This paper summarizes an article on abnormal psychology appearing in the sixty-fourth volume of the Archives of General Psychiatry as authored by Moffitt, Harrington, Caspi, Cohen, Goldberg, Gregory, & Poulton. Moffitt et al. (2007) investigated the cumulative and sequential comorbidity of depression and generalized anxiety disorder in a birth cohort that was followed prospectively to the age of 32 years. The authors prepared the article in response to a call for research to describe the ideal way of characterizing generalized anxiety disorder (GAD) in relation to co-existing major depressive disorder (MDD) in future diagnostic systems. The research problem of the article was whether the two closely related disorders should be classified together and whether GAD should be described as a sub-syndrome of MDD versus an independent disorder.
The Study and its Findings
To review the cumulative and sequential comorbidity between MDD and GAD, Moffitt et al. (2007) applied the prospective longitudinal cohort research design to study the participants who were sampled from the New Zealand Dunedin Multidisciplinary Health and Development Study. They consisted of 91 per cent eligible infants born in Dunedin within the period spanning from April 1, 1972 to March 31, 1973 who took part in the first follow-up at 3 years of age, forming the base sample for the longitudinal investigation. These participants represented all the socioeconomic classes of New Zealand’s South Island and attended full day data collection at ages 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, and recently at 32 years when 1015 (96 per cent) of the study members were still alive.
To measure the two variables (GAD and MDD), diagnoses of MDD in each past year were done at ages 11, 13, 15, 18, 21, 26, and 32 years. Diagnoses at ages 11, 13, and 15 years followed the DSM-III criteria while those at older ages were made in accordance with the Diagnostic Interview Schedule. The Diagnoses at ages 18 and 20 utilized the then-current DSM-III-R and the DSM-IV criteria was used to make diagnoses at ages 26 and 32 years (Moffitt et al., 2007). In a similar manner, GAD diagnoses were made according to the ages and diagnosis criteria used for MDD. In order to make possible the study of comorbidity, MDD was diagnosed regardless of the existence of other disorders. Additionally, the indicators of the use mental health services were assessed by way of life history calendar.
To analyze the data collected for this study, the relationships between MDD and GAD were assessed by use of ORs and 95 per cent CIs, while testing the sex differences in the associations and the findings were tabulated in tables accordingly. To augment statistical power comparison among groups, the sexes were combined but the sex was also controlled as it was a covariate measure. To ensure accuracy and validity of the study, 945 individuals were studied, 8.6 per cent with 1 missing data points and 91.4 per cent with all data points present; cohort members with 2 or more of the diagnostic data points among the critical age periods of 18, 21, 26, 32 years and juvenile were excluded from the analysis (Moffitt et al., 2007).
The results of the study showed that sequentially, anxiety ensued before or concurrently in 37 per cent of depression cases while depression started before or concurrently in 32 per cent of anxiety cases (Moffitt et al., 2007). Cumulative findings showed that 72 per cent of lifetime cases of anxiety gave a history of depression, whereas only 48 per cent of lifetime cases of depression coexisted with anxiety. In adulthood, 12 per cent of the cohort were reported to have comorbid MDD and GAD, with 66 per cent of them having recurrent MDD, 47 per cent having recurrent GAD, 64 per cent reporting use of mental health services, 47 per cent having taken medication, 8 per cent hospitalized and 11 per cent having attempted suicide. Moffitt et al. (2007) report that among the comorbid group, depression ensued first in a third of the cohort members, anxiety started first in one third, and both anxiety and depression began concurrently in the remaining one third of the participants.
The research conducted by Moffitt et al. (2007) presents interesting findings that categorically negate the common notion that depression is preceded by and develops eventually from generalized anxiety. Additionally, it is important to note that the relationship between MDD and GAD is quite strong, indicating that these disorders could be classified into the same category as distress disorders. Another lesson from the findings of the study is that the developmental association between the two conditions is evidently more symmetrical than it could be possibly presumed. This indicates that MDD does not necessarily take precedence over GAD in the hierarchy of diagnosis. Moreover, the prospective longitudinal study shows that lifetime prevalence of MDD and GAD may be possibly underestimated by retrospective studies due to the backward gathering of information about the members, most of which may be missing. To sum up, it is also evident that comorbid GAD and MDD make up a greater burden of mental health than previously assumed by psychiatrists.
Moffitt, T., Harrington, H., Caspi, A., Cohen, J., Goldberg, D., Gregory, A. & Poulton, P. (2007). Depression and generalized anxiety disorder: Cumulative and sequential comorbidity in a birth cohort followed prospectively to age 32 years. Archives of General Psychiatry, 64, 651-660.
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