Inequality is something of a fixation these days; all too many people think that addressing inequality via forced reallocation of the wealth that exists is more important than generating more wealth for all through technological progress. That way lies only ruins. The growth of capabilities and wealth provided by technological progress must be the most important goal, above all others, particularly if we are to develop and benefit from rejuvenation biotechnologies.

Still, all too many people focus on inequality to the exclusion of progress, and inequality, not progress, is the hot button topic of the moment. Thus this paper on variability of human life span over time is presented as a discussion on inequality. Nonetheless, after skipping the rhetoric, the data is quite interesting. The years since 1950 have seen staggering advances in the state of medical technology, unevenly distributed between regions of the world, but the long term direction near everywhere is onward and upward. Despite this uneven distribution of wealth and technology, it seems that most of the variation in human life span is not found between wealthy and less wealthy regions, which may be a surprise to some observers.

Living a long and healthy life is among the most highly valued and universal human goals, so the unparalleled longevity gains recorded all over the world during the last few decades are cause for celebration. While a huge body of scholarship has shed considerable light on the 'efficiency part' of the process (i.e., the global, regional and national trajectories in life expectancy over time are very well documented), much less is known about the 'equality part'. Since mortality can arguably be considered the ultimate measure of health, lifespan inequalities should be seen as the most fundamental manifestations of health disparities.

Studies on lifespan disparities usually focus their attention on differences occurring either between or within countries. The former approach typically compares the average health performance among a cross-section of countries (most often by comparing the corresponding life expectancies) and aims at understanding why population health is better in some countries than in others. In contrast, the latter approach explores the lifespan differences that might exist among the individuals within a given country. Surprisingly, the study of global lifespan inequality - that is, the study of variations in individuals' lifespan both within and between all world countries - is largely underdeveloped.

Our findings indicate that (i) there has been a sustained decline in overall lifespan inequality, (ii) adult lifespan variability has also declined, but some plateaus and trend reversals have been identified, and (iii) lifespan inequality among the elderly has increased virtually everywhere. All these changes have occurred against a backdrop of generalized mortality reductions. While such an increase in elderly lifespan inequality should be expected in the context of increasing longevity, it is nonetheless important to document which countries or regions are spearheading the process and which ones are lagging behind.

The increase in lifespan variability among the elderly was previously investigated in a selected group of highly industrialized countries. According to the authors of that study, the systematic increases in longevity alter the health profile of survivors in fundamental ways: advances in medicine, socioeconomic conditions, and public health planning have facilitated frailer individuals reaching more advanced ages, thus increasing the heterogeneity in health profiles among the elderly. As shown in this paper, it turns out that such mechanisms might have been operating not only in high-income settings but also across all world countries and regions, irrespective of their stage in the demographic or epidemiological transitions.

Decomposing global lifespan inequality levels into within- and between-country components, we observe that most of the world variability in ages at death can be explained by differences occurring within countries. Depending on the inequality measure and the period we choose, the within-country component explains approximately 85% and 95% of the total variation. This suggests that traditional narratives in global health disparities focusing on international variations in life expectancy neglect the major source of lifespan inequality: the source that takes place within countries. This is precisely the component that has experienced the most dramatic changes during the last six decades. Indeed, our counterfactual analyses suggest that the observed changes in global lifespan inequality can be largely attributable to the changes in within-country lifespan distributions, while the contributions of increasing longevity and differential population growth have played a relatively minor role.

Since most lifespan variability takes place within countries, focusing on the trends of central longevity indicators alone disregards the major source of variability, thus potentially arriving at overly simplistic conclusions. During recent decades, much progress has been made in increasing longevity while reducing age-at-death variability across the full lifespan and, to a lesser extent, across adult ages. However, we now appear to face a new challenge: the emergence of diverging trends in lifespan inequality among the elderly around the globe. While lifespan inequality is increasing among the elderly across virtually all world countries, longevity and heterogeneity in mortality among the old has increased faster in the richer regions of the globe.