Four key findings emerged from the above analyses. First, respondents generally attributed more disability to their mental than physical disorders. Second, the higher disability of the mental than physical disorders held as strongly in developing as in developed countries. Third, the higher aggregate disability of mental than physical disorder was much more pronounced for disability in social and personal relationships than in productive (work and housework) roles. Fourth, the proportion of cases in treatment at the time of interview was much lower for mental than physical disorders in developed countries and even more so in developing countries both in the total sample and when we focused exclusively on cases rated severely disabling. These findings substantially extend the results of previous studies, none of which documented comparability in the disabilities associated with such a varied a set of physical and mental disorders or disaggregated disability into the domains considered here to detect the greater relative impact of mental than physical disorders in social-personal domains than productive role domains.2, 4-7
These results are limited by a number of sampling and measurement problems. With regard to sampling, results could be influenced by a truncation of the severity spectrum of physical disorders. For example, persons facing the end stage of chronic physical disease might be institutionalised or not willing or able to participate in an interview to a greater extent than people with severe mental disorders, leading to under-estimation of the relative disability of physical compared to mental disorders. Whether such a difference in sample bias actually exists, though, is unknown.
There were a number of measurement problems in the analysis. One is that the physical conditions checklist did not include the infectious diseases that play such an important part in the morbidity of developing countries. Our results consequently can be generalized only to chronic cardiovascular, digestive, metabolic, musculoskeletal, pain, and respiratory conditions. Despite this limitation, though, the conditions considered are important sources of morbidity even in developing countries and the results are consequently relevant to those countries despite the exclusion of infectious diseases.
Another measurement problem is that the physical disorders were assessed by simple self-report rather than by abstracting medical records or administering medical examinations. Mental disorders were assessed more comprehensively with a fully structured lay-administered diagnostic interview. The more superficial assessment of physical disorders could have led to the inclusion of more sub-threshold cases than mental disorders, introducing an artificial lowering of the estimated disability of physical disorders, although we addressed this in our analysis of treated physical conditions. It could also have led to artificial overlap between the assessments of mental and physical disorders to the extent that core symptoms of some physical conditions (e.g., headache, unexplained chronic pain) are markers of underlying mental disorders, although this would have attenuated physical-mental differences by increasing overlap between the two classes of disorders. In addition, the use of a self-report checklist almost certainly led to an under-estimation of undiagnosed silent physical conditions. As the latter are likely to be less disabling than symptom-based conditions or diagnosed silent conditions, though, this bias presumably led to an artificial increase in the estimated disability of physical disorders.
Some of the WMH physical disorder prevalence estimates are lower than those in gold standard assessments. For example, the population prevalence of diabetes has been assessed in a number of community surveys using glucose tolerance tests from blood samples.26
A meta-analysis of these studies suggests that the prevalence of diabetes is highest in North America (9.2%) and Europe (8.4%), lower in India and most of Latin America (5-8%), and lowest in most of Africa and China (2-5%).27
The WMH prevalence estimates, 4.6% in developed countries and 3.9% in developing countries, are lower than these gold-standard estimates, presumably reflecting the fact that the latter include undiagnosed cases.
In other cases the WMH prevalence estimates are higher than those in gold standard assessments. For example, cancer prevalence data have been assembled from various administrative databases and registries in a number of countries.28
Meta-analysis of these data suggest that cancer is much more common in developed than developing countries, with the highest prevalence in North American (1.5% of the population ages 15 and older diagnosed within the past 5 years), followed by Western Europe (1.2%), Australia and New Zealand (1.1%), Japan (1.0%), Eastern Europe (0.7%), Latin America and the Caribbean (0.4%), with a much lower estimated prevalence in the rest of the world (0.2%). The much higher cancer prevalence estimates in the WMH data, 4.0% in developed countries and 0.6% in developing countries, presumably reflect the fact that cancer survivors who were diagnosed and treated more than five years ago, although not counted in cancer prevalence estimates because they have the same survival rates as the general population, often consider themselves still to have cancer and report this in community surveys.
Based on comparison such as these with gold standard assessments, caution is needed in interpreting the WMH prevalence estimates of physical disorders. However, the fact that the same general pattern of higher disability among mental than physical disorders held in comparisons of treated physical disorders argues strongly that the finding of higher SDS disability associated with mental than physical disorders is not due to imprecision in the measurement of physical disorders.
Another measurement problem involves the fact that disability was assessed with brief self-report scales rather than clinical evaluations. This might have introduced upward bias in the reported disability caused by mental disorders compared to physical disorders to the extent that people with mental disorders gave overly pessimistic appraisals of their functioning. This would seem to be an unlikely interpretation, though, in that the associations of SDS ratings with reported numbers of days out of role – a more objective indicator of disability than the SDS ratings – were found to be equivalent for mental and physical disorders. Furthermore, within-person comparison, which controlled for individual differences in perceptions, found similar results.
Another possibility is that the SDS questions might have been biased in the direction of assessing the disabilities associated with mental more than physical disorders. This would seem unlikely, though, as the SDS questions are quite broad and cover all the main areas of adult role functioning. Another possible limitation is that the SDS focused on the “worst month” in the past year, introducing recall error that possibly was more extreme for physical than mental disorders. In addition, between-disorder differences in persistence were not taken into consideration, which means that particular disorders might have been more dominant in severity ratings than suggested here if they were more persistently severe than others. The aggregate disability estimates should be interpreted cautiously due to these limitations regarding the recall period.
A final measurement problem concerning the assessment of disability relates to our use of a condition-specific
measurement approach. This is an attractive approach from a statistical perspective in comparison to an unconditional measurement approach (i.e., an approach that simply assesses overall disability without asking the respondent to make inferences about the conditions that caused the disability) because it produces condition-specific estimates directly, avoiding the need to rely on multivariate equations that adjust for the effects of comorbidity in predicting overall disability. However, this advantage in analytic simplicity is achieved by requiring respondents with comorbid conditions to perform the difficult task of making judgments about the effects of individual conditions on their functioning. Because of likely imprecision in these assessments, it would be useful to replicate the results reported here in multivariate analyses that evaluated the separate and joint effects of comorbid conditions in predicting an unconditional measure of disability. Unfortunately, the statistical methods needed to estimate models of this sort are very complex,29
making it difficult to carry out such analyses.
Within the context of these limitations, the results reported here are consistent with previous comparative burden-of-illness studies in suggesting that musculoskeletal disorders and major depression are the disorders with the largest contribution to disability at the individual level both in developed and developing countries. Previous studies have documented this pattern only for the US,30-32
although the importance of depression has also been documented throughout the world in the World Health Surveys (WHS).7
The current report replicates the WHS results regarding depression and documents for the first time the cross-national importance of musculoskeletal disorders. As noted above, the WMH results also suggest that mental disorders are especially disabling to personal relationships and social life, which implies that they are disabling more because they create psychological barriers than physical barriers to functioning. Amongst these barriers are limitations in cognitive and motivational capacities, affect regulation, embarrassment and stigma,33
and a tendency to amplify physical symptoms34
and associated disability.35
Given this greater disability of mental than physical disorders, it is disturbing to find that only a minority of even severe cases of mental disorder receive treatment and that treatment was substantially more common among comparably severe physical disorders. In developed countries, seriously disabling mental disorders were only about half as likely to be treated as seriously disabling physical disorders (35.3% vs. 77.6%), while they were only about 20% as likely to be treated as comparably severe physical disorders in developing countries (11.9% vs. 64.0%). This low treatment rate is consistent with the low rate of recognition and treatment of mental disorders in primary care, especially if comorbid with physical disorders.36, 37
In combination with the burden of disability that mental disorders produce, the low treatment rates call for more attention to mental disorders.
Implications of the WMH findings for treatment are not clear because, even though treatment effectiveness trials document that common anxiety and mood disorders can often be successfully treated,38, 39
uncertainties exist regarding long-term outcomes. Another limitation of existing trials is that they focused on symptoms and did little to assess the effects of treatment on reduced disability.38, 39
In particular, long-term functional outcomes are important to track because residual disability and recurrence of disability are major problems with chronic mental disorders.40
Despite this uncertainty about long-term outcomes, though, the results reported here argue strongly that, on the basis of population disease burden associated with disorder-specific disability, more attention should be given to the treatment of mental disorders and that this is especially so in developing countries.