Childhood disruptiveness is one of the most important antecedents of heavy substance use in adolescence, especially among boys. The first aim of the present study is to verify whether parental monitoring and friend conventionality protect disruptive boys from engaging in heavy substance-use in adolescence. The second purpose is to examine whether these protective effects are strengthened by attachment to parents or friends respectively. Finally, the third objective is to verify whether the expected protective effect of parental monitoring could be mediated through exposure to conventional friends. A sample of 1037 boys from low socioeconomic neighbourhoods was followed from childhood (age 6) to adolescence (age 15). Parent, teacher, and self-reported measures were used to measure disruptiveness, parental monitoring, family attachment, friend conventionality, and attachment to friends. Results suggest that parental monitoring and friends' conventionality mitigated the relationship between childhood disruptiveness and adolescence heavy substance use. Exposure to conventional friends further mediated the protective effect of parent monitoring. The postulated enhancement of attachment quality on the protective effect of parents or peer behaviors was not confirmed, but low attachment was related to heavier substance use in highly monitored disruptive boys. Parental monitoring, family attachment, and peer conventionality are factors amenable to intervention, and thus represent promising targets for future prevention strategies aimed at-risk boys. Our results underscore the importance of simultaneously addressing the behavioral and the affective dimensions in interventions with parents.

RESEARCH HIGHLIGHTS:
• Parent monitoring and peer conventionality protected disruptive boys from heavy substance use.
• Disruptive boys who were highly monitored but poorly attached to parents showed heavier use.
• Peer conventionality mediated the protective effect of parent monitoring on disruptive boys.
• Peer conventionality buffered the association between attachment to friends and heavy use.