Tracking a Cohort Through Life
A life table follows a hypothetical cohort — typically 100,000 individuals — from birth to the last survivor. At each age, it records the probability of dying, the number surviving, the person-years lived, and the expected remaining lifetime. This systematic accounting, first formalized by John Graunt in 1662 and refined by Edmund Halley in 1693, remains the cornerstone of actuarial science, public health planning, and demographic analysis.
The Gompertz Law of Aging
In 1825, Benjamin Gompertz discovered that adult mortality risk increases exponentially with age — a pattern so consistent it has been called a law of nature. After age 30, the annual probability of dying roughly doubles every 8-9 years. This exponential hazard function captures the fundamental biology of aging: cumulative damage to DNA, proteins, and cellular machinery progressively degrades the body's ability to maintain homeostasis and resist disease.
The Rectangularization of Survival
Pre-modern survival curves dropped steeply in infancy, then declined gradually throughout life. Modern curves have 'rectangularized' — remaining nearly flat until ages 60-70, then plunging steeply. This dramatic reshaping reflects the elimination of premature death from infection, malnutrition, and childbirth, concentrating mortality into a narrow age window near the biological lifespan limit. Whether this limit can be extended remains one of demography's most debated questions.
Life Expectancy: A Misunderstood Metric
Historical life expectancy of 35 years does not mean most people died at 35 — it means the average across all deaths, heavily weighted by infant mortality. A Roman who survived childhood could expect to live into their 60s. Today's life expectancy gains increasingly come from pushing back elderly mortality rather than reducing infant deaths, meaning each additional year of life expectancy requires progressively more medical intervention and expense.