  In the past six decades, lifespan inequality has varied greatly within and
among countries even while life expectancy has continued to increase. How and
why does mortality change generate this diversity? We derive a precise link
between changes in age-specific mortality and lifespan inequality, measured as
the variance of age at death. Key to this relationship is a young-old threshold
age, below and above which mortality decline respectively decreases and
increases lifespan inequality. First, we show that shifts in the threshold's
location modified the correlation between changes in life expectancy and
lifespan inequality over the last two centuries. Second, we analyze the post
Second World War trajectories of lifespan inequality in a set of developed
countries, Japan, Canada and the United States (US), where thresholds centered
on retirement age. Our method reveals how divergence in the age-pattern of
mortality change drives international divergence in lifespan inequality. Most
strikingly, early in the 1980s, mortality increases in young US males led
lifespan inequality to remain high in the US, while in Canada the decline of
inequality continued. In general, our wider international comparisons show that
mortality change varied most at young working ages after the Second World War,
particularly for males. We conclude that if mortality continues to stagnate at
young ages, yet declines steadily at old ages, increases in lifespan inequality
will become a common feature of future demographic change.
