Humans, and many other species, suffer senescence: mortality increases and fertility declines with adult age. Some species, however, enjoy sustenance: mortality and fertility remain constant. Here we develop simple but general evolutionary-demographic models to explain the conditions that favor...

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There is a unique threshold age separating early deaths from late deaths such that averting an early death decreases life disparity, but averting a late death increases inequality in lifespans.

In a stationary population, age composition and the distribution of remaining lifespans are identical. This equivalence can be used to estimate age structure if information is available on time to death.