In April, the death of Nabi Tajima in a town called Kikai, on an island at the southern tip of Japan, rippled out across the world’s media. At 117, the great-great-great-grandmother was the world’s oldest person.
Deaths of such “supercentenarians” naturally raise the question of just how far we can go. Is there really a biological limit to the human life span? Some high-powered researchers have been slugging it out in the quest for an answer.
In 2016, a study published in the journal Nature found the maximum age at death increased from the early 1970s but plateaued, then decreased slightly around 1995, prompting the authors to conclude, “our results strongly suggest that human lifespan has a natural limit”.
Then in December last year, an analysis in the journal Extremes demurred, finding that after the age of 110 the risk of death stayed constant, at around 47% per year. This, the authors concluded, “does not support that there is a finite upper limit to the human lifespan”.
A bugbear for researchers has always been the accuracy of data. The old, for example, have an unfortunate tendency to exaggerate just how old they really are. It’s a problem new research published in the journal Science, and led by statistician Elisabetta Barbi from the Sapienza University of Rome in Italy, claims to have addressed in coming to its own conclusion on the vexed question.
The researchers availed themselves of a relatively new database from the Italian National Institute of Statistics that collated survival figures between 2009 and 2015 on every Italian aged over 105.
They claim the data, which required not only birth certificates but a “certificate of survival” for those yet to turn in their clogs, “allow estimation of mortality at extreme ages with accuracy and precision that were not possible before”.
The supercentenarian tally was 3836 people, 2883 of whom died during the study period. The team then analysed death rates across the cohort. In younger people these tend to follow the so-called Gompertz model (after nineteenth century British mathematician Benjamin Gompertz) of an exponential increase with age.
The team found that, while death rates do rise exponentially until the age of 80, they decelerate thereafter and plateau on or about the age of 105. For the group born in 1904, for example, that levelling out occurs at an annual probability of dying of 47.5%.
The result, then, places the authors firmly in the ranks of lifespan ceiling sceptics.
“The increasing number of exceptionally long-lived people and the fact that their mortality beyond 105 is seen to be declining across cohorts… strongly suggest that longevity is continuing to increase over time and that a limit, if any, has not been reached,” they conclude.
Paul Biegler is a philosopher, physician and Adjunct Research Fellow in Bioethics at Monash University. He received the 2012 Australasian Association of Philosophy Media Prize and his book The Ethical Treatment of Depression (MIT Press 2011) won the Australian Museum Eureka Prize for Research in Ethics.
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