and environments—the ways in which, for example, stressors may provoke depression but depression further influences social environments, often a vicious cycle that promotes chronic or recurrent depression. A further aspect of this bidirectional influence is the frequent co-occurrence of depression and other disorders, which may complicate its course and treatment. It is noted that some individuals are remarkably resilient in the face of adversity, and a further challenge to the field is to understand such processes.

The first topic to address is that not all depressions are alike; therefore, different etiological models and perspectives are likely to apply to different expressions of depressive disorder.

Age of onset of major depressive disorder and lifetime course are two factors that have etiological as well as treatment and outcome implications.

First onset can occur at any time. Diagnoses of childhood depression are relatively rare (Birmaher et al., 1996; Egger and Angold, 2006), although many preadolescents including preschoolers have significant internalizing symptoms of dysphoria and distress (e.g., Cole et al., 2002; DuBois et al., 1995; Gross et al., 2006). Most diagnosed depressions first appear in adolescence and early adulthood (Andrade et al., 2003; Burke et al., 1990; Kessler et al., 2005)—especially among those born in more recent decades (e.g., Kessler et al., 2003). For example, in recent community studies up to one-third of adolescents met criteria for major depressive disorder (Kessler and Walters, 1998; Lewinsohn, Rohde, and Seeley, 1998).

Age of first onset has both clinical and etiological implications. Clinically, earlier age of onset of depression is generally thought to be associated with a worse course of depression, with greater chances of recurrence, chronicity, and impairment in role functioning (e.g., Hollon et al., 2006; Zisook et al., 2004). Those with adolescent-onset depression include a significant proportion among both treatment and community samples who go on to have recurrent episodes and significant impairment (e.g., Hammen, Brennan, and Keenan-Miller, 2008; Lewinsohn et al., 1999, 2000; Pine et al., 1998; Weissman et al., 1999a).

Evidence increasingly suggests that childhood, adolescent, adult, and older adult first onsets may reflect different causal factors. Childhood depressions may be a mixture of subgroups: those with true genetically familial early-onset recurrent depression; those exposed to significant psychosocial adversity, such as abuse, parental disorder, criminality, and family disruption who continue to experience social maladjustment and other