The human mortality experience has changed fundamentally as a result of the mortality transition. Not only are humans today living longer than their ancestors on average, they also experience greater certainty about the eventual timing of their death. This greater certainty is due to the considerable compression of the distribution of ages at death, which characterizes the mortality transition and results in lower life span disparity at the population level. In this dissertation, I investigate two key issues, which lie at the intersection of the mortality compression and mortality disparities literatures. First, I explore the recent transition from an era of mortality compression to a new era of mortality change, the shifting mortality era, in which life expectancy continues to increase but variability of age at death remains constant. Secondly, I examine differentials in life span disparity between the sexes and across countries over the course of the epidemiological transition. I adopt an age and cause-specific approach in analyzing trends in variability of age at death and differences in life span disparity across groups.

To account for age and cause-specific effects, I take advantage of recent methodological advances in demographic analysis and employ computationally based decomposition, perturbation, and simulation methods. Using an extensive collection of period life tables from the Human Mortality Database (HMD), I am able to explore aggregate level all-cause mortality change over the past one hundred twenty years in ten Western European countries. For some of my analyses, I take advantage of the full collection of historical mortality data available in the HMD, which includes populations in thirty-seven distinct geographic areas on five continents. Using a long series of cause-of-death data from France, which extends back to 1925, I am able to examine how sex-specific trends in variability of age at death were influenced by changes in age and cause-specific mortality during the epidemiological transition.

In documenting sex-specific trends in variability of age at death, I find that, similar to the emergence of the sex gap in life expectancy, a sex gap in variability of age at death developed around the midpoint of the 20th century with females experiencing lower life span disparity in comparison to males. Using decomposition techniques, I demonstrate that the gap emerged because females experienced greater reductions in premature mortality in comparison to males. Cause-of-death decomposition results for France reveal that this is due to (1) declines in infectious disease, which young females had suffered disproportionately, and (2) male disadvantage in external cause mortality across the stages of the epidemiological transition. Examining cross-country differences in life span disparity between Sweden and the United States, I find that a gap in variability of age at death emerged between these two countries with Sweden exhibiting lower variability due to (1) Sweden gaining an advantage in premature mortality and (2) the United States gaining an advantage in old age mortality.

Using measures of variability of age at death, I also examine trends in mortality compression and possible explanations for the recent transition to an era of shifting mortality. To this end, I investigate how initial mortality conditions interact with the age pattern of mortality change to produce mortality compression, expansion, or shifting. I find that proportional mortality change that is fixed at rates unvarying across age does not necessarily lead to a parallel shift in the death distribution. I discover that certain initial mortality conditions are particularly primed for compression and that there is a significant change in the age-pattern of the sensitivity of measures of variability of age at death to proportional changes in age-specific mortality rates over the course of the mortality transition. I demonstrate that the potential for compression still exists in more developed countries despite the current shifting trends and that trends in cancer related mortality may play a particularly important role in determining future trends in variability of age at death.