ways that reduce the threshold at which stressors may trigger a further episode—possibly to the point of autonomous episodes of depression. A review of studies of stress-depression associations in first and later episodes found some support for the model (Monroe and Harkness, 2005). Truly longitudinal within-person studies to test this hypothesis are quite rare, although one such investigation by Kendler, Thornton, and Gardner (2000) studied nearly 2,400 female twins over 4 waves separated by at least 13 months each. They found evidence of a diminishing association between life events and depression as the person experienced increasing numbers of episodes (up to about 6–8 episodes). They suggested that whether the involved mechanism is biological or psychological, it appears to occur intensively in the first few episodes after initial onset, and then the kindling process slows or stops. The stress-depression relationship not only may vary over time with increasing numbers of episodes but also may differ according to genetic risk for depression (Kendler, Thornton, and Gardner, 2001).

Mild, chronic depression—termed dysthymic disorder—may also be very disruptive and enduring. It may be highly predictive of major depressive episodes, and, especially if its onset is early in life, it is associated with slow recovery and high rates of relapse or continuing symptoms (Klein, Shankman, and Rose, 2006). Early-onset dysthymic patients had relatively high rates of poor-quality early home environments (Lizardi et al., 1995) and a relatively elevated exposure to early adverse conditions, including physical and sexual abuse, as well as ongoing stressful life conditions (Riso, Miyaktake, and Thase, 2002). Chronic depression is also associated with higher rates of familial depression than is episodic major depression (Klein et al., 2004), which suggests an etiological subtype.

Key features of the course of depression have significant implications for families. Most depressions first occur in adolescence and young adulthood, periods during which critical developmental accomplishments may be disrupted, such as academic attainment and job planning, peer integration and acquisition of effective social skills, and romantic relationship formation. Obviously, childbearing years are affected as well. Young people who are depressed may select into, or default into, problematic environments that are stressful and may further overwhelm impaired coping capabilities. Depression may become recurrent for biological as well as social and psychological reasons, and thus it may become harder to manage and treat. All members of the family are affected, and children are the most vulnerable to the negative impact of parental depression. Another important observation that comes from this evidence is that prevention programs may be particularly valuable and are probably best targeted at those most vulnerable to depression: those with extensive family history, those with symptoms of depression, and those with multiple risk factors for depression (e.g., poverty, exposure to violence, social isolation).

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