Also, in our analyses we combined studies using samples of varying age ranges, while results of earlier studies have suggested that from adolescence to adulthood the effect of shared environment gradually declined, while genetic influences gradually increased (69). Younger individuals might have limited access to marijuana and experience less peer pressure. However, because of the low number of independent samples, it was not possible to meaningfully distinguish between different age groups or to do other sub-analyses. The inclusion of adolescents and adults in one analysis might have modestly influenced our results, but estimates from the adolescent samples did not differ markedly from those from adult samples, so the effect would be small. Compared to the results of our analyses, heritability estimates for cannabis use initiation were relatively low and shared environmental estimates relatively high for some adolescent samples (27, 28). However, Rhee et al. (34) found a very high A estimate of 72%, and a relatively low C estimate for adolescent females.
