Mohammed T. Abou-Saleh

EPIDEMIOLOGY OF DEPRESSION 391Some investigators, however, have explored historical trendsvia the so-called ‘‘cohort effect’’. Klerman 21 noted that, incontrast to the ‘‘age of anxiety’’ following the Second WorldWar, modern Western society may be entering an ‘‘age ofmelancholy’’, precipitated by social factors, such as the threatof nuclear warfare, a perceived threat of environmentalpollution and economic instability. Estimates of prevalence byage from the Epidemiologic Catchment Area Studies in theUSA reveal a significant decrease in both current and lifetimeprevalence of major depression by age cross-sectionally. In aseparate study, Klerman et al. 22 , analyzing family history datafrom subjects in the Psychobiology of Depression Study, founda progressive increase in the rates of depression in successivelyyounger birth cohorts throughout the twentieth century, withan earlier age of onset of depression with each successive birthcohort. However, these cross-sectional studies, which suggest an‘‘age of melancholy’’, must be considered in the context ofhistorical studies.To interpret historical trends, one must consider the cohorteffect. Four findings suggest that younger birth cohorts carrya higher prevalence of depression than older cohorts. First,longitudinal studies have typically demonstrated a consistentburden of depression across the life cycle for each birthcohort 23 . That is, the prevalence of depression within a birthcohort does not change with increasing age. Second, youngerbirth cohorts appear to experience higher prevalences ofdepression than the older birth cohorts in the latter twodecades of the twentieth century 24,25 . Third, suicide rates doincrease with age, which in turn drives the overall rate ofsuicide up with age 26 . Fourth, the changes in rates of suicideand depression by age group over the past 50 years havebeen of such magnitude that the varying rates are bestexplained by psychosocial rather than biological or evolutionaryfactors.Those factors that contributed to a relative protection of olderbirth cohorts from depression and suicide in the 1990s in Westernsocieties, compared to younger cohorts, and that contributed tothe relative increase in depression and suicide among youngercohorts in the 1990s, are unknown (but subject to muchspeculation). Interpreting these data, investigators and cliniciansmust take care to recognize that significant methodologicalproblems in historical studies remain. Even a clear and decisiveendpoint, such as suicide, can be misleading, for suicide rates areobtained from a review of death certificates, which are subject toconsiderable bias. Methods for estimating the prevalence ofdepression change through time, and therefore it is difficult tocompare studies from many years in the past with current studies(due to an evolving instrumentation for measuring depression incommunity and clinical samples).CONCLUSIONIn summary, it appears that results from cross-sectional studies inWestern societies estimate the prevalence of depression in olderadults to be lower than the prevalence in young and middle-agedadults. These findings are most striking in the USA (particularlythe Epidemiologic Catchment Area Studies) but also appear inother studies. Nevertheless, these findings must be interpretedwith caution. Methodological problems in identifying depressionacross the life cycle may contribute to these difficulties. 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