Cohort effect
DEKohorteneffekt
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.
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