Age, period, and cohort effects on adult mortality due to extrinsic causes of death

After decades of improvement, life expectancy momentarily declined during 2014-15 in several high income countries, with subsequent reversals in some cases. The main sources of this stagnation have been increases in mortality from influenza and drug overdoses, mainly for the baby-boomer generation. This trend is unexpected because it has long been assumed that extrinsic mortality, which is due to causes originating outside the body – in opposition to intrinsic mortality from degenerative diseases at old ages –, plays a negligible role in life expectancy changes. For this reason, the temporal patterns of extrinsic mortality have received little attention in demographic research. Period crises such as influenza epidemics and the opioid crisis are considered the main determinants of variations of extrinsic mortality. However, despite recent evidence suggesting that cohort effects have an important role in modulating extrinsic mortality, little is known about this relationship.  The main objective of this dissertation is to help fill this gap by examining cohort influences on extrinsic mortality change, with a particular emphasis on influenza and behavioral causes. More specifically, we aim (1) to quantify cohort differences in mortality from influenza and the influence of early life exposures to the virus on subsequent influenza mortality; (2) to analyze the baby boomers’ disadvantage in mortality in Canada and the United States, while identifying the contributions of behavioral causes to this disadvantage; and (3) to develop a methodological tool that can be used to both conduct visual analysis of the temporal dynamics of nonlinear Age-Period-Cohort (APC) effects, and compare these dynamics across various phenomena or populations. To achieve these goals, we use micro-level mortality data from vital statistics in Canada and the United States. We also employ death and fertility rates from various countries to generalize the visual analysis of nonlinear effects to other demographic phenomena. The analyses were conducted by applying Serfling models for the estimation of influenza mortality, demographic measures for the decomposition of cause-specific mortality changes, smoothing techniques for the identification of trends, and statistical and visual approaches on the Lexis configuration for the analysis of APC effects.  The results, in the form of three scientific articles, show that despite marked fluctuations over calendar years (periods), birth cohorts have an independent and sustained influence on influenza and mortality from behavioral causes. The main results from the first paper suggest that two mechanisms modulated influenza mortality over cohorts. For the young and adult population, the mortality risks over cohorts depend of the contrast between the first virus to which individuals were exposed (the virus producing an antigenic imprinting) and the virus encountered in adulthood during the observed epidemic. For this age segment, significant changes in risk were found during influenza epidemics among cohorts born during important antigenic shifts (e.g., a decrease in risk for cohorts born between 1957 and 1968). For older ages, we did not identify such “punctual” cohort effects but rather a smooth and monotonic change in cohort effects that might have driven a progressive decline in influenza mortality between 1959 and 2016. Inspired by so-called cohort morbidity phenotype and technophysio evolution theories, we attributed this decline to changes produced earlier, i.e., to the sharp sanitary improvements occurred between 1900 and 1930, when the concerned cohorts were born and when they could have benefited. Findings from the second paper revealed that most of the baby boomers’ excess mortality in Canada and the United States is driven by behavioral causes of death. The “boomer disadvantage” resulted from multiple cohort effects on behavioral-related mortality, and not from punctual period effects affecting the same cohort at different ages. Among the baby boomers, the risk of dying from hepatitis C was almost three times higher, and the risk of dying from drug-related causes was almost two times higher, than among the adjacent cohorts. These results were obtained using an innovative methodology developed in the third paper, which allowed us to analyze the dynamics of nonlinear effects over time through APC curvature plots. This technique provides greater flexibility than statistical models or other Lexis plots, and it has been shown to be applicable to other demographic phenomena, such as fertility. The findings presented in this dissertation offer evidence of the importance of analyzing cohort effects on extrinsic mortality. Our results indicate that even in the presence of substantial period disturbances affecting extrinsic mortality at most ages, cohort effects were sustained over time. These findings also suggest that public policies can significantly improve the health of the population by formulating policies that take into account the differential sensitivity of cohorts to risk factors and by providing social support to the most vulnerable cohorts.

Keyphrases: mortality, age-period-cohort effects, extrinsic causes, influenza mortality, baby boomers, behavioral mortality, drug overdoses, birth cohort effects, generational identity, Lexis surfaces