Additionally, early substance use has been repeatedly linked to externalizing behaviors, such as police contact and sexual intercourse, as well as clinical diagnoses, such as conduct disorder, all of which occur with greater frequency in males (31–33). While this may help explain males consistently using a greater diversity of substances than females throughout adolescence, it does not necessarily account for the differing levels of genetic influence observed between the sexes across ages. While these specific genetic factors remain highly correlated across age, it is possible that these same factors overlap to some extent with risk for frequency of use and misuse (34). Despite environmental unknowns and possible externalizing covariates, earlier substance use may nevertheless signal an earlier display of a developmentally stable genetic predisposition, as the specific genetic factors remain stable across ages, rather than a unique etiology.
