Estimating the force of mortality (mortality hazard) at the highest ages is a key to understanding the mechanisms that govern human mortality. It has been recently showed that human death rates increase exponentially up to about age 80, thereafter decelerate and reach or closely approach a plateau after age 105. The plateau seems to decline somewhat across cohorts. However, several important research questions are left unsettled. Is selective survival the sole responsible for mortality deceleration and the convergence to the plateau at very old ages? Are there other underlying mechanisms possibly contributing to this process? How is increasing longevity determined? Is mortality at the plateau decreasing over time or is rather the convergence to the plateau that is postponed to older and older ages?
To answer these questions, we will device a statistical model incorporating mortality selection and mathematically implying a mortality plateau at the extreme ages, and simultaneously accounting for yearly health and mortality improvements. Statistical strategies will be also developed to overcome possible estimation problems arising from such complex mortality models. The resulting model will be applied to surfaces of mortality rates over age and time by sex for Italy and possibly for other European countries.
The research proposed here is expected to answer the open questions about the biodemography of human longevity. Decomposing a surface of death rates in its main components, namely age factors (senescence/debilitation), period factors (improvement of current conditions) and cohort factors (selective survival), will shed light on the underlying mechanisms of longevity. For the first time, it will be possible to measure the impact of the selective survival on longevity and verify the existence of possible other explanations of extreme ages plateaus. Furthermore, we will estimate the shaping of the mortality curve and its temporal dynamics at extreme ages, contributing so to the lively international debate about the existence of biological limits to human lifespan. Finally, we contribute to knowledge providing the necessary clarity about the shaping of human longevity, an empirical clarity which is crucial for ongoing theoretical progress.