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 the likelihood of depression and other disorders. There is also the common factor theory which proposes that there are genetic, intrapersonal, environmental or social factors or an interaction between all of these factors that underpin the co-occurrence of disorders (Kendler et al., 2003b). Whilst this study cannot be interpreted as support for any of the theories of comorbidity, the levels of comorbidity observed in this sample argue against one unidirectional theory. It is more likely that the comorbidity between substance use disorders and psychiatric disorders seen here is the result of a combination of direct causal relationships and common genetic and environmental risk factors. There is evidence that externalising disorders such as substance use disorders and ASPD can be represented as a spectrum of severity or liability, and internalising disorders form a separate but related severity spectrum (Krueger etc). The idea behind these models is that it is these latent severity spectra that give rise to comorbidity across numerous mental
