, and individual environmental contributions.

Replicating the heritability of drug use in Norway, a country with very different drug policies and much lower prevalence than the US or Australia, strengthens the conclusion that genetic vulnerability is a robust finding across cultural and legal contexts, not an artifact of specific social environments.

The consistency of heritability findings across countries with vastly different drug use rates and policies suggests that genetic vulnerability to substance use disorders is a fundamental human characteristic, not culturally specific. The absence of shared environmental effects (family, neighborhood) in best-fit models was particularly notable.

The low prevalence of drug use in Norway limited statistical power, particularly for individual substance dependence analyses. Twin studies assume equal environments for identical and fraternal twins. The young adult age of the sample means some participants may not yet have developed substance problems.