Cohort Life Expectancy Projects Up to Ten Years Longer Lifespans Than Period Measures

A recent discussion on social media, initiated by a tweet from Crémieux, has highlighted a critical distinction in how life expectancy is calculated, emphasizing that longitudinal (cohort) comparisons offer a more optimistic outlook than cross-sectional (period) projections. The tweet pointed out that "These divots in life expectancy tend to wash out. The issue is with longitudinal vs cross-sectional comparisons." This nuanced understanding is vital for accurate demographic analysis and long-term planning.

Period life expectancy, the most commonly cited metric, provides a snapshot of mortality rates in a specific year. It assumes a hypothetical group experiences the age-specific death rates observed in that single year throughout their entire lives. While useful for comparing mortality trends across different regions or time periods, this method often underestimates actual lifespans as it does not account for future improvements in medical science or living conditions.

In contrast, cohort life expectancy tracks the mortality experience of a specific group of people born in the same year as they age. This measure incorporates both observed past mortality rates and projected future improvements, offering a more realistic prediction of how long individuals within that cohort are expected to live. Because it anticipates ongoing advancements, cohort life expectancy typically presents a higher and less volatile estimate.

The discrepancy between these two measures can be substantial. Data from the Office for National Statistics (ONS) for England and Wales indicates that for cohorts born between 1950 and 2000, cohort life expectancy at birth has been approximately eight to ten years higher than period life expectancy. Similarly, future cohort life expectancy is projected to be seven to eight years greater than period estimates. This difference underscores Crémieux's observation that if you track the same people over time, "life expectancy looks more optimistic and less chaotic."

Experts widely regard cohort life expectancy as a more appropriate measure for understanding how long a person is truly expected to live, particularly for financial planning and policy-making related to pensions and healthcare. The ongoing improvements in healthcare and lifestyle factors mean that a person born today is likely to experience lower mortality rates at older ages than those observed in current period tables, making the cohort approach a more accurate reflection of future longevity.