These findings indicate that intervention in substance use during early adolescence, when environmental factors are overwhelmingly more influential on the diversity of substances used, may be more effective than intervention in later adolescence, especially in males. The developmental process of increasing genetic influence is not unique to substance use (16, 17) and its consistency highlights the importance of affecting change by early adolescence while environmental effects still account for a majority of population variance in behavior. The sex differences in etiology suggest that interventions focusing on family-level factors may be more successful with females throughout adolescence, while early individual-level interventions with males would be key. As substance use has been repeatedly linked to a range of externalizing behaviors (e.g. 35, 36), this understanding of the growing effect of genetic factors on substance use may help inform treatments of the cluster of externalizing behaviors, which present a similar pattern of weak heritability early in adolescence, but increasing heritability later in adolescence (33). Earlier prevention and intervention techniques may be especially important, as earlier substance use has been linked with more negative abuse and dependence outcomes later in life (15).
