“There is a basic distinction between life expectancy and life span,” says Stanford University historian Walter Scheidel, a leading scholar of ancient Roman demography. “The life span of humans – opposed to life expectancy, which is a statistical construct – hasn’t really changed much at all, as far as I can tell.”

Life expectancy is an average. If you have two children, and one dies before their first birthday but the other lives to the age of 70, their average life expectancy is 35.