Introduction

Extensive co-occurrence of major depressive disorder (MDD) with other psychiatric disorders, particularly anxiety, personality, and alcohol and substance use disorders has been well documented within clinical and community samples (Blazer et al., 1994; Corruble et al., 1996; Markowitz et al., 1992; Rohde et al., 1991; Sanderson et al., 1990, 1992). Based on estimates from the National Institute of Mental Health Epidemiological Catchment Area Study (Robins et al., 1991) and the US National Comorbidity Survey (Kessler et al., 1996) as many as one-half to three-quarters of individuals in the community with MDD suffer from an additional one or more psychiatric disorders. Psychiatric comorbidity appears to be particularly high among juvenile depressed populations (Angold & Costello, 1993; Biederman et al., 1995; Birmaher et al., 1996) and among adults with an early age of onset of first depression (Alpert et al., 1994; Fava et al., 1996; Kasch & Klein, 1996). Moreover, results of the US National Comorbidity Survey suggest a disproportionate increase across recent birth cohorts in the prevalence of MDD with comorbidity, but not depression without comorbidity or ‘pure’ depression (Kessler et al., 1996). This trend was found to be largely accounted for by an increase in MDD secondary, in order of chronology, to other psychiatric disorders. In addition, individuals with lower educational attainment, lower income, and non-white ethnicity/race were reported to have an increased risk of comorbid compared with ‘pure’ depression (Blazer et al., 1994).