Academic journal article Population

Minimum Mortality: A Predictor of Future Progress?

Academic journal article Population

Minimum Mortality: A Predictor of Future Progress?

Article excerpt

Many authors have sought to determine the threshold beyond which gains in life expectancy might stall, once humans have exhausted their remaining biological reserves. Although a range of different methods and hypotheses have been used, most authors have turned out to be hugely pessimistic in the limits they propose, many of them having already been exceeded by the time they published their results.

Extremely cautious limits

Louis Dublin, probably one of the first to attempt the exercise,(1) wrote in 1928 that it was highly unlikely for human life expectancy to exceed 64.75 years, not knowing that statistics would soon reveal that New Zealanders(2) had already gone past 66 that same year. He returned to the question ten years later with Alfred Lotka, using New Zealand data, and suggested a maximum of 69.93 years (Dublin and Lotka, 1936). Icelanders promptly contradicted him in 1941. Undeterred, Dublin published a study that year where a "hypothetical life table" capped life expectancy at 70.78 years. Shortly afterwards, however, he seems to have given up the quest as fruitless, writing that: "In every case, within a relatively short period of years, actual mortality had improved close to or even beyond the point assumed, on what was thought at the time a reasonably optimistic basis. Thus, the most recent of these hypothetical life tables, constructed in 1941, exhibited an expectation of life at birth of 70.78 years, only one half year in excess of the figure actually experienced by white females in the United States in 1946" (Dublin et al. 1949, p. 168). In his memoirs published in 1951, he concluded: "Experience has shown that our optimistic views regarding prospects for improved longevity are generally conservative" (Dublin, 1951, p. 392).

This did not stop Jay Olshansky from making his own estimate of the probable upper limit for life expectancy, after deciding ex-ante on a minimum level below which age-specific mortality rates were highly unlikely to fall. Olshansky thus stated in 1986 that life expectancy would probably never exceed 85 years (Olshansky and AuIt, 1986). Later developments cast much doubt on this conclusion, and Olshansky finally admitted in 2001 that, based on the trends observed over the 1985-1995 period, the 85-year threshold could be attained by the Japanese in 2010 and by the French in 2014. He even acknowledged that a life expectancy of 100 years was not totally out of the question, but that this could only be reached by French women in 2106 and by Americans in 2577 (Olshansky, Carnes and Désesquelles, 2001). In fact, the life expectancy of Japanese women topped 85 years as early as 2002, and estimating American life expectancy in 570 years into the future solely on the basis of a ten-year observation period seems risky at best.

Eliminating exogenous mortality, then premature mortality

Using a different method, Jean Bourgeois-Pichat started working in the early 1950s to establish a "biological limit life table" by drawing a distinction between what he called exogenous mortality, attributable to external causes and which may be reduced, if not eliminated, through medical progress and broader access to healthcare, and endogenous mortality, due to deficiencies in the human body itself, and difficult to reduce given the current state of medical knowledge (Bourgeois-Pichat, 1952). After using a specific adjustment method to isolate endogenous infant mortality (Bourgeois-Pichat, 1951a, 1951b), the author noted that in Norway, the most advanced country at the time, this type of mortality appeared to be levelling off. He calculated that it was unlikely to drop below 13 per 1,000 for boys and 9 per 1,000 for girls. He then worked to separate endogenous and exogenous adult mortalities using Norwegian causeof-death statistics for ages where he deemed this separation to be least controversial (between 30 and 89 years). Bourgeois-Pichat found that endogenous age-specific mortality rates for the group followed the Gompertz law, so he simply retropolated data below age 30 until he met the infant mortality rate, and extrapolated beyond age 90 to come up with his biological limit life table, which placed life expectancy for men and women at 76. …

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