Origins of depression in later life. 
Despite the burden of depression in late life, its origins present a paradox to investigators and clinicians alike.  We review biological (genetics and heredity factors, neurotransmitter dysfunction, endocrine changes, vascular disorders, and medical co-morbidities), psychological (personality attributes, neuroticism, cognitive distortions, and the lack of emotional control and self-efficacy) and social (stressful life events, bereavement, chronic stress or strain, socio-economic disadvantage and impaired social support) origins of late-life depression based upon an extensive though not exhaustive review of the extant literature.  In addition, modifying psychological and social factors are discussed.  Older adults appear to be at greater risk for major depression biologically, such as depression resulting from vascular changes, yet the frequency of depression is lower compared to younger adults.  Older adults may be protected psychologically due to factors such as socio-emotional selectivity and wisdom, compared to younger adults, and perhaps relatively protected from social risks.  A biopsychosocial approach to evaluating the origins of late-life depression is heuristically valuable, a continual reminder of the many factors that contribute to the onset and persistence of clinically significant symptoms in late life.