Certain regions are repeatedly cited as having unusual numbers of people living past ninety. The observation is real in some cases, but the conclusions drawn from it stretch further than the evidence supports.
The observation starts with a counting problem
Claims about exceptional longevity depend on birth records. Where record-keeping was poor, ages at the top of the range are unreliable.
Some of the most cited clusters have shrunk considerably once birth registers were audited, particularly where pension incentives encouraged age exaggeration.
This does not dismiss the whole field, but it means the size of the effect in any given region is a first question rather than a settled fact.
The research design is observational
These studies look at populations that already exist and ask what they have in common. They cannot randomise anything.
Long-lived communities differ from comparison populations in dozens of ways at once, including diet, activity, altitude, genetics, social structure and historical exposure to famine and disease.
Isolating the contribution of any single factor from that tangle is not possible with this design, however carefully the analysis is done.
Shared traits are not the same as causes
The features usually highlighted, such as plant-heavy diets, daily physical work and dense social ties, are plausible and consistent with other evidence.
They are also the features an observer expects to find, which introduces a selection effect into what gets reported and repeated.
Less appealing shared traits, including limited healthcare access and physically demanding labour, tend to receive less attention in popular accounts.
Selection at the top of the age range distorts everything
People who reach very old age are survivors of their birth cohort. Their habits are recorded late, after the outcome has already been determined.
Whatever killed their contemporaries is invisible in the sample, so the surviving group can appear to demonstrate the safety of behaviours that were not survivable for others.
This is why researchers treat centenarian studies as generators of hypotheses rather than as sources of recommendations.
What the field does contribute
Where these populations have been studied genetically, they have pointed towards specific biological pathways that are now examined in controlled work.
They also provide a useful counterweight to the assumption that longevity is bought through medical intervention, since the clusters generally predate modern healthcare.
The reasonable reading is that they show what is possible under particular conditions, not that copying a fragment of those conditions will reproduce the result.