An alternative explanation for the comorbidity of substance dependence and other psychiatric illness is a shared cause. The heritability of major psychiatric illness is high, with estimates of 64% in schizophrenia,61 59% in bipolar disorder,61 and 42% in major depressive disorder.62 The combination of the heritability of substance dependence, the heritability of psychiatric illnesses, and comorbidity of substance dependence with psychiatric illness indicates that specific genes may be pleiotropic (ie, contribute to substance dependence and other psychiatric illness).
