The Relationship between Period and Cohort Life Expectancy

Joshua R. Goldstein, Princeton University
Kenneth Wachter, University of California, Berkeley

We map the life expectancy observed in periods to specific birth cohorts. We find that the life expectancy of the current period in low mortality populations is about equal to that of cohorts born 50 years earlier. The period lag behind cohorts has been growing over time. A century ago, period life expectancy was only about 10 or 20 years behind cohort life expectancy. A century from now, the lag will be some 60 or 70 years. We present empirical findings and a mathematical model explaining cohort-period lags. We find that lags do not depend so much on the pace of mortality improvement as on a measure related to the entropy of the life table. Our approach offers an alternative to Bongaarts and Feeney's "tempo-adjusted" life expectancy and solves a problem set forth by Ryder in his classic work on tempo-cohort translation.