An Examination of Recent Historical Variability in Life Spans

The long upward trend in human life expectancy derives from progress in medicine in its broadest definition. The varied technologies and techniques involved have very different contributions to the shape of life expectancy, however. To pick the obvious examples: control of infectious disease improves the proportion of the population who can expect to reach adulthood, while better treatments for age-related disease improve the quality and remaining length of life of the old. In a number of past studies, researchers have worked through data sets on human life span to quantify the effects of various facets of medical progress. One such work estimates the upward trend in life expectancy is produced by an equal contribution from (a) reduction in premature deaths, such as via infectious disease, and (b) reduction in late life mortality rates, such as through better treatments for age-related disease.

If there was a more equal mortality rate at all ages, then life spans would vary more widely. In the past there was a high mortality rate at all ages due to infectious disease. In the future there will be a very low mortality rate at all ages, thanks to medical technologies capable of maintaining youth through periodic repair of the causes of aging, the various forms of cellular and molecular damage that accumulate over the years. In both cases the outcome would be a larger variation in life spans than is presently the case. Today, however, the state of medicine has created a distribution of mortality rates that runs from low in youth to high in old age, and this acts to decrease the variability of life span. Many more people make it to old age than was the case, and old age is where they die. The point to bear in mind is that this is both unusual considered in the broader scope of history and also a transient state of affairs: it wasn't the case in the past, and it won't be the case in the future.

Is it fair that some people will live much longer than others? Is life fair at all? Can it be made to be fair? No to all three counts. In a world in which aging is treatable and life is only limited by fatal accidents, some people will live for thousands of years longer than others. That is simply the way it will be, even if everyone sets out to keep accident rates as low as possible. Unfairness is looked upon with grave displeasure these days; ours is a culture that drinks deeply of the shallow waters of egalitarianism. The prospect of unfairness is often put forward in opposition to forms of development that might improve matters for everyone. It is shameful that some people would choose to ensure a continuation of present death and suffering because they feel uncomfortable about the inherently unfair nature of accident statistics. Yet this is not an unusual position.

If you are among the population of demographers who like to dig more deeply into the data, then there are numerous epicycles to be found beyond the high-level picture noted above, some counter-intuitive:

Why do lifespan variability trends for the young and old diverge? A perturbation analysis

For much of human history, mortality rates at all ages were relatively high and the length of human life was highly variable. During the course of the demographic transition, mortality rates declined, life expectancy rose, and the variability of the distribution of lifespans, or ages at death, changed in response. Two stages in the history of lifespan variability have been identified. In the first stage, spanning the late nineteenth and early twentieth century "the level of mortality fell ... resulting in a very large reduction in the disparities of life spans." The second stage, starting in the 1950s, was one in which "the increase in life expectancy is no longer associated with a reduction in the dispersion of life spans - or with only a very small reduction." A closer examination of variability trends suggests another key, yet overlooked, aspect of this story. In high-longevity populations, survival improvements have taken place at all ages, including the oldest, but trends in the variability of the distribution of ages at death have not exhibited a uniform pattern. Variation in the length of life has declined as life expectancy at birth has risen, but the variation in lifespan among survivors to older ages (e.g. 65 and above) has increased.

Previous research has shown that lives saved at younger ages reduce lifespan disparity, while lives saved at older ages increase it, with the threshold demarcating early and late ages changing in response to changes in the mortality schedule and the historical contingencies that shape it. Our results likewise indicate that early and late deaths have different implications for the variability of lifespan conditional on survival to successive ages. Perturbation analysis enabled us to quantify this relationship, showing the differential responses of variability measure conditional on survival to younger and older ages to the parameters defining the course of child, adult, and background mortality levels. In particular, we showed that lifespan variability decreases for younger ages because of its sensitivity to the childhood mortality parameters, and that lifespan variability at older ages has increased because its sensitivity to the decline in adult mortality is in fact negative.

Notably, the expansion of lifespan variability at older ages takes place despite the fact that deaths are being concentrated into older ages. While the classic description of mortality compression predicts that lifespan variability will decline as life expectancy rises, this decline in variability doesn't materialize at older ages because, as we show, the relationship between mortality rates and lifespan variability is negative at those ages. While the data leave no doubt that deaths are indeed being delayed into increasingly older ages, our analysis shows that the implications of such changes for lifespan variability patterns are not pre-determined, but rather depend in intricate ways on the specific pattern of mortality change by age and over time.

The longitudinal nature of our analysis further highlights the impact of the temporal pattern of mortality change (i.e. an initial decline in childhood mortality followed by a decline in adult mortality some decades later) on the differential trends in lifespan variability at younger and older ages. Mortality has declined at all ages, but not at the same time or to the same extent. For the successive cohorts aging through the dramatic population changes of the twentieth century, survival has improved at all ages, but more so in early life than in adulthood. At the same time, each successive cohort is reaching older ages with added benefits of lower mortality (and likely better health) throughout the life course, suggesting that the period trends we describe here may also be explained by cohort effects and changing distributions of health and vulnerability to mortality within cohorts.

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