OBJECTIVE: Social isolation is highly common in late life and is associated with devastating mental health and physical outcomes. This study investigated whether components of social isolation (marital status, perceived social support, and interpersonal problems) predict change in depression severity over the course of a brief adherence intervention delivered in a primary care setting.

METHOD: A sample of 189 older adults with major depressive disorder were randomized to either an adherence intervention, "Treatment Initiation Program," or treatment as usual. Marital status, perceived social support and interpersonal problems were assessed at baseline. A mixed-effects regression was used to test whether these factors predicted the change trajectory in depression severity over 24 weeks.

RESULTS: Being married (F(2,176) = 6.60; p = 0.001), reporting higher perceived social support (F(2,177) = 4.70; p = 0.01), and fewer interpersonal problems (F(2, 176) = 4.34; p = 0.01) predicted lower depression severity on average over the course of 24 weeks.

CONCLUSION: Social variables such as living in partnership, perceiving others as supportive, and reporting few interpersonal problems may reduce older adults' vulnerability to depression and enhance their ability to benefit from treatment. These findings can guide development of interventions that will target these social factors early in treatment to increase efficacy.