The aetiology of such comorbidity has been the topic of extensive research. Many theories have focused on the co-occurrence of two disorders. Some argue for a direct causal relationship whereby a mental illness causes substance use problems. A subset of this theory is the self-medication hypothesis. An alternative explanation is that substance use problems cause a mental illness, such as the case put forward for alcohol causing secondary depression (Fergusson et al., 2009), or cannabis triggering psychosis in vulnerable individuals (Degenhardt et al., 2003b, Muller-Vahl & Emrich, 2008, McGrath et al., 2010). There are other feasible explanations for comorbidity, such as an indirect causal relationship whereby the presence of one disorder affects a third variable, which in turn increases the risk for the development of a second disorder. Early-onset substance use disorders decrease the likelihood of completing school and other education, and increase the likelihood of teenage parenthood and marriage breakdown (Kessler et al., 1995, Kessler et al., 1997a, Kessler et al., 1998). This may in turn lead to difficulties establishing stable relationships and earning an income, which could increase
