Human Longevity, Death Rates and the “Mortality Plateau”

A new report published in the journal Science suggests that there may not be a “biological limit” to human longevity. This is just the latest addition to a contested issue within the demographic community.

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The report analyzed survival probabilities of about 4,000 “super-elderly” people in Italy. All subjects were aged 105 or older. The research team, led by demographer Elisabetta Barbi of Sapienza University and Francesco Lagona of University of Roma Tre, found that the risk of death essentially flatlines after age 105. The researchers say that the odds of dying from one year to the next are about 50:50 from 105 onward, creating what they call a “mortality plateau.”

“If there is a mortality plateau, then there is no limit to human longevity,” says Jean-Marie Robine, a demographer at the French Institute of Health and Medical Research in Montpellier. Robine was not involved in the study.

The debate regarding whether or not our species has an upper age limit is not new. Scientists generally concur that probability of death increases as we age. This continues at a steady rate until about age 80. Disagreement abounds, however, about the nature of aging as people hit their 90s and 100s.

Some demographic studies suggest there’s a fixed shelf-life for humans and that mortality rates continue to grow. Other conclusions support the idea that the risk of death plateaus once we reach the highly-elderly years. In this sense, human lifespans do not appear to have a biological upper limit.

Data Collection

Researchers looked at records of every Italian 105 years or older between 2009 and 2015 for the recent study. They gathered death, birth and survival certificates to minimize the chances of “age exaggeration,” a common issue among the oldest populations. They then tracked each individual’s likelihood of survival from year to year.

Since the study focused solely on Italy, researchers were able to prevent data variations among different regions. According to Kenneth Howse, health-policy researcher at the Oxford Institute of Population Ageing in the United Kingdom, “these data provide the best evidence to date of extreme-age mortality plateaus in humans.”

Death rates appear to slow down when people reach 105 years and older.
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Ken Wachter is a mathematical demographer from the University of California, Berkeley, and one author of the recent report. He thinks that earlier quarrels about the patterns of old-age mortality were results of sub-par information. “We have the advantage of better data,” he says. “If we can get data of this quality for other countries, I expect we’re going to see much the same pattern.”

Still, this small study does not provide definitive answers to the age-limit and longevity questions. Mr. Robine, for instance, mentions that unpublished data from France, Japan and Canada provides evidence that a “mortality plateau” is not altogether certain. He suggests a global study to determine whether the Italy data reflect a “universal feature of human aging.”

Questions Abound

There are currently roughly 500,000 people in the world aged 100 years or older. This stat is expected to nearly double with each coming decade.

The reasons why mortality rates seem to level off in extreme old age are still unknown. Siegfried Hekimi, a geneticist at McGill University in Montreal, Canada, posits that cells eventually reach a point where repair mechanisms can offset further damage. “Why this plateaus out and what it means about the process of aging — I don’t think we have any idea,” Hekimi says.

Even with research like this recent report, there is little consensus among the scientific community regarding human age-limits and a mortality plateau.

Brandon Milholland, who co-wrote a paper published in Nature in 2016 claiming human aging had reached its natural limit, says evidence for a mortality plateau is “marginal.” He cites the fact that this new study looked at less than 100 people who lived to be 110 years or older. Furthermore, small inaccuracies in the Italian longevity records could lead to illegitimate conclusions.

Thus, debates regarding human age-limits and longevity will surely continue.