The above results must be interpreted in the context of several limitations: First, individuals with severe physical or neurocognitive impairment may have been less likely to participate, presumably reducing our estimates of the associations between MDE and physical conditions more among elderly than younger respondents. Second, physical conditions were assessed with a checklist, whereas mental disorders where assessed with a comprehensive diagnostic interview, possibly leading to greater attenuation of estimated associations involving physical than mental disorders. Third, the fact that the assessments were made with fully-structured interviews rather than clinical interviews might have artificially inflated comorbidity estimates between diagnoses of MDE and the other disorders due to overlap in core symptoms (e.g., lethargy, insomnia), although we tried to minimize this problem by not using organic exclusion rules in making diagnoses of mental disorders.
Notwithstanding these limitations, our results are consistent with previous studies in showing that recent MDE is less prevalent among older than younger respondents in developed countries.[2
] The finding that MDE generally does not decrease with age in developing countries is also consistent with the small number of previous studies that have examined this pattern in developing countries;[33
] although those studies generally found depression to increase with age whereas we found that association with age to be for the most part insignificant. We did not investigate reasons for the differing relationship between age and depression between developed and developing countries, but this should be the focus of future study. Our findings of higher mean MDE AOO and longer time lag between AOO and current age with increasing age are both substantively plausible and inconsistent with evidence of age-related recall bias in previous epidemiological surveys.[31
] We attribute this difference between the WMH results and the results of previous survey to the use of an innovative AOO probing technique in the WMH surveys that has been shown experimentally to reduce recall bias.[37
The finding that retrospectively reported number of lifetime episodes increases with age among respondents with a history of MDE is, like the findings for AOO in the preceding paragraph, substantively plausible and consistent with previous research.[38
] The finding that age is positively related to duration of 12-month depressive symptoms among 12-month cases, in comparison, is inconsistent with previous research that found no association between age and 12-month duration of depressive episodes.[39
] It should be noted, though, that these earlier studies were based on much smaller samples than the WMH series. Our results regarding longer episode duration among the oldest respondents are also consistent across types of countries. We also found consistently across countries that despite the longer duration of recent episodes, recent MDE was reported to cause less role impairment than among younger depressed people. At least two previous studies also found that symptom severity and severity of role impairment due to 12-month depression are both inversely related to age.[6
] One plausible interpretation of the lower impact of depression on role impairment with increasing age is that role demands decrease with age. However, this interpretation does not explain the finding that symptom severity also decreases significantly with age in developing countries. Another possibility is that depression subtypes change with age and that the subtypes more typical of older people are less severe and impairing than those more typical of younger people.[42
] Although no attempt was made here to examine depressive symptom profiles by age to investigate this interpretation, this would be a useful extension of the current results.
Our finding that 12-month prevalence of some mental disorders decreases with age while prevalence of most physical disorders increases with age is consistent with much previous research.[13
] We are aware of little previous research, though, other than earlier WMH analyses[15
] on age differences in the associations of depression with comorbid disorders. Our findings that these associations generally increase with age when they involve comorbid mental disorders but decrease with age when they involve comorbid physical disorders are consequently of special interest. The most plausible interpretation of the generally increasing are-related ORs with other mental disorders is that comorbid cases have a more persistent course than pure cases. Although it is beyond the scope of the current report to investigate the reasons for such an effect, it is noteworthy that this could also be implicated in the longer duration of depressive episodes among the elderly. The fact that the role impairment associated with depression is lowest among the elderly is all the more striking in light of the greater persistent and higher comorbidity of MDE with other mental disorders among the elderly.
The generally decreasing age-related ORs of MDE with physical disorders are more interpretable because the age patterns in prevalence are different for MDE (decreasing prevalence with age) and most physical disorders (increasing prevalence with age). In a situation of this sort, it is likely that the decreasing ORs are at least partially attributable to a decrease in the causal effects of physical disorders on MDE. Whether or not causal effects of MDE on comorbid physical disorders also decrease with age is difficult to say because the implications of such a decrease on the prevalence of physical disorders would be negligible in light of the much lower prevalence of MDE than chronic physical conditions among the elderly. In either case, though, the existence of these patterns argue against the suggestion that the low prevalence of MDE among the elderly in developed countries is due to increased confounding of depression symptoms with symptoms of chronic physical conditions.
Our results shed no light on why physical disorders might have decreasing effects on MDE among the elderly. One possibility proposed in the literature is that elderly people are more accepting than those of younger ages of the inevitability of physical illness, resulting in the otherwise adverse psychological effects of physical disorders being buffered.[6
] A related suggestion is that elderly people are “immunized” from the negative psychological effects of adversity by prior life experience.[45
] Although we are aware of no direct test of these hypotheses, elderly people have been shown to be more likely than younger people to cope with adversity by using strategies that accept and adapt rather than try to change their situations[46
] and that disengage from stressful situations in ways that reduce adverse emotional effects.[47
] Other research has shown similar age differences in coping with physical illness,[48
] but has not investigated whether these differences lead to reductions in the causal effects of physical disorders on depression. Investigation of these buffering effects is an important next step that, while beyond the scope of the present study, might help delineate positive patterns of response to the increasing physical infirmity of advanced age. Another possibility is that elderly people might have reduced capacity to register or express mood states due to autonomic, neuroendocrine, or cognitive dysfunction that lead to reduced prevalence of mood disorders in old age.[49
] It is unclear, though, how this or any of the other explanations proposed in the literature would account for the fact that age-related decline in depression prevalence is largely confided to developed countries. New theorizing is required to explain this specification.