Perceived sibling polydrug use (but not use of a single substance) was associated with general vulnerability for psychopathology. Sibling substance use has an influence on development of adolescent substance use (Duncan et al., 1996). Given that sibling polydrug use is also a marker of familial vulnerability for substance use, its association with all classes of offspring psychopathology is consistent with classical twin studies demonstrating common genetic vulnerability to substance use disorders, affective disorders and conduct disorder (Lyons et al., 2008; True et al., 1999; Xian et al., 2000; Kendler et al., 2003; Scherrer et al., 2008; Slutske et al., 1998).
