Population Council Research that makes a difference

The measurement of human longevity is one of the oldest and most important topics in demography. Estimates of the life expectancy at birth (i.e., the average age at death under current mortality conditions) are now routinely provided by national and international statistical agencies.

For example, the United Nations Population Division publishes such estimates for all countries in the world, ranging from a low of 37 years in Sierra Leone to 80 years in Japan for the period 1995–2000. Life expectancy at birth worldwide has risen sharply over the past half century and is estimated at 63 for males and 67 for females.

Life expectancy of a birth cohort may be calculated directly if survival in this cohort is observed until the last person dies. Life expectancy at birth is simply the average age at death. This calculation is unsatisfactory for many purposes, however, because it provides a selective summary of mortality over nearly a century, an interval during which mortality conditions are likely to have changed. Most obviously, life expectancy at birth calculated in this way is not useful for studying mortality change over periods of less than a century.

The study of mortality change over shorter time spans is generally based on age-specific death rates calculated for a single year or for periods of several consecutive years. Most life expectancy statistics in the demographic literature are calculated from such rates by life table methods that have been standard in the field for well over 100 years. They may be referred to as period life expectancies to distinguish them from the cohort or generational life expectancies calculated for groups of persons observed over long time periods.

This project documents that, for contemporary countries with high life expectancy, the conventional approach to the measurement of longevity is unsatisfactory whenever mortality is changing. It demonstrates that these conventional estimates of period life expectancy are affected by an undesirable “tempo effect.” This distortion is positive when the mean age at death is rising and negative when the mean is declining.

Estimates of the effect for females in three countries with high and rising life expectancy range from 1.6 years in the United States and Sweden to 2.4 years in France for the period 1980–95. This result implies that we do not live as long as we thought we did.