with lower parental monitoring.43 These analyses suggest that when adolescents receive little parental monitoring, it creates an environment that allows for greater opportunity to express genetic predispositions. The moderating effects of peer alcohol use on adolescent drinking has been shown to operate in a similar fashion: among adolescents with a larger number of peers who used alcohol, there was greater expression of genetic predispositions.44 These findings may reflect a situation in which environments characterized by low parental monitoring or high peer substance use create opportunity for adolescents to express genetic predispositions. These results support previous findings from the Finnish Twin Studies, which indicated that in neighborhoods in which there is less stability, presumably engendering less community monitoring, there was greater evidence of genetic influence.45 Conversely, in more supervised and restricted environments, there was less opportunity to express genetic predispositions and greater influence of environmental effects.34,45 Hicks and colleagues46 examined the specificity of each of these environmental risk factors on externalizing spectrum disorders, including substance dependence/abuse. They concluded that, in the context of environmental adversity, broadly defined, genetic factors become more important in the etiology of externalizing disorders. In addition, their results suggest a general mechanism of environmental influence on externalizing
