Whether or not there is a limit to the human lifespan has been a subject of debate for millennia. Historical estimates of the maximum possible lifespan strongly suggest that it has increased substantially over recorded history.
A key but unresolved issue in the study of human mortality at older ages is whether mortality is being compressed (which implies that we may be approaching a maximum limit to the length of life) or postponed.
A new study- published in the journal PLOS ONE- analyzed the mortality of older individuals in richer countries using the Human Mortality Database. They analyzed the mortality by birth cohort rather than by period. Using cohort data follows a fixed set of individuals over time and is, therefore, most suited to clarify the biological mechanisms underlying mortality. In particular, using cohort data may avoid the conflation in period data of changes in mortality rates over time and/or age with changes across cohorts.
Using Gompertz law, scientists estimated the age (Gompertzian Maximum Age or GMA) at which individuals first reach an assumed mortality plateau and test whether this age has changed across birth cohorts or nots.
Scientists noted, “This approach allows us to disaggregate changes in remaining life expectancy at 50 in historical and current cohorts between postponement and compression explicitly using an analytical method. We emphasize, however, our conclusions on compression and postponement do not depend on either the existence of this plateau or on its assumed level.”
“Our analysis reveals that over much of our data, the GMA appears to have remained unchanged in some countries for centuries. But we find past and current episodes where mortality postponement has occurred and the GMA has risen.”
“In particular, for cohorts born between 1910 and 1940, we project that the GMA will increase rapidly, confirming the finding that in recent data, longevity does not appear to be approaching an upper limit.”
“In contrast to this work, however, we show that old-age mortality patterns can be well explained by cohort effects rather than period effects. These cohort patterns show further why longevity records have not changed in recent decades despite the well-documented improvements in mortality at older ages across much of the industrialized world in recent years.”
“We show that confidence intervals for the length of life of the longest-lived person in each cohort in each country derived using our approach fit historical data on extreme longevity very well. Extrapolating these results forward, we show that it is likely that longevity records will rise as cohorts born after 1910 reach advanced ages, although our projections of how much they will rise to depend on our modeling assumptions.”
As these cohorts attain advanced ages in the coming decades, longevity records may increase significantly. The results confirm prior work suggesting that if there is a maximum limit to the human lifespan, we are not approaching it.