Age differences in major depression: results from the National Comorbidity Survey Replication (NCS-R)

Community surveys consistently find that elderly people have much less clinical depression than younger people (Blazer & Hybels, 2005; Jorm, 2000). A number of explanations have been proposed for this finding, most focusing on the possibility that depression is underestimated among the elderly. Suggested biases include age-related differentials in recall, mortality, selection out of the household population into nursing homes, willingness to participate in surveys, and willingness to admit psychiatric symptoms in interviews (Schoevers et al., 2008; Snowdon, 1997). However, evidence for these methodological interpretations is weak (Ernst & Angst, 1995), leading some commentators to conclude that the low estimated prevalence of depression among the elderly is genuine (Blazer & Hybels, 2005). One issue that complicates analysis of late-life depression is that many physical disorders become increasingly prevalent in old age, making boundaries with depression sometimes unclear and raising the possibility that depression is under-estimated because it is confused with the symptoms of physical disorders (Drayer et al., 2005). This issue is complicated by the fact that some somatic disorders that increase with age can induce depression (Bremmer et al., 2008; Salaycik et al., 2007) while late-life depression can increase risk of some physical disorders (Bremmer et al., 2007; Petronijevic et al., 2008). In an effort to shed light on the possible age-related under-estimation of depression in epidemiological studies due to confounding with physical disorders, we analyzed age-related changes in associations of physical disorders with DSM-IV major depressive episodes (MDE) in the National Comorbidity Survey Replication (NCS-R) (Kessler & Merikangas, 2004). Parallel analysis was carried out with comorbidity between MDE and other DSM-IV mental disorders to determine if the patterns for physical disorders are unique or the same as for mental disorders. MDE was defined without either organic exclusions or diagnostic hierarchy rules to facilitate investigation of comorbidity. Our primary aims were to see if the estimated prevalence of MDE declines with age and to evaluate the extent to which comorbidity of MDE with other disorders changes with age. In studying comorbidity, we focused on associations rather than conditional prevalence, as data on age-related changes in prevalence exist (Blazer et al., 1987; Kennedy et al., 1990) but not data on age-related changes in associations. An examination of these associations is of interest because we would expect them to increase with age if the assessment of MDE is confounded by physical disorders. As a preliminary to the analysis of comorbidity, we examined basic MDE prevalence by age to show that an inverse age-MDE relationship does, in fact, exist in the NCS-R. We then examined age differences in the ratios of recent (30-day and 12-month) to lifetime prevalence, expecting that recall bias would produce higher ratios among elderly than younger respondents whereas substantive patterns would produce the opposite pattern. We also investigated whether MDE age-of-onset distributions differ by age, expecting that recall bias would result in these distributions being skewed more to the right among older than younger respondents. We then compared age-related differences in median numbers of lifetime MDE episodes among respondents with a history of MDE, expecting that recall bias would result in these values not increasing with age in a substantively plausible way. We then examined age differences in the persistence and severity of 12-month MDE, which we assumed would be less influenced by recall bias than data on lifetime MDE. We are unaware of any previous attempt to investigate these specifications in the context of an analysis of comorbidity between MDE and other disorders.