Second, contrary to prior studies of this question (Hicks et al. 2004; Kendler et al. 2003, 2011), we found shared environmental influences on the externalizing disorders. Furthermore, these effects were considerably stronger in males than in females. We would hypothesize that this results from a greater impact of peer influences on externalizing behaviors in males than females. This hypothesis is consistent with several lines of evidence. We previously showed that cohabitation effects in childhood and adolescence are stronger on risk for DA in males than in females (Kendler et al. 2013). In comparison to females, males are more motivated to use psychoactive substances to conform to subgroup values, more influenced by peers in their intake of drugs and later delinquency is better predicted by deviant peer groups (Borsari and Carey 2001; Graziano et al. 2012; Piquero et al. 2005). In a particularly relevant Swedish survey of high-school students, Svensson (Svensson 2003) found males to have consistently higher levels of exposure to deviant peers than females and found that this arose because parents of girls monitored their offspring’s social network more
