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Data InsightsOn average, people have lived much longer than the period life expectancy at their time of birth
May 28, 2024Saloni Dattani

On average, people have lived much longer than the period life expectancy at their time of birth

The chart shows a comparison between period and cohort life expectancy. Cohort life expectancy (the actual average lifespan) is higher than period life expectancy. This is because period life expectancy is calculated by assuming people will experience the current year’s mortality rates at each age at the corresponding ages in their lifetime.

But in reality, mortality rates declined throughout the 20th century, so people actually lived longer than what’s implied by period life expectancy.

Another reason for the difference is that period life expectancy is partly a reflection of conditions of the past that continue to affect older generations’ death rates today.

You can also see that the trendline of cohort life expectancy ends decades ago. It can only be measured retrospectively, because researchers need to wait for data on deaths of the population who were born more recently.

The data on this chart comes from the Human Mortality Database.

It shows that in 1930, people in France had a period life expectancy of 57 years. Period life expectancy is a metric that summarizes age-specific mortality rates in one particular year. This means that newborns would live 57 years on average if they experienced the same death rates at each age of their lives as those seen at each age in the population that year.

However, these newborns actually faced lower death rates than previous generations and lived an average of 69 years. This second measure — the average lifespan of a birth cohort — is called cohort life expectancy. It can only be calculated once all cohort members have died.

Period life expectancy is the more commonly reported “life expectancy” measure. However, these two measures are very different, and the gap can be large, as this data shows.

Read more on the difference between period and cohort measures