Abstract
Birth cohort patterns in mortality are often used to infer long-lasting impacts of early life conditions. One of the most widely accepted examples of a birth cohort effect is that of tuberculosis mortality before the late 1940s. However the evidential basis for claims of cohort-specific declines in tuberculosis mortality is very slight. Reanalysis of original or enhanced versions of datasets used previously to support claims of cohort effects in tuberculosis mortality indicated that: 1. where the initial decline in tuberculosis mortality occurred within the period of observation, onset of decline occurred simultaneously in many age groups, in a pattern indicative of ‘period’ not cohort-dependent effects. 2. there was little evidence of ‘proportional hazard’-type cohort patterns in tuberculosis mortality for any female population studied. Therefore any mechanisms proposed to underlie this type of cohort pattern in male mortality must be sex-specific. 3. sex ratios of tuberculosis mortality at older ages peaked in cohorts born around 1900, and resembled cohort sex ratios of lung cancer mortality. This analysis indicates that age-specific patterns in the decline in tuberculosis mortality before 1950 are unlikely to reflect improvements in early life conditions. The patterns observed are generally more consistent with the influence of factors that reduced mortality simultaneously in most age groups. Additional influences, possibly smoking habits, impeded the decline of tuberculosis in older adult males, and produced the sex-specific shifts in age distributions of mortality that were previously interpreted as evidence of cohort-dependent mortality decline.
Highlights
Birth cohort effects in epidemiology can be defined as variations in risk according to year of birth and arising from influences unique to particular birth cohorts, or which affected some birth cohorts more than others [1]
Numerous authors have reported birth cohort patterns in tuberculosis mortality decline (Table 1), and these patterns have generally been interpreted as evidence of progressive improvements in early life conditions, that increased the survival of successive cohorts at all ages
In the male populations of Norway, Scotland and Ireland there was little evidence of a shift to the right, there was a reduction in the relative size of the peak in early adulthood
Summary
Birth cohort effects in epidemiology can be defined as variations in risk according to year of birth and arising from influences unique to particular birth cohorts, or which affected some birth cohorts more than others (for example because the effect varied with age at exposure) [1]. In an influential review of early life effects, Elo and Preston concluded that a number of childhood health conditions carried implications for survival at older ages, ‘‘but only in the case of respiratory tuberculosis has the demographic importance of a specific mechanism been established by cohort studies’’ [7]. Numerous authors have reported birth cohort patterns in tuberculosis mortality decline (Table 1), and these patterns have generally been interpreted as evidence of progressive improvements in early life conditions, that increased the survival of successive cohorts at all ages. Cohort patterns in mortality from respiratory tuberculosis in the late nineteenth and early twentieth centuries are often used to exemplify cohort effects in epidemiology textbooks, and have been proposed to underpin the cohort patterns observed in historical all-cause mortality decline in England and Wales [2], [7]. The term ‘cohort’ was first used in the demographic sense of a birth cohort in the context of tuberculosis mortality [8]
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