Definition
Life expectancy at birth (e₀) is a period summary measure of mortality: the average number of years a newborn would live if current age-specific death rates remained constant throughout their lifetime. It synthesizes an entire age schedule of mortality rates into a single number and is the dominant benchmark for tracking population health.
Key Ideas
- Period vs. cohort: Period life expectancy (LE) uses current-year death rates and is a hypothetical measure — no actual cohort experiences those exact rates. Cohort (generational) life expectancy tracks a real birth cohort forward in time and requires long-term mortality forecasts. Period LE typically understates cohort LE during secular mortality improvement.
- Life expectancy at age 65 (e₆₅): The additional years expected at retirement age; used for Social Security solvency analysis and pension actuarial work.
- Standard components: The National Vital Statistics System (NVSS) computes U.S. period LE annually from age-specific death rates; standard population weighting applies for age-adjusted comparisons.
- Year 2020: U.S. e₀ fell from 78.8 to 77.0 — a 1.8-year drop, the largest single-year decline in 75+ years — driven primarily by COVID-19, unintentional injuries, heart disease, homicide, and diabetes.
- Long-run trend: Best-practice female LE rose approximately 0.24 years/year from 1840 to 2000 with no clear deceleration. U.S. LE lagged peer countries, and Social Security Administration (SSA) mortality projections historically forecasted at roughly half the international rate of improvement.
How It Works
Period LE is computed from a life table: convert observed age-specific death rates (mₓ) to probabilities of death (qₓ), then accumulate survival fractions to produce the complete life table. Summing life-years lived (Lₓ) over all ages and dividing by the radix gives e₀. Lee-Carter and related models project future mₓ to generate probabilistic LE forecasts.
Why It Matters
e₀ is the most widely cited health indicator for international comparison and the central target of Social Security Trustees' long-term demographic assumptions. Small errors in LE projections compound over 75-year projection horizons, creating large financing uncertainty for pay-as-you-go systems.
Open Questions
- Whether the secular 0.24 y/yr improvement trend will continue, slow, or reverse given obesity, drug overdose deaths, and COVID-19 sequelae.
- How to reconcile period and cohort LE in welfare analysis of Social Security progressivity.
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