Demographic Trends: Beyond the Limits?

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Caselli Graziella, Vallin Jacques.- Demographic Trends: Beyond the Limits? Life expectancy is clearly set to continue rising in many countries over the coming decades. The United Nations projects that it should converge around a maximum limit of 85 worldwide. If world fertility itself converges towards the absolute minimum replacement level of just under 2.1 children per woman with a life expectancy of 85, all the world's populations would stabilize not just in size (around 10 to 11 billion people by the turn of this century) but also age structure, at 24% under 20 years of age, 46% aged 20-59, and 30% aged 60 and above. But while this life expectancy of 85 years is an absolute limit for some authors, for others it is a gloomy prognosis on two counts. First, even if longevity increases no further, there is still a sufficiently wide gap between this putative limit and Jeanne Calment's age of 122 which the average person can hope to bridge. But there is also no proof that human longevity is an intractable constant. On the contrary, there is evidence that its boundaries are not fixed. That allows freerein to speculation. We have taken a conservative tack, hypothesizing a rise in life expectancy to around 150 years. All other things being equal, that would double the final pre-stabilization population size, but obviously at the cost of an unprecedented population ageing: at 2.1 children per woman, there would be only 14% of young people aged under 20, and 27% of adults aged 20 to 59, but 59% aged 60 and over (and even 30% aged 100-plus). But there is no evidence that fertility is set to level off permanently at 2.1 children per woman. Were the sporadically observable one-child trend to spread, then the population would be doomed to die out within a matter of centuries, but not before its age structure had altered even more spectacularly: with a life expectancy of 150, the under- 20s would account for less than 2%, the 20-59 year-olds under 7%, and the 60+ more than 91%. Even more acutely, 74% would be centenarians. Other hypotheses - especially as to birth sex ratios - add to the complexities of the projected outcomes.