# Cohort effect

A cohort is a group of people born in the same years. A cohort effect is a pattern that belongs to a birth cohort rather than to a place or an age: people born around 1900 in a village may reach 100 unusually often while people born in 1945 in the same village do not. Regional longevity claims often mix the two up. A count of living centenarians per 100,000 residents blends many cohorts and rises when young people leave, because the denominator shrinks. A cohort measure follows one birth cohort and asks what share of it reached a given age, so emigration of the young cannot inflate it. Rosero-Bixby's 2023 analysis of Costa Rica's Nicoya peninsula is the textbook case: men born around 1905 had a 33 percent lower death rate than other Costa Ricans, men born in 1945 a slightly higher one. The advantage was a cohort, not a postcode. Okinawa splits along the same line, with people born before the Second World War dying later than the rest of Japan and those born after it dying earlier.

## Sources

- Rosero-Bixby L. (2023). The vanishing advantage of longevity in Nicoya, Costa Rica: A cohort shift. Demographic Research. https://doi.org/10.4054/DemRes.2023.49.27
- Poulain M, Herm A. (2024). Exceptional longevity in Okinawa: Demographic trends since 1975. Journal of Internal Medicine. https://doi.org/10.1111/joim.13764

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_Canonical: https://longevity-germany.com/en/glossary/cohort-effect · Part of Longevity Cities · Updated 2026-09-07_