I don’t want to be that guy but isn’t this kind of an obvious result? The main claim is that life expectancy improvements in the past century are mostly due to decreases in childhood mortality.
During the Roman period, the average life expectancy was only 22-25 years old because so many babies were dying prematurely.
If you could make it past the age of 10, then you were expected to make it to about 50, which almost doubles life expectancy.
https://en.m.wikipedia.org/wiki/Demography_of_the_Roman_Empi...
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