Heritability of cannabis initiation was higher in teenagers than young adults because environmental influences grew with age | Research

The takeaway

In 7,753 Dutch twins and siblings aged 11-25, cannabis initiation heritability was 40% at age 16.5, but environmental influences (shared and unique) increased with age, making genetics relatively less important in young adults.

Read this if you are interested in how genetics versus environment shapes when young people first try cannabis.

Environmental influences on cannabis initiation grew with age

What the researchers found

Researchers examined 6,208 twins and 1,545 siblings from 3,503 Dutch families to understand how the genetic-environmental balance for cannabis initiation changed with age.

At the median age of 16.5, genetic factors explained 40% of individual differences in cannabis initiation. Twins resembled each other more than non-twin siblings, even after accounting for age differences.

Environmental influences increased with age across all categories. Factors shared only between twins accounted for 47% of variance, factors shared by all siblings in a family for 24%, and individual-specific factors for 13%. All of these environmental components grew larger as young people aged.

The result was that heritability was proportionally higher in adolescents than in young adults, not because genetic influence decreased, but because environmental exposure opportunities expanded with age.

Why it matters

The finding that environmental influences on cannabis initiation grew with age helped explain why prevention efforts targeting social environments may be increasingly important for older adolescents and young adults.

The numbers in context

7,753 total participants from 3,503 families. At age 16.5: genetic = 40%, twin-shared environment = 47%, family-shared = 24%, unique environment = 13%. All environmental components increased with age.

How the study worked

Twin-sibling study using genetic structural equation modeling. 6,208 twins (ages 13-20) and 1,545 siblings (ages 11-25) from 3,503 families in the Netherlands Twin Register. Self-reported cannabis use from the Dutch Health Behavior Questionnaire. Age moderation of genetic and environmental variance components tested.

What this study cannot tell us

Dutch sample where cannabis is relatively accessible, potentially affecting environmental component estimates. Self-reported cannabis use. Cross-sectional age comparisons rather than longitudinal tracking.

How to read the evidence

Large twin-sibling study with age moderation analysis providing robust estimates of how genetic and environmental influences shift across adolescence.

When this study was published

Published in 2011. Understanding of age-dependent heritability of substance use has continued to develop.

The bigger picture

The age-dependent shift from genetic to environmental influence suggested that cannabis initiation in older teens and young adults was increasingly driven by social context, peer groups, and availability rather than genetic predisposition.

Questions still open

Do the same age-moderation patterns apply to progression from initiation to problematic use? Would restricting cannabis availability change the heritability estimates?

Common questions

Is trying cannabis genetic or environmental?

Both, but it changes with age. At 16.5, genetics explained 40% of who tried cannabis. As people got older, environmental factors (friends, social context, availability) became increasingly important.

Why did twins resemble each other more than regular siblings?

Twins share more environmental factors (same age peers, same school classes, same social timing) than siblings born in different years. This "twin-specific" shared environment accounted for 47% of variance in cannabis initiation.

Read the original research

Age moderates non-genetic influences on the initiation of cannabis use: a twin-sibling study in Dutch adolescents and young adults.

Addiction (Abingdon, England), 106(9), 1658-66

View on PubMed ↗Publisher’s paper ↗

Citation

Distel, Marijn A; Vink, Jacqueline M; Bartels, Meike; van Beijsterveldt, Catharina E M; Neale, Michael C; Boomsma, Dorret I. (2011). Age moderates non-genetic influences on the initiation of cannabis use: a twin-sibling study in Dutch adolescents and young adults.. Addiction (Abingdon, England), 106(9), 1658-66. https://doi.org/10.1111/j.1360-0443.2011.03465.x