Exposure to substance use can also be a risk factor for increases in drug involvement. Family influences that increase risk for the initiation of substance use during adolescence include parents or older siblings who use drugs (Ellickson et al. 2004b) and parents or other first-degree biological relatives with histories of substance use disorders (Brook et al. 1999; Clark et al. 2005; Hawkins et al. 1992; Hill et al. 2000; Höfler et al. 1999; Morojele and Brook, 2001; Newcomb and Felix-Ortiz, 1992; Novins and Mitchell, 1998; Petraitis et al. 1995; Tarter et al. 2003; von Sydow et al. 2002; Wills et al. 1996). And peer influences, including peer approval of alcohol or other substance use (Ellickson et al. 2004b; Kosterman et al. 2000; Novins and Mitchell, 1998), and offers of marijuana (Novins and Mitchell, 1998; von Sydow et al. 2002), predict the initiation (Hawkins et al. 1992) and escalation (Morojele and Brook, 2001) of substance use. Finally, personal experience using alcohol and tobacco predicts the initiation and escalation of marijuana and other illicit substance use (Bentler et al. 2002; Boyle et
