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

The takeaway

A study of 1,386 Norwegian twin pairs found heritability of illicit drug use ranged from 58% to 81% across substances, replicating findings from the US and Australia in a country with much lower drug use rates.

Read this if you want to understand how much of drug use risk is genetic versus environmental.

58-81% heritability for drug use replicated in Norway despite much lower prevalence than US/Australia

What the researchers found

Researchers assessed lifetime use, abuse, and dependence across five illicit drug categories (cannabis, stimulants, opiates, cocaine, psychedelics) in 1,386 young adult Norwegian twin pairs. Only 6.4% reported significant lifetime use (10+ times) of any illicit substance, reflecting Norway's low drug use prevalence.

Despite the low prevalence, twin model fitting consistently found that resemblance between twins was due largely or entirely to genetic factors rather than shared environment. Best-fit models included only genetic and individual-specific environmental effects, with heritability estimates ranging from 58% to 81% across all analyses.

Meaningful abuse/dependence analyses were only possible for cannabis and for any substance combined, as too few twins met criteria for individual substance dependence diagnoses. The genetic findings replicated results previously reported from the US and Australia.

Why it matters

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 numbers in context

1,386 twin pairs studied. Only 6.4% reported significant lifetime illicit drug use (10+ times). Heritability estimates: 58-81% across analyses. Best-fit models: genetic + individual environmental factors only (no shared environmental contribution). Replicated US and Australian findings.

How the study worked

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.

What this study cannot tell us

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.

How to read the evidence

Large twin study with personal interviews and standard twin modeling methodology. Cross-cultural replication strengthens the genetic findings.

When this study was published

Published in 2006. Genome-wide association studies have since begun identifying specific genetic variants associated with cannabis use and substance use disorders.

The bigger picture

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.

Questions still open

Why did shared environment (family factors) not contribute to twin resemblance in drug use? Do the same specific genetic variants contribute to drug use risk across different cultural contexts?

Common questions

How much of drug addiction risk is genetic?

This study found genetic factors accounted for 58-81% of the variation in illicit drug use and dependence. The remaining variation was due to individual-specific environmental factors (personal experiences), not shared family environment.

Does growing up in a low-drug-use country protect against genetic risk?

Norway has much lower drug use rates than the US or Australia, yet the genetic contribution to drug use risk was similar. This suggests genetic vulnerability operates regardless of the overall social context, though the lower prevalence means fewer people are exposed to triggers.

Read the original research

Illicit psychoactive substance use, abuse and dependence in a population-based sample of Norwegian twins.

Psychological medicine, 36(7), 955-62

View on PubMed ↗

Citation

Kendler, Kenneth S; Aggen, Steven H; Tambs, Kristian; Reichborn-Kjennerud, Ted. (2006). Illicit psychoactive substance use, abuse and dependence in a population-based sample of Norwegian twins.. Psychological medicine, 36(7), 955-62.