Cannabis initiation was 44% heritable in the Netherlands, similar to countries with stricter laws | Research

The takeaway

A Dutch twin study of 3,115 twins found 44% genetic influence on cannabis initiation, with 31% shared environment and 24% unique environment, similar to findings from countries where cannabis is illegal.

Read this if you are interested in how cannabis policy affects the role of genetics versus environment in cannabis use.

44% heritability regardless of liberal Dutch cannabis policy

What the researchers found

Researchers examined cannabis initiation in 3,115 Dutch twins with a mean age of 27.4 years. The Netherlands permits small-scale cannabis use, providing a natural experiment to test whether liberal drug policy changes the genetic-environmental balance.

Genetic influences accounted for 44% of individual differences in cannabis initiation. Shared environmental factors (family, peers, neighborhood) explained 31%, and unique environmental factors explained 24%.

These proportions were remarkably similar to those found in the United States, Australia, and United Kingdom, where cannabis policies were more restrictive. The authors concluded that the relative importance of genetic and environmental factors was not meaningfully different in a country with more liberal cannabis policy.

Why it matters

The finding that heritability proportions were similar regardless of legal context suggested that genetic vulnerability to cannabis use operates independent of policy environment.

The numbers in context

3,115 twins, mean age 27.4 (SD 4.7). Heritability: 44%. Shared environment: 31%. Unique environment: 24%. Results comparable to US, Australia, and UK estimates.

How the study worked

Twin study of 3,115 twins (mean age 27.4) from the Netherlands Twin Register. Standard genetic modeling to estimate additive genetic (A), shared environmental (C), and unique environmental (E) contributions to cannabis initiation.

What this study cannot tell us

Single-country study with self-reported cannabis use. The Netherlands policy specifically applies to small amounts; results might differ under full legalization. Twin studies assume equal environments for identical and fraternal twins.

How to read the evidence

Well-powered twin study from a unique policy environment, though limited to a single country and self-reported data.

When this study was published

Published in 2010. More countries have since liberalized cannabis policy, enabling further cross-national comparisons.

The bigger picture

This cross-national comparison suggested that while drug policy affects overall prevalence, the relative balance between genetic and environmental influences on who within a population tries cannabis remains stable across different policy environments.

Questions still open

Would full legalization with commercial sales change the heritability balance? Does permissive policy mainly increase overall rates while leaving genetic vulnerability unchanged?

Common questions

Does legal cannabis change how much genetics matters?

In the Netherlands, where cannabis use was permitted, the genetic contribution (44%) was similar to countries where it was illegal. This suggests drug policy changes who is exposed but not how genes influence individual vulnerability.

How much of cannabis use is due to peer pressure or family?

Shared environmental factors (family, peers) accounted for 31% of cannabis initiation in this study, suggesting social influences are important but genetics explains a larger portion of individual differences.

Read the original research

Heritability of cannabis initiation in Dutch adult twins.

Addictive behaviors, 35(2), 172-4

View on PubMed ↗Publisher’s paper ↗

Citation

Vink, Jacqueline M; Wolters, Liselot M C; Neale, Michael C; Boomsma, Dorret I. (2010). Heritability of cannabis initiation in Dutch adult twins.. Addictive behaviors, 35(2), 172-4. https://doi.org/10.1016/j.addbeh.2009.09.015