Four limitations of the NCS-R are relevant to the analyses reported here. First, the sample under-represents several important population segments, including the homeless, those in institutions, and those who cannot speak English. The first two of these exclusions reduce prevalence estimates. In addition, mentally ill people might be more reluctant than others to participate in a mental health survey. This is relevant because the 70.9% response rate means that nearly 30% of eligible respondents are not represented in the sample. Evidence for selection bias related to mental illness has been reported in other community surveys,29–31
although no evidence for it was found in an NCS-R non-response survey.13
To the extent that this bias exists, it will make NCS-R estimates conservative.
Second, participants might have under-reported 12-month prevalence. This possibility is consistent with evidence in the methodology evidence that embarrassing behaviors are often under-reported.32
Experimental studies show that this under-reporting bias can be reduced by using strategies aimed at decreasing embarrassment3, 33
a number of which were used in the NCS-R.10
To the extent these strategies were unsuccessful, the NCS-R estimates are likely to be conservative.
Third, the WMH-CIDI is a lay-administered interview. As reported elsewhere,34
though, a clinical reappraisal study using the Structured Clinical Interview for DSM-IV (SCID)16
found generally good individual-level concordance between the WMH-CIDI and SCID and conservative estimates of prevalence compared to the SCID.
Fourth, the NCS-R did not include all DSM-IV diagnoses. Schizophrenia and other non-affective psychoses (NAP) are notably missing. NAP was excluded from the NCS-R core because previous studies have shown it is dramatically over-estimated in lay-administered interviews.35–39
These same studies showed that the vast majority of respondents with NAP meet criteria for CIDI anxiety, mood, or substance disorders and are consequently captured as cases. If severity is under-estimated in the WMH-CIDI, though results will be conservative.
Within the context of these limitations, NCS-R results are generally consistent with the earlier Epidemiologic Catchment Area (ECA) Study and National Comorbidity Survey (NCS)1
in finding 12-month mental disorders to be highly prevalent. The 26.2% estimate of any disorder in the NCS-R is very close to estimates of 28.1% in the ECA2
and 29.5% in the NCS.1
This great similarity should not be over-interpreted, though, as the three surveys differed greatly in sampling frames, age ranges, diagnostic systems used to define disorders, and measures that it is impossible to draw firm conclusions about time trends in prevalence from these comparisons. It is nonetheless noteworthy, in light of these different design elements, that the three most prevalent NCS-R disorders (specific phobia, social phobia, and major depressive disorder) are identical to the three most prevalent disorders in the NCS and to two of the three in the ECA. The exception is social phobia, which was not comprehensively assessed in the ECA.
The NCS-R findings that anxiety disorders are more prevalent than mood disorders and that mood disorders are more prevalent that substance disorders are also consistent with both ECA and NCS findings. The NCS-R prevalence estimates can also be directly compared to those in over a dozen countries that participated in the WHO World Mental Health (WMH) Survey Initiative.8
NCS-R prevalence estimates are consistently higher than in these other countries. However, as with the ECA and NCS, within-country differences in disorder prevalence in the NCS-R are quite similar to those reported so far in other WMH countries.40, 41
The externalizing disorders in NCS-R have been much less well studied than anxiety, mood, and substance disorders in previous adult surveys. The limited evidence on intermittent explosive disorder42
is consistent with the NCS-R prevalence estimate of 2.6%, but we are aware of no comparable information on other impulse-control disorders among adults. These disorders are routinely assessed in surveys of children.43–45
NCS-R 12-month prevalence estimates of all but one of the childhood-onset impulse disorders are much smaller than in surveys of youth. The exception is ADHD, with 12-month NCS-R prevalence approximately 50% as high as the estimates in surveys of youth. This is consistent with independent evidence that as many as half of children with ADHD continue to have symptoms as adults.46
The NCS-R results regarding severity support the secondary analyses in showing that many mental disorders are mild. Indeed, nearly twice as high a proportion of NCS-R cases are mild (40.4%) as serious (22.3%). Nonetheless, the 14.0% of respondents with serious or moderate disorder is substantial. The 5.7% with a serious disorder (22.3% of the 26.2% overall prevalence) is almost identical to the estimated prevalence of Serious Mental Illness (SMI), using the SAMHSA definition of that term, in the baseline NCS.47
The finding that mood disorders are more likely than anxiety disorders to be classified serious is consistent with a cross-national comparative analysis of five earlier CIDI surveys that used a less precise measure of severity7
as well as with the results of the more recent WMH Surveys.8
Patterns of bivariate comorbidity are broadly consistent with the ECA and NCS in showing the vast majority of disorders positively correlated. Relative magnitudes of associations are also quite similar across the three surveys, with high rank-order correlations of odds-ratios among comorbid pairs in the NCS versus published odds-ratios48
in both the NCS (.79) and the ECA (.57). Major internal patterns of comorbidity are also quite consistent across surveys, such as the stronger odds-ratios within the mood disorders than the anxiety disorders, very high odds-ratios between anxiety and mood disorders, and odds-ratios between anxiety and mood disorders generally being higher than between pairs of anxiety disorders.
The factor analysis found a very similar two-dimensional solution as in the NCS.49
A similar structure was found in a stud of comorbidity among primary care patients.50
The log-linear analysis showed clearly, though, that powerful interactions exist among NCS-R disorders that are not captured by the additive model on which factor analysis is based. LCA was used to study these profiles. This is a departure from the confirmatory factor analysis approach used in other recent studies of comorbidity (CITES: Kreuger papers #1–2 that are already in the bib; Vollebergh WA et al. Arch Gen Psych 2001). The LCA results documented progression within and overlap between internalizing and externalizing disorders, with a clear divergence from a simple two-dimensional progression due to panic and phobia being considerably more prevalent in the comorbid internalizing class than in the highly comorbid internalizing and externalizing classes. This is an intriguing specification that was also found a decade ago in an LCA analysis of the NCS data.51
It is conceivable that this pattern reflects a protective effect of comorbid panic and phobias against externalizing disorders, possibly through risk aversion.
The NCS-R LCA results share several other features with the earlier NCS LCA results. Both include separate classes of pure and comorbid internalizing disorders with low prevalence of bipolarity. Both have highly comorbid classes with a small proportion of the sample (4.9% in NCS and 7.3% in NCS-R) having a high concentration of severe cases. The implicit progression among these classes warrants a more fine-grained investigation of transitions in lifetime comorbidity. Such an investigation goes beyond the scope of the current report.
The results regarding socio-demographic correlates are broadly consistent with previous surveys in finding that mental disorders (i.e., low probability of membership in Latent Class I) are associated with a general pattern of disadvantaged social status, including being female, unmarried, and having low socioeconomic status.8, 52–59
The finding that Non-Hispanic Blacks and Hispanics have significantly lower risk of disorders is inconsistent with this general pattern, but the same relationship was found in the baseline NCS.1
It is not clear whether the associations of achieved social statuses (i.e., marital status, socioeconomic status) with prevalence are due to effects of environmental experiences on mental disorders, to effects of mental disorders on achieved social status, to unmeasured common biological causes, or to some combination. In the case of the ascribed social statuses (i.e., sex and race-ethnicity), the causal effects clearly flow from the statuses and their correlates to the disorders, although the relative importance of environmental and biological mediators is unclear. The significant associations of race-ethnicity, marital status, education, and income with positive disorder classes are largely confined to predicting highly comorbid major depression (Class VI). This means the associations of these important socio-demographic variables with 12-month DSM-IV disorders are due largely to effects on a comparatively rare (16% of the population) profile of highly comorbidity.