Present findings must be interpreted in light of two important study limitations. First, all disorders included in these analyses were assessed by fully-structured interviews administered by professional lay interviewers. Such interviews often generate more reliable diagnoses than semi-structured clinical interviews (Wittchen, 1994
), and their prevalence estimates typically correspond well to diagnoses yielded by semi-structured clinical interviews (Kessler et al., 2005a
). Nevertheless, fully-structured assessments limit the sorts of symptom clarification and cross-disorder questions that facilitate differential diagnosis, and this may have led to inflated associations between GAD and other disorders. Second, the temporal order of disorders was determined from retrospective age-of-onset reports. While we used a probing strategy that has been shown to improve recall of age of onset (Knauper et al., 1999
), there were doubtlessly residual errors, perhaps especially when distinguishing GAD from disorders with overlapping symptoms. To help mitigate this concern, we performed sensitivity analyses within subsamples reporting an onset of GAD at least five years before the comorbid disorder. Prospective longitudinal research would be needed to more definitively establish the temporal priority and prospective associations of GAD with other disorders.
Within the context of these limitations, we found that broadening the GAD diagnosis in the three ways considered here would more than double prevalence of the disorder. Most of the increase in prevalence comes from reducing the minimum duration to one month and, to a lesser extent, from eliminating the excessive worry requirement. Prevalence is minimally affected by requiring two rather than three associated symptoms. As the GAD definition is broadened, there is a slight but consistent decrement in the comorbidity of GAD with other disorders. This small decrement is apparent both in cross-sectional analyses of lifetime comorbidity and in longitudinal analyses estimating the odds of subsequent disorder onsets. However, few of the differences are sufficiently large to reach statistical significance. The overall pattern of results is the same regardless of whether GAD severity is controlled and even when the onsets of GAD and later disorders are separated by at least five years.
These results join a growing dialogue about the optimal definition of GAD and the implications of revising some of its more controversial criteria in DSM-V and ICD-11. Our finding that relaxing these criteria leads to a sizable influx of new GAD cases inevitably raises questions about how high a rate of GAD is plausible and whether new cases are sufficiently severe to merit a diagnosis. It is clear that a broadened definition would lead to more widespread diagnosis than the GAD definitions in DSM-III, III-R, and IV, which have an estimated lifetime prevalence of 4% to 7% in the US population (Blazer et al., 1991
; Grant et al., 2005
; Kessler et al., 2005a
; Wittchen et al., 1994
). Perhaps the more critical issue, though, is whether broadly-defined GAD is likely to be more valid and clinically useful than DSM-IV GAD. Theorists disagree about the optimal diagnostic criteria for distinguishing normal from pathological general anxiety (Barlow & Wincze, 1998
; Rickels & Rynn, 2001
) and even about whether GAD is best conceived as a clinical syndrome, an anxious temperament type, or some combination of the two (Akiskal, 1998
; Rickels & Schweizer, 1995
). Such disagreement is particularly pronounced in debates over the DSM-IV excessiveness criterion, which has been criticized for focusing on characterological anxiety and excluding people whose anxiety is associated with severe or chronic stress (Kessler & Wittchen, 2002
). As in the case of depression, it is possible that distinguishing between endogenous and reactive anxiety is not useful for optimal diagnosis and treatment, and that a broader GAD construct would have greater clinical utility than the current narrowly-defined syndrome. This is supported by indications that some cases falling below the GAD threshold are sufficiently similar to diagnosed cases that their inclusion may improve the validity of the diagnosis. Important similarities have previously been observed in functional impairment, socio-demographic features, family history of GAD, and other key correlates between the full GAD syndrome and GAD lasting as little as one month (Bienvenu et al., 1998
; Carter et al., 2001
; Hunt et al., 2002
; Kendler et al., 1992
; Kessler et al., 2005b
; Maier et al., 2000
; Wittchen et al., 2002
) or involving non-excessive worry (Bienvenu et al., 1998
; Ruscio et al., 2005
Consistent with these results, we found few significant differences in the associations of GAD with later disorders as a function of GAD duration or excessiveness. This finding casts further doubt on the DSM-IV six-month duration and excessive-worry requirements, which appear to miss individuals who not only suffer from significant generalized anxiety, but have an elevated risk of developing additional disorders. While our results suggest similar conclusions about the requirement of three associated symptoms, it remains to be seen whether requiring fewer symptoms results in a clinically significant syndrome that is distinguishable from normal anxiety and transient stress reactions (Breslau & Davis, 1985
; Spitzer & Williams, 1984
). An alternate possibility is that requiring more
associated symptoms would result in a more valid GAD diagnosis that more powerfully predicts important outcomes. It is noteworthy in this regard that, in a clinical sample, Brown et al. (1995)
found a threshold of four rather than three associated symptoms to maximize diagnostic sensitivity (i.e., correspondence with DSM-III-R GAD diagnoses) and specificity (i.e., distinction from other anxiety and mood disorders). Thus, there is a need for systematic study of all possible thresholds along the associated symptoms criterion in relation to a wide range of correlates in both clinical and community samples. Finally, and crucial for the issue of predictive validity, we found that individuals missing two or more GAD criteria did not differ significantly from diagnosed cases in their risk for most subsequent disorders. There is a need to examine the severity and disability experienced by such individuals, who in our sample comprised more than one-quarter of new cases of broadly-defined GAD. There is also a need to determine whether early diagnosis and treatment of broadly-defined GAD would be effective in decreasing symptoms, improving associated disability, and preventing the subsequent development of comorbid disorders.
Present results run counter to concerns that less restrictive diagnostic criteria may lead to poorer differentiation of GAD from other disorders. If anything, we found the opposite effect: a slight reduction in associations with most other disorders as these GAD criteria are relaxed, with the most consistent reduction observed for later bipolar spectrum conditions. One possible explanation is that broadening the definition of GAD brings less severe cases into the diagnosis, and that lower rates of psychiatric comorbidity among less severe cases (Andrews et al., 2002
; Kessler et al., 2005c
) diminish the comorbidity associated with GAD. This explanation is partly supported by the association of GAD severity with risk of later disorders, especially mood and anxiety disorders, in our sample. In contrast to this general pattern, we found the odds of subsequent dysthymic disorder to be substantially higher for non-excessive than excessive GAD. Previous studies have documented very strong associations between GAD and dysthymia in both community (Andrews et al., 2002
) and clinical (Pini et al., 1997
) samples, suggesting that dysthymia may be a particularly challenging boundary condition for GAD and that broadening the GAD diagnosis to include non-excessive worriers may further impede differentiation of these conditions. Alternatively, the higher odds may reflect a genuine increase in the co-occurrence of these two disorders. It is possible, for example, that an individual exposed to a chronic, objectively severe stressor may at first experience a proportionate (non-excessive) anxiety response and, over time, develop persistent feelings of hopelessness and dysphoria. Future research will need to distinguish these possibilities, and to consider further refinements to the GAD definition that facilitate its distinction from neighboring conditions and improve its detection and appropriate treatment.