Depression and generalized anxiety disorder: cumulative and sequential comorbidity in a birth cohort followed prospectively to age 32 years.

CONTEXT The close association between generalized anxiety disorder (GAD) and major depressive disorder (MDD) prompts questions about how to characterize this association in future diagnostic systems. Most information about GAD-MDD comorbidity comes from patient samples and retrospective surveys. OBJECTIVE To revisit the sequential and cumulative comorbidity between GAD and MDD using data from a prospective longitudinal cohort. DESIGN Prospective longitudinal cohort study. SETTING New Zealand. PARTICIPANTS The representative 1972-1973 Dunedin birth cohort of 1037 members was followed up to age 32 years with 96% retention. MAIN OUTCOME MEASURES Research diagnoses of anxiety and depression were made at ages 11, 13, 15, 18, 21, 26, and 32 years. Mental health services were reported on a life history calendar. RESULTS Sequentially, anxiety began before or concurrently in 37% of depression cases, but depression began before or concurrently in 32% of anxiety cases. Cumulatively, 72% of lifetime anxiety cases had a history of depression, but 48% of lifetime depression cases had anxiety. During adulthood, 12% of the cohort had comorbid GAD + MDD, of whom 66% had recurrent MDD, 47% had recurrent GAD, 64% reported using mental health services, 47% took psychiatric medication, 8% were hospitalized, and 11% attempted suicide. In this comorbid group, depression onset occurred first in one third of the participants, anxiety onset occurred first in one third, and depression and anxiety onset began concurrently in one third. CONCLUSIONS Challenging the prevailing notion that generalized anxiety usually precedes depression and eventually develops into depression, these findings show that the reverse pattern occurs almost as often. The GAD-MDD relation is strong, suggesting that the disorders could be classified in 1 category of distress disorders. Their developmental relation seems more symmetrical than heretofore presumed, suggesting that MDD is not necessarily primary over GAD in diagnostic hierarchy. This prospective study suggests that the lifetime prevalence of GAD and MDD may be underestimated by retrospective surveys and that comorbid GAD + MDD constitutes a greater mental health burden than previously thought.